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Article

Advancing Research on Overlooked Invertebrates in Biological Control: A Case Study of Local Hoverflies and Wolf Spiders

by
Rosemary A. Knapp
1,*,
Robert McDougall
1 and
Paul A. Umina
1,2,*
1
Cesar Australia, 95 Albert Street, Brunswick, VIC 3056, Australia
2
Bio21 Institute, School of BioSciences, The University of Melbourne, Parkville, VIC 3010, Australia
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(5), 1203; https://doi.org/10.3390/agronomy15051203
Submission received: 5 April 2025 / Revised: 9 May 2025 / Accepted: 12 May 2025 / Published: 16 May 2025
(This article belongs to the Special Issue Advances in Beneficial Insects Research for Conservation Biological Control)
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Abstract

:
Preserving natural enemies in agricultural landscapes is a cornerstone of biological pest control, and avoiding insecticides and miticides that harm non-target species is a key strategy to support naturally occurring populations in the field. Current research on the impacts of these chemicals is often biased toward a small number of commercially cultured species, leaving important knowledge gaps for those groups that naturally occur at local scales. Hoverflies (Diptera: Syrphidae) and wolf spiders (Araneae: Lycosidae), both globally important invertebrates in agricultural systems, have been under-researched due to challenges in the field collection and laboratory cultivation of local species. This study helps to address these gaps by evaluating the effects of several widely used chemicals on Australian hoverflies (Melangyna sp.) and wolf spiders (Venatrix spp.) as case study species, with detailed descriptions of laboratory rearing and testing methodologies. The results from standardised chemical toxicity testing showed Venatrix spp. were relatively tolerant to various chemicals, highlighting their potential role in Integrated pest management (IPM) strategies that combine chemical and biological control methods. In contrast, Melangyna sp. was sensitive to numerous chemicals tested, including some that are widely regarded as safe for non-target species. These findings emphasise the need to expand research on underrepresented natural enemy groups to effectively support biological control efforts at local scales. Specifically, the methodologies developed in this study can be adapted to facilitate further research on locally occurring hoverfly and spider species in other regions.

1. Introduction

In agroecosystems, naturally occurring predators and parasitoids help protect crops from damage by controlling invertebrate pests [1,2]. Diversity among these natural enemy assemblages can enhance pest control efficacy through functional redundancy and niche complementarity [3]. As such, agricultural management strategies, such as conservation biological control (CBC), focus on supporting the biodiversity of resident natural enemy populations through habitat and resource provision and minimising the off-target impacts of insecticides and miticides [4]. Henceforth, throughout this manuscript, we use the term “chemicals” in reference to the diverse suite of insecticides and miticides applied to crops for pest control.
Hoverflies (Diptera: Syrphidae) and spiders (Araneae) represent two of the most abundant and widely distributed groups of predators occurring in agroecosystems [5]. Spiders are a diverse group of generalist predators that feed on a variety of prey, allowing them to colonise crops early in the cropping cycle [6]. As such, they can reduce pest populations before more specialised natural enemies, such as ladybirds and hoverflies, arrive later in the cropping cycle [6,7,8]. Hoverflies primarily prey upon aphids, with females able to detect and oviposit next to aphid colonies [9,10]. Emerging larvae are highly voracious, with some species consuming more than 1000 aphids during larval development [11]. As adults, hoverflies feed on nectar and pollen, providing additional benefits to many crops through pollination [12,13]. Hoverflies and spiders are generally utilised through CBC, with the exception of two hoverfly species, Sphaerophoria rueppelli and Eupeodes corollae, which are mass-reared for augmentative biological control (ABC) in European greenhouses [12].
Optimal crop protection often requires incorporating both biological and chemical control within integrated pest management (IPM) frameworks [4]. Harm to natural enemies can be minimised by avoiding chemicals with broad-spectrum neurotoxicity, such as synthetic pyrethroids, organophosphates, and carbamates, and opting instead for selective alternatives. Field and laboratory studies generally support the idea that selective chemicals are less harmful to natural enemies compared with broad-spectrum chemicals [4,14,15,16]. However, certain selective chemicals may still cause harm to natural enemies, and recent studies highlight that toxicity cannot be universally extrapolated across species [16,17,18]. Ongoing assessments of newer selective chemicals are, therefore, needed to understand their impacts on diverse natural enemy species.
Few studies have examined the impacts of chemicals on hoverflies and spiders. In the field, broad-spectrum chemicals are known to reduce the diversity and abundance of spider communities [6]. However, standardised laboratory tests on chemicals typically exclude spiders [19,20,21], leading to a lack of species-specific toxicity data for this group, particularly for selective chemicals [22]. Similarly, studies on the toxicity of chemicals to hoverflies are limited. Recent work has investigated the toxicity of neonicotinoids to adult hoverflies (S. rueppellii and Eristalinus aeneus) using methodologies adapted from honey bee toxicity testing [23]. However, research on larvae—the most important life stage for biological control and likely the most susceptible to chemical impacts—is also required. Only one study in the past 20 years has assessed the toxicity of selective chemicals to hoverfly larvae [24], leaving several newer chemicals unstudied.
Research on hoverflies and spiders has likely been hindered by challenges in the laboratory cultivation of local species. Efficient techniques for the field collection, rearing, and maintenance of both taxa are underdeveloped or labour-intensive [22,25]. This reflects a broader trend in ecotoxicology, where a limited number of species mass-reared for ABC constitute the majority of studies, as noted for predatory mites [16,26]. To support local-scale CBC efforts, there is a need for chemical toxicity assessments targeting resident natural enemies.
To address this need, we focus on hoverflies and spiders commonly found in Australian cropping systems: Melangyna sp. (Diptera: Syrphidae) and Venatrix spp. (Araneae: Lycosidae). Melangyna is the most abundant hoverfly genus in grain crops [27], orchards [28], and urban gardens [29] in south-eastern Australia. Although the Melangyna genus was once thought to consist of at least five Australian species, recent genetic analyses suggest the genus consists of a single polymorphic species [30], which we refer to as Melangyna sp. Wolf spiders are the most abundant ground-hunting spiders in Australian agricultural landscapes [31,32,33]. Venatrix konei is thought to be abundant in cotton crops [34], while other Lycosid species are found in vegetable crops, pastures, and shelter-belts [35,36]. Unfortunately, taxonomic keys and molecular diagnostics currently available are insufficient to confidently distinguish among Venatrix species in Australia. In this study, we evaluated the effects of broad-spectrum and selective chemicals used in the Australian grains industry on Melangyna sp. and Venatrix spp., following International Organisation for Biological Control (IOBC) methodologies. Additionally, we describe field collection and laboratory cultivation methods to support future research on these important natural enemy groups.

