Adult Chironomid (Chironomidae: Diptera) Positive Phototactic Behaviour—A Cue for Adult Population Management and Impact on Insect Biodiversity at Lake Trasimeno, Central Italy

Abstract

The positive phototaxis showed by adults of some pestiferous chironomid species, annoying to waterfront residents and businesses, was investigated at Lake Trasimeno (Italy) to develop a strategy against their massive swarms. Two experimental devices (ChiroTraps), located at Passignano sul Trasimeno (PA) and at Sant’Arcangelo (SA), were employed in 2019 and 2020. The total biomass attracted by the traps amounted to 6498.78 g at PA and to 8597.05 g at SA. Chironomids biomass constituted 99.66% and 96.59% of the biomass in these sites, respectively. Only a few specimens of other fauna except chironomids were found at PA. In contrast, the values at SA were considerable, being 91- and 35-fold (number of taxa and weight, respectively) higher than in PA. These results demonstrated that exploiting the light attraction behaviour of adult chironomids is an efficient method for managing their pestiferous populations, thereby reducing the necessity of using insecticides. By comparing the biodiversity in the two sites, it was evident that the differences were linked primarily to the environmental conditions. Finally, it is suggested that light trapping systems should be located in urban centres or floated on the lake surface to maximise the efficiency of trapping chironomids and minimising the impact on biodiversity.

1. Introduction

Numerous investigations provide evidence of the impact of artificial light at night on crepuscular and nocturnal biodiversity by examining the influence of positive phototaxis on the biology and behaviour of many insect groups (see review in [1]). Even though light sources attract nocturnal flying insects, some lamps attract more insects than others due to their different spectral composition, light intensity, and the different sensitivity of the various species [2,3]. However, different environmental conditions can also affect insect attraction to light as proven by Thein and Choi (2016) [4], who concluded that insect assemblages depend upon habitat characteristics, such as altitude and vegetation type. Therefore, differences in attraction between different insect groups can be expected in terms of abundance and richness [5,6].

A new incentive in investigating light attraction has been provided by the introduction of light-emitting diode (LED) lights, which are a new generation of broad-spectrum, energy-efficient light sources in substitution of the usual gas-discharge lamps. Yet, there are conflicting pieces of evidence concerning the efficiency of LED lamps in insect attraction when compared with traditional lighting methods. Indeed, Pawson and Bader (2014) [7] captured 48% more insects by using LED light traps, while Van Grusven et al. (2014) [6] stated that LED attracted approximately half of the number of insects compared to mercury vapour lights, whereas Zemel and Houghton (2018) [8] observed no difference in richness, total abundance, and abundance of most common orders between the different light types they used.

In this regard, it has been ascertained that light trapping has become a common method for studying insect biodiversity [4,6,9,10,11] and for monitoring and capturing insects, such as invasive agricultural pests [12,13], or insect vectors of medical importance [14].

The attraction and capturing of some non-biting pestiferous midge species using artificial light was investigated long ago [15,16,17]. Indeed, insects’ attraction to light has been considered as appropriate for developing an integrated strategy to control the massive swarms of pestiferous insects, such as chironomid midges [18], which are widely distributed freshwater insects with a key role in the food chain of many ecosystems [19,20,21,22,23]. This method, which is useful for chironomid population management, exploits the positive phototaxis of adult chironomids to reduce midge nuisance [17,18,24,25]. In fact, the short-lived chironomid adults emerge in exceptionally large numbers, forming huge swarms that, in some parts of the world, cause great annoyance and inconvenience to tourist activities as well as waterfront residents and businesses. These swarms can also cause soiling of surfaces and monuments, they can represent a potential hazard for vehicular and air traffic, and, in the worst-case scenario, they can cause health issues such as allergies or conjunctivitis [21,26,27].

In Lake Trasimeno (central Italy), many species of chironomid midge occur; in particular, the dominant and most pestiferous species is Chironomus plumosus (Linnaeus, 1758), as demonstrated by previous investigations carried out in this biotope [24,28,29,30]. In the aforementioned investigations, particular attention has been paid to the effect of massive emergences of adult chironomids from this habitat, resulting in nuisance problems and severe economic losses for the waterfront residents and businesses. The control of these massive swarms at Lake Trasimeno has been attempted by using different techniques, such as controlling larval population through the use of the biological larvicide Bacillus thuringiensis var. israelensis (Bti), and targeting the adults directly by diverting their populations from the waterfront businesses and residential areas through the use of outdoor lighting (tofo lamps) set up along the shoreline, as well as the use of bats as biological control, by the installation of bat boxes in waterfront areas.

