Consuming Parasitized Aphids Alters the Life History and Decreases Predation Rate of Aphid Predator

Simple Summary

Intraguild predation is a common phenomenon between predators and parasitoids. Despite numerous studies on the performance of intraguild predators by consuming on intraguild prey, the entire two-sex life table and predation rates of intraguild predators fed on intraguild prey remain poorly known. In this study, we investigated the effect of parasitized Myzus persicae aphids by Aphidius gifuensis (Ashmead) on the entire two-sex life table and predation rates of Aphidoletes aphidimyza (Rondani). Our results showed that feeding on parasitized aphids did not influence the survival rates of immature A. aphidimyza individuals but significantly increased the development time of A. aphidimyza individuals and markedly reduced their longevity. The predation rate of immature A. aphidimyza individuals was also adversely affected by feeding on parasitized aphids. These results provide basic data for the potential use of A. aphidimyza in combination with A. gifuensis in M. persicae control programs.

Abstract

Intraguild predation interactions have substantial theoretical and practical implications for the dynamics of natural competitor populations used for biological control. Intraguild predation on parasitized aphids not only has a direct, negative effect on the parasitoid species, but it may indirectly influence the predator’s development, survival, reproduction and predation rates. In this study, we used two-sex life table theory, life table parameters and predation rates of Aphidoletes aphidimyza (Rondani) to compare when its populations fed on aphids (Myzus persicae Sulzer) (Hemiptera: Aphididae) that were either unparasitized or parasitized by Aphidius gifuensis (Ashmead) (Hymenoptera: Braconidae). Our results showed that individuals of A. aphidimyza were capable of completing their development and attaining maturity when they fed on parasitized aphids. Although feeding on parasitized aphids did not influence the survival rates of immature A. aphidimyza, it did significantly slow their development and extended their longevity, thereby reducing the fecundity and predation rates of A. aphidimyza. These findings may be pivotal for better understanding the sustained coexistence of predators with parasitoids in the biological control of aphids.

1. Introduction

Intraguild predation (IGP) arises when natural competitors of a shared resource also engage in predation or parasitism in food webs of prey and their natural enemies at their same trophic level [1,2]. A common case of IGP interaction within aphidophagous guild involves three species: A shared resource (aphids), a predator of the aphid such as a predatory midge or coccinellid beetle [3], and a parasitoid of the aphid. IGP also occurs when the predator consumes the parasitoid offspring developing in the aphid. In this respect, IGP may have the potential to greatly disrupt the distribution, abundance, evaluation, and control efficiency of predators [1]. Predatory brown lacewing Micromus variegatus (Fabricius) and lady beetle Coccinella septempunctata L. prefers to consume parasitized aphids over unparasitized aphids [4,5]. However, the consumption of intraguild prey by the intraguild predator can also negatively affect the development, survival, and oviposition of such predators [6,7,8,9,10]. Conversely, under greenhouse conditions, the combination of M. variegatus and Aphidius ervi Haliday did not result in intraguild predator densities and shared prey compared with intraguild predator alone [4]. The simultaneous application of predators and parasitoids could improve the efficacy of aphid control under greenhouse operations [11,12]. Yet, there is little information about how parasitism of aphids may affect the predator’s life history or its predation rates.