2. Methods

2.1. Collection and Rearing of Wolf Spiders

Adult female Venatrix spp. were collected from parks and gardens in Melbourne (Victoria, Australia) between September and January in 2021/22 and 2023/24. During this time, it is common to observe females carrying egg sacs or hatched spiderlings on their abdomens in clutches of 100–200, and those not visibly harbouring offspring often later produce egg sacs in the laboratory if gravid (R. Knapp, pers. obs.). Laboratory culturing of gravid or egg-sac-bearing females, therefore, provides an effective method of obtaining large numbers of juveniles for laboratory studies.
Collections took place at night, approximately 1 h after sunset, using a head lamp to identify eye shine; in wolf spiders, light is reflected by the tapeta, a layer of light-reflecting cells in their eyes, causing a bluish or greenish shimmer that reveals their presence [37]. Wolf spiders were collected into 50 mL screw-cap clear plastic containers and morphologically identified as Venatrix spp. in the field based on characteristic patterning on the ventral side of the abdomen [38]. All Venatrix adult females found were transported back to the laboratory within 24 h of collection. While some females already harboured egg sacs or hatched spiderlings, those without were assumed to potentially be gravid and also transported.
In the laboratory, Venatrix adult females were maintained individually in plastic containers at room temperature, with indirect exposure to natural daylight and photoperiod (~14:10 h L:D in Melbourne during January). A variety of container sizes and substrates were trialled initially. The most effective housing was a clear plastic container (~126 mm long, 208 mm wide and 178 mm high), with mesh windows that allowed ventilation and prevented the movement of individuals into and out of the containers. The optimum substrate was a mixture of potting mix (Scotts Osmocote Premium, Spring Hills, Australia) and leaf litter (~40 mm deep) to enable burrowing, with a small section of cardboard egg-carton placed on top (~50 mm long and 80 mm wide) to provide further shelter and opportunity to hide. Humidity was maintained by spraying the soil and leaf litter substrate with water twice weekly [39]. After introduction into the containers, all spiders were provided live Drosophila melanogaster as a food source ad libitum. Gravid females harbouring mature egg sacs were frequently observed to burrow into a web-like nest, either within the substrate or under the egg cartons, typically not emerging until after spiderlings had hatched.
Using the above approach, gravid females collected from the field between September and January continued to produce successive clutches of spiderlings until late April in both years. Some females produced as many as three clutches during this period.