The present investigation aimed to develop a strategy against the massive swarms of pestiferous chironomids. Two experimental devices (named ChiroTraps) that exploit the positive phototaxis behaviour of adult chironomids were tested, to verify the differences linked to the two different habitats selected for the devices’ installation; to test the efficiency of two different lighting systems; and to highlight the possible impact that these experimental devices can have on the local biodiversity.

2. Materials and Methods

2.1. Study Area

Lake Trasimeno is a laminar lake located in the Umbria Region (Central Italy). It is the fourth-largest lake in Italy; it has an extension of 124 km² and a maximum depth of 6.3 m. For a detailed description of the study area and the benthic community of this environment, see [28,29,30].

2.2. Sampling Campaign

Adult chironomids were collected using experimental devices named ChiroTraps (registered trademark; Figure S1), developed by the Italian Company Laika Lab s.r.l. [29]. Two traps, connected to the electrical grid, were provided with a light system that exploited the positive phototactic behaviour of adult chironomids. The traps were equipped with a suction chamber with an opening of 1 × 1 [in m] and an internal support surface of 4 m² (Figure S2). The ChiroTraps were employed along the coast of Lake Trasimeno, one located at Passignano sul Trasimeno (PA) (geo-coordinates: 43°11′1.2804″ N, 12°8′40.488″ E) and the other at Sant’Arcangelo (SA) (geo-coordinates: 43°5′22.722″ N, 12°9′23.1876″ E) (Figure 1). They were activated from the spring (March/April) to early autumn (October) of 2019 and 2020 (

Table 1. Results of the collections made with the two ChiroTraps, one located at Passignano sul Trasimeno (PA) and the other at Sant’Arcangelo (SA), central Italy. For each sample, the year (2019 and 2020), the period, and the collection days are indicated. As for the biomass, it is reported as total biomass (g); total daily biomass (g/day); chironomid biomass (g); chironomid daily biomass (g/day); other biomass (g); and other daily biomass (g/day). In the last two columns, the identified other biomass (g) and the identified other daily biomass (g/day) are reported. At the end of each ChiroTrap/year, the descriptive statistics (total, mean, minimum, and maximum) are presented.