The green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), is the most devastating pest aphid worldwide, capable of attacking more than 400 plant species, including key Solanaceae, Cruciferae, and Leguminosae crops [13,14]. This aphid species have a short lifespan, rapid reproduction, and causes significant damage to plants by injecting viruses into them, which causes severe losses in crop production yields [15,16]. Myzus persicae can directly extract sap from young leaves and excretes honeydew, which contaminates foliage and fruit and induces sooty mold [17]. Aphidoletes aphidimyza (Rondani), aphidophagous gall midge that can attack more than 80 aphid species [18]. Third instar larvae of A. aphidimyza can kill 4–50 M. persicae nymphs per day [19]. Releasing A. aphidimyza pupae in a predator:aphid ratio (1:10 and 1:15) causes exceeded 95% loss rates of M. persicae nymphs on tobacco in the field [20]. The control efficacy of A. aphidimyza on aphids appears high when implemented with parasitoids [4]. Another biocontrol agent of aphids, the solitary endoparasitoid Aphidius gifuensis (Ashmead) (Hymenoptera: Braconidae), has been extensively used as a biocontrol agent against M. persicae in China [21,22,23]. Notably, A. gifuensis has been successfully mass-reared and incrementally released to control M. persicae populations in fields of tobacco and vegetables [16]. Both A. gifuensis and A. aphidimyza often co-occur in host-crop fields, and the latter can easily replace colonies of A. gifuensis on tobacco in greenhouses in Leshan Town of Zunyi City in Guizhou Province, China (personal observations). The IGP interaction between this predatory midge and parasitoid is unidirectional, with A. aphidimyza killing parasitoids [24]. Aphidoletes aphidimyza ate aphids parasitized by Aphidius colemani Viereck and thereby reduced the production of parasitoids [25]. Intraguild interactions are mostly detrimental to the aphid parasitoid population [24]. However, it remains unclear whether these ecological interactions could adversely influence the life history and consumption of A. aphidimyza populations.

Life table analysis can provide comprehensive information on population growth rates, and life table parameters are important tools for comparing the fitness of populations and understanding the effects of external factors on fitness (e.g., temperature or diet) [26,27]. Chi and Liu (1985) replaced the traditional female life table with the age-staged two-sex life table, which allows for the projection of populations and provides a clearer division of life history phases than classical life tables [28]. To date, however, no previous studies have investigated the effect of parasitized aphids on two-sex life tables and predation rates to assess the net effect on the predator population.

Previous studies showed that A. aphidimyza has a negative preference for parasitoid mummies [11]. We hypothesized that feeding on young parasitized aphids also would adversely affect A. aphidimyza’s survival, development, reproduction, and predation rate. Therefore, in this study, we chose young parasitized aphids to understand their effects on life history and rate of predation by A. aphidimyza to provide information for the simultaneous application of two biocontrol agents in the greenhouse.

2. Materials and Methods

2.1. Insect Colonies

The M. persicae aphids were collected from tobacco (Nicotiana tabacum) leaves at the Natural Enemy Breeding Center, in Jinhua Town of Fenggang County (Guizhou Province, China), on 1 May 2017. The reared colonies were kept on tobacco (“K326”) plants in mesh cages (45 cm × 45 cm × 30 cm) in the greenhouse belonging to the Institute of Entomology, Guizhou University.

The A. gifuensis parasitoids were originally obtained from M. persicae in a tobacco farm located in Leshan Town (Zunyi City, Guizhou Province), and then, in a greenhouse, reared on M. persicae as a host for more than five generations. We transferred more than 400 M. persicae onto tobacco plants enclosed by synthetic fine-nylon mesh cages, after which we introduced 30–40 pairs of A. gifuensis adults into each cage at 25 °C ± 1 °C; 24 h later, we removed these adults from the mesh cages, and young parasitized fourth instar M. persicae nymphs were prepared for experiments 2 days later. The young parasitized aphids moved very slowly, and their abdomen backs were lightly colored. The percentage of parasitized aphids exceeded over 98% in the experiment.

We obtained the A. aphidimyza larval predators from the same location as A. gifuensis. Aphidoletes aphidimyza was reared with M. persicae aphids on tobacco for three generations in an artificial climate chamber. Thirty pairs of A. aphidimyza adults were allowed to oviposit on fresh and healthy tobacco plants with enough M. persicae aphids for one day in a mesh cage (45 cm × 45 cm × 30 cm). After all A. aphidimyza adults were removed from the plant, A. aphidimyza eggs were prepared for the experiment. All experiments were conducted in an artificial climate chamber at 25 °C ± 1 °C, 65% ± 5% relative humidity (RH), with a 14-L:10-D photoperiod.