2.2. Collection and Rearing of Hoverflies

Adult Melangyna sp. were collected between September and November in 2021 and again in 2023 from parks, gardens, canola fields, and shelter belts in and around Melbourne (Victoria, Australia). Melangyna sp. are attracted to yellow-flowering plants (e.g., Taraxacum spp., Brassica spp., Acacia spp., and Arctotheca calendula) and were captured by hand netting individuals that landed on these flowers. Once collected, individuals were placed in clear plastic containers (~175 mm long, 120 mm wide, and 43 mm high) with moist paper towel and plant material from the collection site. The containers were kept cool and transported back to the laboratory within 24 h. Melangyna sp. were morphologically identified based on abdomen shape and characteristic thoracic patterning [40].
Adult Melangyna sp. were housed in colonies of 10–15 individuals in Bug-Dorm insect rearing cages (475 mm wide, 475 mm long, 930 mm high; 4F4590 series, Australian Entomological Supplies, Australia). Adults were sexed by their external genitalia and the sex ratio in colonies was ~1:1. They were provided with cubed sugar, bee pollen (Bee SustainedTM), and water by adding damp paper towel. Colonies were supplemented with additional field-collected individuals weekly. Colonies were initially maintained in a controlled temperature (CT) cabinet at 20 °C and 16:8 h L:D photoperiod. To induce oviposition in females, we introduced a potted bok choy plant (Brassica rapa subsp. chinensis) infested with green peach aphids (Myzus persicae) for a 12 h period, twice a week. Leaves on which eggs had been laid were excised and placed into 1% agar in 90 mm Petri dishes lined with filter paper (Whattmann, 90 mm diameter) to maintain turgidity. These dishes were maintained in a CT cabinet at 20 °C and a photoperiod of 16:8 h L:D. Eggs were monitored daily, and hatched larvae were supplied ad libitum with additional live M. persicae.
This method proved successful between September and October, with colonies of 10–15 adults producing ~100–200 eggs per plant introduction. However, starting in November, this approach became ineffective, likely due to the absence of gravid females being collected in the field and the lack of mating inside the insect rearing cages, consistent with previous observations [41]. To encourage mating and oviposition from November onwards, the insect rearing cages were maintained at conditions emulating the field conditions of Melbourne in September. These conditions included a 10.5:13.5 h L:D photoperiod and temperatures ramping from a minimum of 10 °C during the dark period to a maximum of 25 °C during the light period. Under these conditions, successive generations of Melangyna sp. were bred without the need to capture additional individuals from the field. Observations of the same field sites where adults had been collected in September and October showed that Melangyna sp. were no longer active in those sites during the summer months (R. McDougall, pers. obs.).

2.3. Chemical Formulations

The chemicals tested this study were chosen because they are commonly used as foliar sprays in Australian grain crops. Chemicals were tested at their maximum registered field rates (MRFR) in grains and are referred to by their active ingredient (a.i.) throughout this manuscript. Detailed information on all chemical formulations and their rates tested (g or mL a.i./ha) is provided in Table 1. Chlorantraniliprole was tested at the MRFR and a second, higher rate (referred to as ‘chlorantraniliprole extra’) in anticipation of future changes to label registrations in Australia.

2.4. Acute Toxicity Bioassays

Acute chemical toxicity was assessed through laboratory bioassays based on IOBC protocols [21] and previous research [16,17,18,42]. Chemical treatments were applied using a Potter Spray Tower (Buckard Manufacturing, Rickmansworth, UK) to evenly coat 35 mm Petri dish bases with spray deposits of 1 mg/cm2, equivalent to a field foliar spray application volume of 100 L/ha. Residues were dried for 30–60 min in a fume hood before introducing test organisms. Venatrix spp. spiderlings used in the bioassays were age-matched based on the time of dispersal from the mother’s abdomen, with all individuals being used within 72 h of this occurrence. Melangyna sp. larvae used in the bioassays were second instar.
A single hoverfly larva or spiderling was transferred to each Petri dish using a sable-hair paintbrush or a mouth pooter, respectively. They were placed on a layer of unsprayed ParafilmTM in the dish lid, with ½ a radish (Raphanus sativus) cotyledon added for humidity. Hoverfly larvae were provided with ~15 live M. persicae nymphs, while spiderlings were provided with ~20 D. melanogaster flies. The sprayed Petri dish bases were then inverted onto the lids and sealed with ParafilmTM to prevent escape. Although neither test individuals nor the food source were directly placed on the sprayed surface, they were frequently observed moving across it. To prevent desiccation and minimise cross-treatment fumigant effects, all dishes within each treatment were housed in 85 L sealed plastic containers (Inadox, Australia) containing a piece of moist paper towel. After 48 h, individuals were scored as alive (actively moving), dead (no movement), or incapacitated (inhibited movement). Incapacitated individuals were pooled with dead individuals for analysis, as they are unlikely to survive or exhibit normal predatory behaviours in the field [43].
Data was collected through multiple bioassays as age-matched cohorts of sufficient larvae and spiderlings became available from laboratory populations. Each bioassay included a negative control (deionised water) and followed consistent methodologies. For Melangyna sp., we generally tested different chemicals in each bioassay, aiming for at least 20 individuals per chemical treatment. For Venatrix spp., we adopted a blocked experimental design, testing a consistent set of chemicals across multiple bioassays, with a combined target of at least 20 individuals per treatment. Further details on each bioassay are available in Table S1, and total individuals tested per chemical are shown in Table 1. Additionally, we prioritised testing chemicals with no prior data available for each natural enemy group, and hence, not all chemicals were tested on both taxa.