Passignano sul Trasimeno (PA)	Period	Collection	Total Biomass	Total Biomass	Chironomid Biomass	Chironomid Biomass	Other Biomass	Other Biomass	Other Biomass (Identified)	Other Biomass (Identified)
2019		(days)	(g)	(g/day)	(g)	(g/day)	(g)	(g/day)	(g)	(g/day)
PA1	8 March–18 March	11	12.97	1.18	9.47	0.86	3.50	0.32	3.097	0.282
PA2	19 March–29 April	42	10.94	0.26	10.63	0.25	0.31	0.01	0.036	0.001
PA3	30 April–17 May	18	47.78	2.65	47.74	2.65	0.038	0.00	0.038	0.002
PA4	18 May–5 June	19	166.11	8.74	165.74	8.72	0.37	0.02	0.178	0.009
PA5	6 June–18 June	13	606.19	46.63	604.93	46.53	1.26	0.10	0.039	0.003
	19 June–2 July
	3 July–9 July
PA6	12 July–01 August	42	697.65	16.61	696.77	16.59	0.88	0.02	0.646	0.015
PA7	2 August–9 August	8	311.97	39.00	311.79	38.97	0.18	0.02	0.009	0.001
PA8	10 August–22 August	13	314.99	24.23	314.42	24.19	0.57	0.04	0.031	0.002
PA9	23 August–6 September	15	145.15	9.68	144.90	9.66	0.25	0.02	0.027	0.002
PA10	7 September–10 September	4	875.01	218.75	874.27	218.57	0.74	0.19	0.137	0.034
PA11	11 September–10 October	30	538.07	17.94	537.51	17.92	0.56	0.02	0.091	0.003
Total		215	3726.83	---	3718.17	---	8.66	---	4.33	---
Mean		19.55	338.80	35.06	338.02	34.99	0.79	0.07	0.39	0.032
Minimum		4	10.94	0.26	9.47	0.25	0.04	0.00	0.01	0.001
Maximum		42	875.01	218.75	874.27	218.57	3.50	0.32	3.10	0.282
Sant’Arcangelo (SA)	Period	Collection	Total biomass	Total biomass	Chironomid biomass	Chironomid biomass	Other biomass	Other biomass	Other biomass (identified)	Other biomass (identified)
2019		(days)	(g)	(g/day)	(g)	(g/day)	(g)	(g/day)	(g)	(g/day)
SA1	8 March–18 March	11	34.06	3.10	14.28	1.30	19.78	1.80	17.98	1.63
SA2	19 March–30 April	43	44.47	1.03	38.68	0.90	5.79	0.13	4.88	0.11
SA3	1 May–17 May	17	118.08	6.95	115.24	6.78	2.84	0.17	2.84	0.17
SA4	18 May–5 June	19	473.86	24.94	466.71	24.56	7.15	0.38	6.84	0.36
SA5	6 June–18 June	13	447.93	34.46	439.95	33.84	7.98	0.61	7.03	0.54
	19 June–29 June
SA6	12 July–1 August	32	744.19	16.61	724.71	22.65	19.48	0.61	17.98	0.56
SA7	2 August–9 August	8	175.43	21.93	171.11	21.39	4.32	0.54	3.93	0.49
SA8	10 August–22 August	13	408.62	31.43	396.07	30.47	12.55	0.97	8.31	0.64
SA9	23 August–6 September	15	182.35	12.16	176.57	11.77	5.78	0.39	5.07	0.34
SA10	7 September–10 September	4	394.29	98.57	386.31	96.58	7.98	2.00	6.99	1.75
SA11	11 September–11 October	31	347.35	11.20	326.53	10.53	20.82	0.67	17.92	0.58
Total		206	3370.63	---	3256.16	---	114.47	---	99.77	---
Mean		18.73	306.42	23.85	296.01	23.71	10.41	0.75	9.07	0.65
Minimum		4	34.06	1.03	14.28	0.90	2.84	0.13	2.84	0.11
Maximum		43	744.19	98.57	724.71	96.58	20.82	2.00	17.98	1.75
Passignano sul Trasimeno (PA)	Period	Collection	Total biomass	Total biomass	Chironomid biomass	Chironomid biomass	Other biomass	Other biomass	Other biomass (identified)	Other biomass (identified)
2020		(days)	(g)	(g/day)	(g)	(g/day)	(g)	(g/day)	(g)	(g/day)
PA1	22 April–4 May	13	362.47	27.88	361.89	27.84	0.58	0.04	0.390	0.030
PA2	5 May–27 May	23	399.11	17.35	398.80	17.34	0.31	0.01	0.027	0.001
PA3	28 May–11 June	15	47.54	3.17	47.54	3.17	0.00	0.00	0.000	0.000
PA4	12 June–23 June	12	57.38	4.78	56.69	4.72	0.69	0.06	0.296	0.025
PA5	24 June–10 July	17	604.28	35.55	597.80	35.16	6.48	0.38	0.639	0.038
PA6	11 July–28 July	18	608.48	33.80	606.79	33.71	1.69	0.09	0.415	0.023
PA7	29 July–7 August	10	233.17	23.32	232.25	23.23	0.92	0.09	0.165	0.017
PA8	8 August–2 September	26	260.53	10.02	259.50	9.98	1.03	0.04	0.321	0.012
PA9	3 September–16 September	14	133.63	9.55	133.42	9.53	0.21	0.02	0.063	0.004
PA10	17 September–9 October	23	65.36	2.84	64.09	2.79	1.27	0.06	0.180	0.008
Total		171	2771.95	---	2758.77	---	13.18	---	2.50	---
Mean		17.10	277.20	16.83	275.88	16.75	1.32	0.08	0.25	0.02
Minimum		10	47.54	2.84	47.54	2.79	0.00	0.00	0.00	0.00
Maximum		26	608.48	35.55	606.79	35.16	6.48	0.38	0.64	0.04
Sant’Arcangelo (SA)	Period	Collection	Total biomass	Total biomass	Chironomid biomass	Chironomid biomass	Other biomass	Other biomass	Other biomass (identified)	Other biomass (identified)
2020		(days)	(g)	(g/day)	(g)	(g/day)	(g)	(g/day)	(g)	(g/day)
SA1	23 April–4 May	12	101.35	8.45	93.56	7.80	7.79	0.65	6.22	0.52
SA2	5 May–27 May	23	482.19	20.96	465.14	20.22	17.05	0.74	14.15	0.62
SA3	28 May–11 June	15	448.40	29.89	434.03	28.94	14.37	0.96	7.78	0.52
SA4	12 June–23 June	12	606.82	50.57	599.35	49.95	7.47	0.62	5.56	0.46
SA5	24 June–10 July	17	1276.53	75.09	1253.40	73.73	23.13	1.36	18.04	1.06
SA6	11 July–28 July	18	1270.88	70.60	1244.20	69.12	26.68	1.48	14.27	0.79
SA7	29 July–7 August	10	220.46	22.05	206.59	20.66	13.87	1.39	11.02	1.10
SA8	8 August–2 September	26	207.35	7.98	185.83	7.15	21.52	0.83	17.54	0.67
SA9	3 September–16 September	14	450.71	32.19	423.43	30.25	27.28	1.95	24.93	1.78
SA10	17 September–9 October	23	161.73	7.03	141.81	6.17	19.92	0.87	16.92	0.74
Total		170	5226.42	---	5047.34	---	179.08	---	136.43	---
Mean		17.00	522.64	32.48	504.73	31.40	17.91	1.08	13.64	0.83
Minimum		10	101.35	7.03	93.56	6.17	7.47	0.62	5.56	0.46
Maximum		26	1276.53	75.09	1253.40	73.73	27.28	1.95	24.93	1.78