2.2. Life Table Analyses

To evaluate the effect of parasitized M. persicae on life-history parameters and predation rates of A. aphidimyza, we maintained a total of 52 freshly laid eggs (<12 h) by A. aphidimyza females in modified Petri dishes (35 mm-diameter) as described above, for each treatment (parasitized vs. unparasitized aphids as food). We recorded the incubation rate of eggs daily until the larvae hatched.

Based on a previous study of the predation ability of A. aphidimyza on M. persicae [19], each hatched larva was provided daily with 30 unparasitized or parasitized fourth instar M. persicae. We observed the larvae every day and determined developmental time and survival rate. We also counted the number of unparasitized and parasitized M. persicae consumed by A. aphidimyza daily before pupation. When A. aphidimyza reached the third instar stage of development, we placed balls of moist cotton on each Petri dish; mature larvae were able to access this moist absorbent cotton for their pupation. We sprayed distilled water daily onto cotton balls where the pupae were developing to keep the cotton moist until the emergence of adults. Newly emerged male and female A. aphidimyza were paired and moved into an artificial fine-nylon mesh cage (45 cm × 45 cm× 30 cm), which contained a tobacco seedling with 120 unparasitized or parasitized fourth instar M. persicae nymphs. New tobacco seedlings with infected aphids (as described above) and absorbent cotton soaked in a 10% honey solution were provided daily for these A. aphidimyza adults. We recorded the preoviposition period, oviposition period, fecundity, and longevity of A. aphidimyza individuals until all of them had died.

2.3. Life Table Data Analysis

For the age-staged life table, age-stage specific survival rate (S_xj), female age-specific fecundity (f_x6), age-specific maternity (l_xm_x), age-specific survival rate (l_x) and age-specific fecundity of the total population (m_x) of A. aphidimyza were calculated using the program TWOSEX-MSChart [29,30]. Mean values and standard errors of developmental time, survival rate, adult preoviposition period, total preoviposition period (calculated from birth to oviposition), oviposition period, and fecundity of A. aphidimyza, as well its predation rates, were estimated with 10,000 bootstrap replicates. A paired bootstrap test was used to calculate life table parameters of A. aphidimyza when it fed on unparasitized or parasitized M. persicae [31]. Age-stage-specific consumption (c_xj), age-specific predation rate (k_x), age-specific net predation rate (q_x), net predation rate (C₀), stable predation rate (ψ), finite predation rate (λ) and transformation rate (Q_p) of A. aphidimyza were analyzed according to the CONSUME-MSChart program [29,32]. We used the TIMING-MSChart program [29,33] to project the population growth, stage growth, total population and total consumption of A. aphidimyza reared on parasitized or unparasitized M. persicae. We used Sigma Plot v12.5 to construct the curves for A. aphidimyza’s survival rates, reproductive value, and predation rates.

3. Results

3.1. Effect of Parasitized Aphids on the Survival of A. aphidimyza

Feeding on parasitized aphids did not influence the survival rates of immature A. aphidimyza individuals (

Table 1. The survival rates, development time (mean ± SE) of various life stages and sexes, reproduction parameters of Aphidoletes aphidimyza that fed on unparasitized and parasitized Myzus persicae by Aphidius gifuensis.