2.5. Statistical Analysis

Data were pooled from all bioassays and analysed using R (Version 4.4.1). As mean percent mortality in negative controls was below 10% (1.36% average for wolf spiders and 7.46% average for hoverflies), Abbott’s correction was not applied. Percent mortality of Melangyna sp. and Venatrix spp. was calculated for each chemical treatment. Binomial 95% confidence intervals (CIs) with Wilson’s adjustment were calculated for mortality percentages using the R package DescTools. Non-overlapping 95% CIs were used to infer statistical significance between chemical treatments (p < 0.05; [44]) within taxa. Where fewer than 20 individuals were tested, reduced statistical power is reflected through wider CIs and increased likelihood of overlap. Additionally, IOBC classifications for each chemical and taxa were derived from percent mortalities following guidelines for laboratory assessments, where 1 = ‘harmless’ (<30%), 2 = ‘slightly harmful’ (30–79%), 3 = ‘moderately harmful’ (80–99%), and 4 = ‘harmful’ (>99%) [21].

3. Results

3.1. Wolf Spiders

Out of the 18 chemical treatments tested on Venatrix spp., 14 showed no significant difference in mortality compared with the negative control (Table 1, Figure 1) and were classified as IOBC category 1 (‘harmless’; <30% mortality). Four chemical treatments had statistically significant impacts on Venatrix spp. mortality compared with the control (non-overlapping 95% CIs; p < 0.05). Methomyl and bifenthrin, both broad-spectrum chemicals, were highly toxic to Venatrix spp., causing >99% mortality and receiving IOBC classifications of 4 (‘harmful’). Dimethoate, another broad-spectrum chemical, resulted in 93.75% (71.67–98.89%) mortality, placing it in IOBC category 3 (‘moderately harmful’; 80–99% mortality), while pirimicarb was classified as IOBC category 2 (‘slightly harmful’; 30–79% mortality).

3.2. Hoverflies

Nineteen chemicals were tested on Melangyna sp. Of these, nine were classified as IOBC category 1 (‘harmless’), with no significant differences in mortality compared with the negative control (Table 1, Figure 1). Cyantraniliprole, NPV, and Bacillus thuringiensis (Bt) were categorised as IOBC category 2 (‘slightly harmful’); however, only Bt showed a statistically significant increase in mortality compared with the control (non-overlapping 95% CIs; p < 0.05). Methomyl, bifenthrin, dimethoate, spinetoram, abamectin, thiodicarb, and chlorpyrifos were all toxic to Melangyna sp., with statistically significant impacts on mortality compared with the control (non-overlapping 95% CIs; p < 0.05). Thiodicarb and chlorpyrifos were the most toxic, receiving IOBC classifications of 4 (‘harmful’), while the remaining chemicals were classified as IOBC category 3 (‘moderately harmful’).

3.3. Comparisons Between Wolf Spiders and Hoverflies

Although the chemicals tested differed slightly between the two taxa, direct comparisons can be made for several chemicals. Flonicamid, chlorantraniliprole, afidopyropen, indoxacarb, diafenthiuron, gamma-cyhalothrin, and paraffinic oil were all classified as IOBC 1 (‘harmless) for both Melangyna sp. and Venatrix spp., while dimethoate, methomyl and bifenthrin were classified as either IOBC category 3 (‘moderately harmful’) or category 4 (‘harmful’) for both.
In contrast, there were considerable differences in the effects of other chemicals between Melangyna sp. and Venatrix spp., with Melangyna sp. generally showing a greater level of sensitivity. Notably, the effects of spinetoram and thiodicarb differed markedly between the taxa; spinetoram caused 83.33% (66.44–92.66%) mortality in Melangyna sp. yet was harmless to Venatrix spp. (0.00%; 0.00–16.11% mortality). Similarly, thiodicarb was highly toxic to Melangyna sp. (100.00%; 83.89–100.00% mortality) while having minimal impact on Venatrix spp. (5.00%; 0.89–23.61% mortality). Melangyna sp. also showed greater sensitivity to NPV and cyantraniliprole compared with Venatrix spp. These chemicals were classified as IOBC category 2 (‘slightly harmful’) towards Melangyna sp., but category 1 (‘harmless’) towards Venatrix spp.