). To maximise the specificity of chironomid collection, the two traps were only active around sunset, generally from 6 p.m. to 11 p.m.

The PA site was characterized by a densely urbanized littoral area; the ChiroTrap was installed inside a port area located within the town of Passignano sul Trasimeno. The SA site showed a more natural environment, characterised by rich lake vegetation; the location of the ChiroTrap was about 1 km from the town of Sant’Arcangelo.

The ChiroTrap located at Passignano sul Trasimeno (PA) was equipped with an LED lamp (100 W, 6000 K, cold white light). In contrast, the ChiroTrap located at Sant’Arcangelo (SA) was equipped with a gas-discharge lamp (250 W, 5200 K). The emission spectra of the two light systems are reported in Figure S3.

2.3. Sample Analysis

Invertebrate specimens collected with the ChiroTraps (collections were usually carried out monthly, both in 2019 and 2020) were processed by air drying them after treatment at low temperatures in a fridge.

Specimens belonging to Chironomidae were separated from those belonging to other zoological groups (indicated as “other biomass”), such as various orders of Insecta, Gastropoda: Pulmonata, and Arachnida: Araneae.

Each sample was weighed and labelled to indicate the site and sampling period.

The non-chironomid fauna was identified to the order level, whereas the species included in the annexes of the “Habitat” Directive, together with the invasive alien species and the most common lepidopteran species, were identified to the species level.

3. Results

The present investigation was carried out at Lake Trasimeno, using the ChiroTraps located at Passignano sul Trasimeno (PA) and Sant’Arcangelo (SA), over 215 and 206 days, respectively, from March to October in the year 2019 (11 samples were collected: PA₁–PA₁₁, SA₁–SA₁₁); whereas in the year 2020, it occurred over 171 and 170 days, respectively, from April to October (10 samples were collected: PA₁–PA₁₀, SA₁–SA₁₀).

In 2019, the total biomass (dry weight) amounted to 3726.83 g at the site PA and 3370.63 g at the site SA; Diptera: Chironomidae corresponded to 99.77% (3718.17 g) and 96.60% (3256.16 g), respectively. In 2020, the total biomass at the site PA was 2771.95 g, whereas this value practically doubled at the site SA, reaching 5226.42 g, where Diptera: Chironomidae constituted 99.52% (2758.77 g) and 96.57% (5047.34 g), respectively.

Therefore, in addition to chironomids, in 2019, the sampled biomass (defined from here on as other biomass) at the site PA amounted to 8.66 g (0.23%) and to 114.47 g (3.40%) at the site SA. In 2020, at the site PA, other biomass amounted to 13.18 g (0.48%) and 179.08 g (3.43%) at the site SA (Table 1).