Parameters	Stage	Unparasitized M. persicae		Parasitized M. persicae		p-Value
		N	Mean ± SE	N	Mean ± SE
	Egg	52	100.0 ± 0	52	100.0 ± 0	-
	1st instar larva	47	92.1 ± 3.7	48	90.4 ± 4.1	0.7276
Survival rates (%)	2nd instar larva	47	100.0 ± 0	45	93.8 ± 3.5	0.0909
	3rd instar larva	47	100.0 ± 0	45	100.0 ± 0	-
	Pupa	47	100.0 ± 0	45	100.0 ± 0	-
	Proportion of female Nf/N (%)		46.2 ± 6.9		55.8 ± 6.8	0.3178
Development time (d)	Egg	52	2.0 ± 0.0	52	2.0 ± 0.0	-
	1st instar larva	47	1.1 ± 0.0	48	1.1 ± 0.0	0.7775
	2nd instar larva	47	1.0 ± 0.0	45	1.0 ± 0.0	0.5524
	3rd instar larva	47	2.0 ± 0.0	45	2.3 ± 0.1	<0.001
	Pupa	47	8.7 ± 0.1	45	9.6 ± 0.1	<0.001
	Total immature stages	47	14.8 ± 0.1	45	16.0 ± 0.0	<0.001
Reproduction parameters	Female adult longevity (d)	24	6.3 ± 0.3	29	4.8 ± 0.2	<0.001
	Male adult longevity (d)	23	3.0 ± 0.2	16	1.8 ± 0.2	<0.001
	Adult preoviposition period (APOP, d)	24	0.3 ± 0.1	29	0.7 ± 0.5	0.2692
	Total preoviposition period (TPOP, d)	24	15.2 ± 0.1	29	16.6 ± 0.2	<0.001
	Oviposition period (d)	24	5.4 ± 0.3	29	3.8 ± 0.3	<0.001
	Fecundity (eggs per female)	24	131.3 ± 5.9	29	98.5 ± 5.8	<0.001

Significant difference within each row calculated by paired bootstrap test with 10,000 replications.

). Of the 52 A. aphidimyza eggs initially collected from the prepared colonies, 47 and 45 individuals developed into adults when feeding on unparasitized and parasitized M. persicae, respectively. The age-stage specific survival rate (S_xj) of individual immature stages of A. aphidimyza overlapped between unparasitized and parasitized M. persicae (Figure 1a). Age-specific survival rate (l_x) of A. aphidimyza showed a similar trend when it fed on unparasitized and parasitized M. persicae (Figure 2b).

3.2. Effect of Parasitized Aphids on Developmental Time, Longevity, and Fecundity of A. aphidimyza

Feeding on M. persicae parasitized by A. gifuensis significantly increased the development time of A. aphidimyza individuals and markedly reduced their longevity (Table 1). Third instar larvae, as well as pupae and the total duration of the immature stage A. aphidimyza that fed on unparasitized M. persicae, developed markedly faster than those arising when the predator fed on aphids parasitized by A. gifuensis. Female and male A. aphidimyza had significantly shorter lifespans when they fed on parasitized than unparasitized M. persicae. However, there was no significant difference in the adult preoviposition period (APOP) of A. aphidimyza feeding on parasitized and unparasitized M. persicae. The total preoviposition period (TPOP) of A. aphidimyza was significantly longer for those that fed on parasitized than unparasitized M. persicae. Recorded oviposition periods were longer, and fecundity was markedly higher of A. aphidimyza that fed on unparasitized M. persicae than the A. aphidimyza that consumed parasitized M. persicae (Table 1). The maximum peak of female age-specific fecundity (f_x6 = 41.58) and maximal age-specific maternity (l_xm_x = 19.19) of A. aphidimyza occurred at 16 days of age when feeding on unparasitized M. persicae; however, age-specific fecundity showed a later maximal peak (m_x), at 18 days, when parasitized M. persicae were the prey (Figure 2).

3.3. Population Parameters

The net reproductive rate (R₀), intrinsic rate of natural increase (r_m), and finite rate of increase (λ) of A. aphidimyza was similar between predator populations feeding on unparasitized and parasitized M. persicae. Nevertheless, the mean generation time (T) of A. aphidimyza was significantly higher when it fed on parasitized than unparasitized M. persicae aphids (

Table 2. Population parameters (means ± SE) of Aphidoletes aphidimyza fed on unparasitized and parasitized Myzus persicae by Aphidius gifuensis.