4. Discussion

Our study provides a timely overview of the acute toxicity of chemicals commonly used in Australian grain crops towards two important natural enemies, hoverflies (Melangyna sp.) and wolf spiders (Venatrix spp.). Our results revealed significant differences in sensitivity between these taxa, with Melangyna sp. generally exhibiting greater sensitivity to certain chemicals, particularly spinetoram and thiodicarb, compared to Venatrix spp. This differential toxicity is important for IPM strategies, which rely on understanding the impacts of chemical applications towards non-target organisms [4]. While several chemicals, including flonicamid and chlorantraniliprole, were deemed harmless to both Melangyna sp. and Venatrix spp., others, such as methomyl, bifenthrin, and dimethoate, caused substantial mortality, highlighting the need for careful selection of pest control products. Additionally, we detail field collection and laboratory cultivation methods for Melangyna sp. and Venatrix spp. to help address the scarcity of published methods and the inherent challenges of conducting laboratory-based research on these important natural enemy groups. These findings contribute to the growing body of knowledge required for effective CBC and underscore the importance of refining chemical usage to achieve pest control and protect biodiversity in agroecosystems [4].
Broad-spectrum chemicals such as organophosphates, synthetic pyrethroids, and carbamates have generally been found to have indiscriminately high toxicity towards natural enemies [16,45,46]. While organophosphates caused mostly high toxicity to Melangyna sp. and Venatrix spp. (‘moderately harmful’ or ‘harmful’), the toxicity of synthetic pyrethroids varied by compound type. Consistent with previous work on parasitoid wasps [18,47], we found Melangyna sp. and Venatrix spp. were more sensitive to the type I pyrethroids (e.g., bifenthrin) than the type II pyrethroids (e.g., gamma-cyhalothrin). This highlights the importance of not generalising toxicity across pyrethroid compounds [18]. Although gamma-cyhalothrin was classified as ‘harmless’ to Melangyna sp. and Venatrix spp., sub-lethal side effects were not examined; hence, CBC and ABC programs should remain cautious in its use. Gamma-cyhalothrin has been demonstrated to cause detrimental sub-lethal effects in other natural enemies, such as reductions in fecundity of parasitoid wasps including Aphelinus abdominalis, Aphidius colemani, and Telenomus remus [42,48]. Furthermore, both field and laboratory studies have shown that applications of the type II pyrethroid lambda-cyhalothrin are damaging to spider populations [22,49,50], suggesting that gamma-cyhalothrin may be similarly detrimental in field settings.
While carbamates are generally characterized by broad-spectrum neurotoxicity (e.g., methomyl is highly toxic to various natural enemies [16,51,52]), pirimicarb is often considered to be selective due to its low toxicity to natural enemy groups such as Anthicidae, Chrysopidae, and Coccinellidae [53,54,55]. In spiders, previous work has also found pirimicarb to be ‘harmless’ towards Erigone atra and Oedothorax apicatus [22]. However, our data classified pirimicarb as ‘slightly harmful’ towards Venatrix spp., indicating species-specific responses among spiders. Previous work demonstrating the sensitivity of various parasitoid wasp and predatory mite species to pirimicarb has also challenged the true selectivity of this chemical [16,18]. The toxicity of pirimicarb towards Melangyna sp. was not assessed in this study, as we prioritized testing chemicals for which no prior data was available for hoverflies. Previous studies have demonstrated that pirimicarb is highly toxic to hoverfly larvae [24,56]. Our findings, together with these previous studies, paint a mixed picture for pirimicarb’s toxicity profile to natural enemies, suggesting its suitability for IPM likely varies on a case-by-case basis.
Several chemicals were classified as ‘harmless’ to both Melangyna sp. and Venatrix spp., including selective chemicals such as flonicamid, afidopyropen, and chlorantraniliprole, with chlorantraniliprole, in particular, showing low toxicity even when tested at twice the current registered field rate in Australian grain crops (i.e., 30 g a.i./ha). This is consistent with a large body of laboratory research similarly classifying these chemicals as ‘harmless’ to various natural enemy groups [16,52,57,58,59,60], including larvae of the hoverfly Episyrphus balteatus [24]. However, flonicamid was found to reduce the reproductive output of E. balteatus adults, leading to an overall classification of this chemical as ‘slightly harmful’ [24]. More broadly, there is accumulating evidence of sub-lethal and transgenerational impacts of both selective and broad-spectrum chemicals on natural enemies, including impairments to physiology, development, behaviour, locomotion, and fecundity [42,61,62,63,64]. Therefore, while the chemicals identified as ‘harmless’ to Melangyna sp. and Venatrix spp. in this study will likely be more compatible with IPM than those receiving higher IOBC classifications, they are not completely without risk. Future laboratory studies should also investigate sub-lethal and transgenerational impacts to gain a comprehensive understanding of chemical risks, as recommended by Desneux and colleagues [61].
Cyantraniliprole, NPV, and Bt, all of which are widely considered selective, were classified as ‘slightly harmful’ towards Melangyna sp. However, it is important to note that only Bt caused a statistically significant increase in mortality. The sensitivity of Melangyna sp. to Bt is noteworthy, given laboratory studies have consistently classified Bt as ‘harmless’ towards numerous natural enemy groups, including predatory mites (Odontoscirus lapidaria), aphid parasitoids (A. colemani, A. abdominalis, and Diaeretiella rapae), and predatory beetles (Dalotia coriaria) [16,17,18,52,65,66]. Bt is a bacterium that produces insecticidal proteins with high specificity towards certain insect groups, owing to the unique toxin combinations produced by each subspecies and strain [67]. The Bt subspecies evaluated in this study, Bt var. kurstaki, synthesises five insecticidal proteins specific to lepidopterans: Cry1Aa, Cry1Ab, Cry1Ac, Cry2Aa, and Cry2Ab [68]. Although primarily used against lepidopteran larvae, these toxins have also been shown to affect dipteran hosts [67], with previous research on Drosophila species indicating susceptibility to Bt var. kurstaki [69]. The sensitivity of Melangyna sp. reported here represents evidence that Bt can have detrimental impacts on hoverflies (see also [70]). Given their vital roles as natural enemies and pollinators, we recommend further studies on the toxicity of various Bt strains to hoverflies. More broadly, Melangyna sp. showed either similar or increased sensitivity to a range of chemicals compared with Venatrix spp. As legless, soft-bodied organisms, Melangyna sp. larvae may have greater vulnerability due to increased contact with chemical residues, as suggested for other hoverfly species [24]. While further study is needed to confirm this, our findings indicate that local CBC strategies should be particularly cautious in applying chemicals when hoverfly larvae are seasonally active in crops (e.g., September–October in Victoria, Australia). On the other hand, Venatrix spp. have mainly tarsal contact with the sprayed surface. The relative resilience of Venatrix spp. observed in this study shows promise for their utility in CBC when used alongside selective chemical applications. However, it is important to note that species-level identification was not possible due to the lack of reliable morphological and molecular diagnostic tools for this genus.
Chemical toxicity assessments on Venatrix spp. and Melangyna sp. are scarce given the challenges of obtaining sufficient numbers of individuals for laboratory studies. For wolf spiders, commercial rearing of individuals for ABC does not occur. The methodologies documented for Venatrix spp. in this study, as well as previous work on Pardosa pseudoannulata [71], represent important advancements for further research into this group. For hoverflies, artificial rearing of pollinating species such as drone flies (e.g., Eristalis tenax) has been well documented [72]. However, the aquatic saprophagous life history of drone fly larvae mean these methods are not applicable to aphidophagous species relevant to biological control in agriculture. Mass-rearing methods for the European aphidophagous species E. corollae have only recently been investigated [73], and methodologies for Australian taxa such as Melangyna sp. have received very little attention. Similar to E. corollae, our cultivation efforts for Melangyna sp. indicate the major elements for mass-rearing larvae from field-collected adults to be (A) the provision of fresh pollen, sugar, and water for adults, (B) the provision of aphid-infested plant hosts for oviposition, and (C) the provision of sufficient aphids for larval feeding [73,74,75]. Rearing aphidophagous hoverflies for testing in laboratory bioassays presents specific challenges that have not been addressed in previous literature, as large numbers of age-matched individuals are required for sufficient statistical power. Here, our methods yielded age-matched cohorts of Melangyna sp. larvae through the provision of aphid-infested plants for a 12 h period and the separation of eggs into Petri dishes to reduce cannibalism among emerging larvae [73]. Although we made progress in the cultivation of both Venatrix spp. spiderlings and Melangyna sp. larvae in the laboratory, these techniques remain labour-intensive, with age-matched cohorts often only available for testing at staggered and unpredictable intervals.