Parameter	M. persicae	Parasitized M. persicae	p-Value
Net reproduction rate, R0 (offspring/individual)	60.58 ± 9.46	54.94 ± 7.54	0.6427
Intrinsic rate of increase, rm (d−1)	0.24 ± 0.01	0.22 ± 0.01	0.1202
Finite rate of increase, λ (d−1)	1.27 ± 0.01	1.24 ± 0.01	0.1197
Mean generation time, T (d)	17.40 ± 0.11	18.48 ± 0.12	<0.001

Significant difference within each row calculated by paired bootstrap test (TWOSEX-MSChart; p < 0.05).

).

3.4. Predation Rate

Parasitized M. persicae significantly affected the rates of predation by immature A. aphidimyza individuals (

Table 3. Predation rate (aphids/day) by Aphidoletes aphidimyza and calculated parameters on unparasitized and parasitized Myzus persicae by Aphidius gifuensis.

Sex/Stage	Unparasitized M. persicae	Parasitized M. persicae	p-Value
Female
L1	1.29 ± 0.09	1.00 ± 0.00	0.0023
L2	2.38 ± 0.12	1.41 ± 0.12	<0.001
L3	8.62 ± 0.41	7.10 ± 0.34	0.0036
Male
L1	1.35 ± 0.12	1.19 ± 0.10	0.2941
L2	2.26 ± 0.21	1.38 ± 0.12	0.0006
L3	7.78 ± 0.33	6.25 ± 0.48	0.0079
Net predation rate, C0	10.8077 ± 0.5182	8.2115 ± 0.4285	<0.001
Stable predation rate, ψ	0.9827 ± 0.0325	0.7129 ± 0.0231	<0.001
Finite predation rate, λ	1.2441 ± 0.0458	0.8855 ± 0.0314	<0.001
Transformation rate, Qp	0.1784 ± 0.0289	0.1495 ± 0.0191	0.3376

Significant difference within each row calculated by paired bootstrap test (TWOSEX-MSChart; p < 0.05).

). Both immature female and male A. aphidimyza consumed more unparasitized M. persicae than those that fed on parasitized M. persicae. The age-stage-specific predation rate (C_xj) of A. aphidimyza was strongly affected by parasitized M. persicae as prey, especially in the second and third larval stages (Figure 3a). The maximum peak of the age-stage-specific predation rate (C_xj) of A. aphidimyza occurred at the age of 4 d when the predator fed on unparasitized M. persicae. Age-specific predation rate (k_x) and age-specific net predation rate (q_x) curves of A. aphidimyza were both higher when unparasitized aphids were eaten instead of parasitized ones (Figure 3b). Taking survival rate and longevity of A. aphidimyza into account, feeding on parasitized M. persicae significantly reduced A. aphidimyza’s net predation rate (C₀), stable predation rate (ψ), and finite predation rate (λ) when compared with its feeding on unparasitized M. persicae. However, eating parasitized M. persicae did not affect the transformation rate (Q_p) of A. aphidimyza (Table 3).

3.5. Population Projection

To evaluate the effect of parasitized M. persicae prey on the population stages’ size and growth rate of A. aphidimyza as well as its predation rates, the data from a two-sex life table and for predation rates were used together to project the growth of A. aphidimyza population and its consumption of aphids (Figure 4, Figure 5 and Figure 6). The total population of A. aphidimyza feeding on unparasitized aphids at 60 days reached 201,113 individuals, which was higher than those predators the prediction obtained at 60 days (150,816 individuals) for A. aphidimyza that fed on parasitized aphids (Figure 6a). Moreover, those A. aphidimyza individuals feeding on unparasitized aphids also featured higher predation ability with those feeding on parasitized aphids at 56 days (Figure 6b). Since predators only have predatory capacity during the larval stage, these consumption values omitted pupal and adult periods.