5. Conclusions

In conclusion, this study provides valuable insights into the acute toxicity of chemicals on two important natural enemies, Melangyna sp. and Venatrix spp. Our findings underscore significant differences in the sensitivity of these taxa to various chemicals, with Melangyna sp. generally exhibiting greater sensitivity to chemicals compared with Venatrix spp. These results are crucial for the development of IPM strategies, which must account for off-target impacts on natural enemies. While some chemicals, such as flonicamid and chlorantraniliprole, were classified as ‘harmless’ to both species, others like methomyl and bifenthrin caused substantial mortality, emphasizing the importance of selective chemical use in IPM programs. Additionally, our work contributes to the small body of knowledge on field collection and laboratory cultivation techniques for these species, addressing challenges often faced when researching wolf spiders and hoverflies in the laboratory.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy15051203/s1. Table S1. Additional information on data generated from each discrete bioassay, including chemicals tested and their rates, the number of individuals tested, and percent mortality of each treatment, including the negative controls (water).

Author Contributions

Conceptualization, R.M. and P.A.U.; Methodology, R.A.K., R.M. and P.A.U.; Formal Analysis, R.A.K.; Investigation, R.A.K. and R.M.; Resources, P.A.U.; Data Curation, R.A.K.; Writing—Original Draft Preparation, R.A.K.; Writing—Review and Editing, R.A.K., R.M. and P.A.U.; Visualization, R.A.K.; Supervision, P.A.U.; Project Administration, R.A.K., R.M. and P.A.U.; Funding Acquisition, P.A.U. All authors have read and agreed to the published version of the manuscript.

Funding

This research was made possible through funding from the Australian Grains Research and Development Corporation (grant numbers UOM1906-002RTX and UOM2404-006RTX).

Data Availability Statement

The original data presented in the study are openly available in zenodo at https://doi.org/10.5281/zenodo.15172773.

Acknowledgments

We thank Ary Hoffmann, Lizzy Lowe, Cait Selleck, Karyn Moore, and Emily Doyle for technical assistance, and the many agrochemical companies who freely provided chemical samples used in this study. We acknowledge the Traditional Custodians of the land on which our research was conducted and pay our respects to their Elders past and present.

Conflicts of Interest

Rosemary A. Knapp, Robert McDougall and Paul A. Umina are employed by the Cesar Australia and all authors declare no conflicts of interest.

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Agronomy 15 01203 g001
Figure 1. Percent mortality of hoverflies (Melangyna sp.) and wolf spiders (Venatrix spp.) after 48 h direct exposure to sprayed chemical residues in laboratory bioassays, where water is the negative control. Error bars denote 95% binomial confidence intervals (CIs), with Wilson’s adjustment. Standardised International Organisation for Biological Control (IOBC) classifications of chemicals based on mortality cut-offs are shown by differing colours, where 1 = ‘harmless’, 2 = ‘slightly harmful’, 3 = ‘moderately harmful’, and 4 = ‘harmful’.
Figure 1. Percent mortality of hoverflies (Melangyna sp.) and wolf spiders (Venatrix spp.) after 48 h direct exposure to sprayed chemical residues in laboratory bioassays, where water is the negative control. Error bars denote 95% binomial confidence intervals (CIs), with Wilson’s adjustment. Standardised International Organisation for Biological Control (IOBC) classifications of chemicals based on mortality cut-offs are shown by differing colours, where 1 = ‘harmless’, 2 = ‘slightly harmful’, 3 = ‘moderately harmful’, and 4 = ‘harmful’.
Agronomy 15 01203 g001
Table 1. Acute toxicity of chemicals towards hoverflies (Melangyna sp.) and wolf spiders (Venatrix spp.) and associated International Organisation for Biological Control (IOBC) classifications. Details for each chemical treatment include the mode of action (MoA) group as per Insecticide Resistance Action Committee (IRAC) classifications, product name, and formulation. Rates tested correspond with the maximum registered field rates of chemicals in Australian grain crops. N denotes the number of individuals tested. Percent mortality and 95% confidence intervals (CIs), with Wilson’s adjustment, are reported at 48 h exposure. Lowercase letters next to percent mortalities indicate non-overlapping confidence intervals between chemicals and are used to infer statistical significance. NA = not applicable.
Table 1. Acute toxicity of chemicals towards hoverflies (Melangyna sp.) and wolf spiders (Venatrix spp.) and associated International Organisation for Biological Control (IOBC) classifications. Details for each chemical treatment include the mode of action (MoA) group as per Insecticide Resistance Action Committee (IRAC) classifications, product name, and formulation. Rates tested correspond with the maximum registered field rates of chemicals in Australian grain crops. N denotes the number of individuals tested. Percent mortality and 95% confidence intervals (CIs), with Wilson’s adjustment, are reported at 48 h exposure. Lowercase letters next to percent mortalities indicate non-overlapping confidence intervals between chemicals and are used to infer statistical significance. NA = not applicable.
Mode of Action (MoA) GroupActive Ingredient (a.i.)Product Name (Distributer)Formulation (g a.i./kg or mL a.i./L)Rate Tested (g or mL a.i./ha)Hoverflies (Melangyna sp.)Wolf Spiders (Venatrix spp.)
Percent Mortality (95% CIs)IOBC ClassificationNPercent Mortality (95% CIs)IOBC ClassificationN
NAWaterNANANA7.46 (4.57–11.95) abNA2011.36 (0.37–4.82) aNA147
CarbamatesMethomylMethomyl 225 (Nufarm)22545090.00 (69.9–97.21) cd320100.00 (81.57–100.00) c417
PirimicarbPirimor WG (Syngenta)500500---36.00 (20.25–55.48) b225
ThiodicarbLarvin 375 (Bayer)375281.25100.00 (83.89–100.00) d4205.00 (0.89–23.61) ab120
OrganophosphatesChlorpyrifosLorsban 500 EC (Corteva)500750100.00 (83.89–100.00) d420---
DimethoateDimethoate 400 (ADAMA)40032096.43 (82.29–99.37) cd32893.75 (71.67–98.89) c316
PyrethroidsBifenthrinTalstar 250 EC (FMC)2508093.75 (83.16–97.85) cd348100.00 (87.94–100.00) c428
Gamma-cyhalothrinTrojan (FMC)1504.519.40 (11.71–30.42) b1673.33 (0.59–16.67) a130
SulfoximinesSulfoxaflorTransform WG (Corteva)5005026.67 (14.18–44.45) b13012.00 (4.17–29.96) ab125
SpinosynsSpinetoramSuccess Neo (Corteva)5005083.33 (66.44–92.66) cd3300.00 (0.00–16.11) a120
AvermectinsAbamectinVantal Upgrade 36 (FMC)365.497.50 (87.12–99.56) d340---
Emamectin BenzoateAffirm (Syngenta)175.1---4.00 (0.71–19.54) a125
PyropenesAfidopyropenVersys (BASF)10056.67 (1.85–21.32) a1307.14 (1.98–22.65) ab128
Bacillus thuringiensisBacillus thuringiensis (Bt) subsp. kurstakiDiPel DF (Sumitomo)2803285.776.19 (61.47–86.52) c242---
DiafenthiuronDiafenthiuronPegasus (Syngenta)50030010.00 (3.46–25.62) ab13015.00 (5.24–36.04) ab120
OxadiazinesIndoxacarbSteward EC (FMC)150600.00 (0.00–11.35) a13010.00 (2.79–30.1) ab120
DiamidesChlorantraniliproleAltacor (FMC)350153.57 (0.63–17.71) a1283.45 (0.61–17.18) a129
Chlorantraniliprole ExtraAltacor (FMC)350303.33 (0.59–16.67) a1308.33 (2.32–25.85) ab124
CyantraniliproleExirel (FMC)1001530.00 (14.55–51.9) b2200.00 (0.00–16.11) a120
FlonicamidFlonicamidMainMan (ISK)500500.00 (0.00–11.35) a1300.00 (0.00–11.35) a130
BaculovirusesNuclear Polyhedrosis Virus (NPV)NearNPV (AgBiTech)7.5 × 10910037.50 (24.22–52.97) b2400.00 (0.00–16.11) a120
NAParaffinic OilCanopy (FMC)778158420.00 (9.51–37.31) ab1300.00 (0.00–16.11) a120
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MDPI and ACS Style

Knapp, R.A.; McDougall, R.; Umina, P.A. Advancing Research on Overlooked Invertebrates in Biological Control: A Case Study of Local Hoverflies and Wolf Spiders. Agronomy 2025, 15, 1203. https://doi.org/10.3390/agronomy15051203

AMA Style

Knapp RA, McDougall R, Umina PA. Advancing Research on Overlooked Invertebrates in Biological Control: A Case Study of Local Hoverflies and Wolf Spiders. Agronomy. 2025; 15(5):1203. https://doi.org/10.3390/agronomy15051203

Chicago/Turabian Style

Knapp, Rosemary A., Robert McDougall, and Paul A. Umina. 2025. "Advancing Research on Overlooked Invertebrates in Biological Control: A Case Study of Local Hoverflies and Wolf Spiders" Agronomy 15, no. 5: 1203. https://doi.org/10.3390/agronomy15051203

APA Style

Knapp, R. A., McDougall, R., & Umina, P. A. (2025). Advancing Research on Overlooked Invertebrates in Biological Control: A Case Study of Local Hoverflies and Wolf Spiders. Agronomy, 15(5), 1203. https://doi.org/10.3390/agronomy15051203

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