4. Discussion

Consumption of parasitized aphids and mummies could adversely affect the development, body size, survival, and behavior of aphid predators [6,7,10,34,35]. Our results showed that consumption of parasitized aphids did not influence the survival rates of immature A. aphidimyza individuals, but it did significantly delay their overall developmental time and also reduced the predator’s longevity, fecundity and predation rates. To our best knowledge, this study is the first to have comprehensively evaluated the effect of parasitized aphids on the entire life history and predation rates of aphid predators.

The survival rates of immature A. aphidimyza were not significantly influenced by feeding on parasitized M. persicae, and over 90% of the predator egg survived to adult emergence. Similar results were reported for Harmonia axyridis and Propylea japonica when each fed on Aphid craccivora parasitized and mummified by A. colemani [34], and also for Coccinella undecimpunctata that fed on “injured” M. persicae mummies parasitized by A. colemani [35]. Compared with parasitized aphids, it seems that mummies do not provide enough nutrition to sustain the development of either C. septempunctata or H. convergens into adulthood [7,35].

Development times for each stage and total immature stage of A. aphidimyza that fed on parasitized M. persicae were longer than those of predator larvae feeding on unparasitized M. persicae. Our results are consistent with studies of C. septempunctata, H. convergens, H. axyridis, and P. japonica that subsisted on mummified aphids, demonstrating that predators could slow their ontogenetic development [7,10,34]. Our study using A. aphidimyza revealed that its longevity, oviposition period, and fecundity were all significantly affected by its feeding on parasitized aphids. Little research has been conducted on the effects of parasitized aphid species on the reproduction parameters of their predators. When fed A. gifuensis mummies, aphid enemy H. axyridis females had a longer pre-oviposition period and did not lay eggs within 30 days of observation [10]. The number of eggs produced by Episyrphus balteatus females was affected by mummified aphids and the exuvia of mummies of Acyrthosiphon pisum, but the number of eggs laid by E. balteatus was nonetheless similar when the female fed with healthy aphids and parasitized aphids [36]. Under a greenhouse choice experiment, E. balteatus females laid significantly fewer eggs on mummified M. persicae colonies parasitized by A. colemani than they do on colonies with unparasitized aphids [37]. Mummies are inferior prey for aphid predators; hence, a qualitative difference in the nutrition between unparasitized and parasitized aphids might influence the development and reproduction of their predators [10,34,35]. Furthermore, the fatty acid content of parasitized cotton aphids could decrease for three days post-parasitization [38]. Compared with the unparasitized aphids, mummies (parasitized ≥9 days) with a dark spot in the abdomen tend to be carbohydrate-poor and richer in proteins and lipids [6,10]. The changed nutritional content of parasitized aphids may be a factor affecting the development and reproduction of predators.

The predation potential of aphid predators is markedly influenced when they feed on parasitized aphids [6,7]. Fourth instar larvae and adults of H. axyridis try to consume more unparasitized aphids than mummies parasitized by A. asychis in 1 h and 3 h [6]. Compared with unparasitized A. craccivora, the larvae of H. axyridis, P. japonica and C. septempunctata consume less parasitized and mummified aphids [34]. However, no previous study has investigated the effects of parasitized aphids upon the daily rate of predation by aphid predators. Our study is the first to demonstrate that the consumption of parasitized aphids can significantly lower the net predation rate, the stable predation rate, and the finite predation rate of A. aphidimyza, leading to a weaker predation ability than for those feeding on unparasitized aphids across population projections.

5. Conclusions

In conclusion, A. aphidimyza could complete its development if feeding solely on parasitized aphids, but it had a longer larval stage development and shorter longevity, longer preoviposition period, and lower reproduction than when feeding on unparasitized aphids. The completed development and reproduction of the predator when feeding on unparasitized and parasitized aphids are both pivotal for maintaining its coexistence with the parasitoid and effective suppression of the aphid in the ecosystem.