The above results revealed that the percentage of other biomass at the site SA was about 15-fold (in 2019) and 7-fold (in 2020) higher than at the site PA.

In 2019, the highest values of other biomass at the site PA were found in the samples PA₁ (March; 0.32 g/day) and PA₁₀ (September; 0.19 g/day), whereas in 2020, the highest value was found in the sample PA₅ (end of June–early July; 0.38 g/day) (Figure 2a). In 2019, the site SA showed the highest values of other biomass in the samples SA₁ (March; 1.80 g/day) and SA₁₀ (September; 2.00 g/day). In 2020 the highest values were found in the samples SA₅ (end of June–early July; 1.36 g/day), SA₆ (July; 1.48 g/day), SA₇ (August; 1.39 g/day), and SA₉ (September; 1.95 g/day) (Figure 2b).

In the years 2019–2020, because of the limited material collected (21.84 g), the analysis of the other biomass showed that, at the site PA, the identification of the taxa was only 31.25% (6.82 g) (Figure 2a). By contrast, at the site SA, the situation was completely different. Indeed, here, the identification of the taxa belonging to the other biomass reached 80.47% (236.20 g), with a low and homogeneous deviation from the whole amount of the other biomass found in the various samples (Figure 2b).

In the years 2019–2020, 15 taxa were included in the other biomass (i.e., Insecta: Blattodea, Coleoptera, Dermaptera, Diptera (except Chironomidae), Ephemeroptera, Hemiptera, Hymenoptera, Lepidoptera, Mantodea, Neuroptera, Odonata, Orthoptera, Trichoptera; Arachnida: Araneae; Gastropoda: Pulmonata). Among the 15 taxa, 12 were commonly found at PA and SA, whereas 3 were restricted to SA.

Only a few specimens were found in the other biomass at PA (102 specimens in the year 2019 and 120 specimens in the year 2020), whereas these values at SA were substantial—5161 and 15,038 specimens in the year 2019 and 2020, respectively. Therefore, during the two-year period, SA showed that, in the other biomass, the specimens were 91-and 35-fold higher than in PA, with respect to both the number of taxa and the weight.

Therefore, it seemed acceptable to limit the analysis to the taxa found in the other biomass at the site SA. Here, in the years 2019 and 2020, Lepidoptera was the prevailing taxon, showing an average (N/day) of 30.69 and 69.79, respectively. As for the year 2019, the average values (N/day) of the main taxa involved Hymenoptera (1.56), Trichoptera (0.96), Diptera (0.84), Ephemeroptera (0.71), Coleoptera (0.46), and Odonata (0.36). By contrast, in the year 2020, the highest average values of the main taxa, except for Odonata, were found from 7 to 10 September (SA₁₀), whereas in 2020, from 3 to 16 September (SA₉), Lepidoptera (273.79) and Trichoptera (23.14) reached their maximum value. In the year 2020 Coleoptera, Hymenoptera, Odonata, and Diptera showed their highest values from 29 July to 7 August (SA₇), and Ephemeroptera from 12 to 23 June (SA₄).

Therefore, Lepidopteran moths, 31 taxa of which were identified (Table S1), represented an interesting field of investigation because they showed positive phototaxis towards artificial lights. At the site SA, these insects constituted 78.31% in number and 72.44% in weight in comparison with the other biomass identified.

Under a conservation perspective, four species of European Interest (“Habitat” Directive 92/43/CEE) were found in the other biomass, though restricted to the site SA— namely, the Lepidopterans Euplagia quadripunctaria (Poda, 1761) (2020: 2 specimens, SA₉) and Proserpinus proserpina (Pallas, 1772) (2019: 1 specimen, SA₅; 2020: 3 specimens, SA₂; 1 specimen, SA₃; 1 specimen, SA₆); the Coleopteran Lucanus cervus (Linnaeus, 1758) (2019: 2 specimens, SA₆), and the Odonatan Lindenia tetraphylla (Vander Linden, 1825) (2019: 9 specimens, SA₆; 2020: 3 specimens, SA₄; 5 specimens, SA₅; 4 specimens., SA₆) (Table S2).

Lastly, an invasive alien species was also found in the other biomass—the brown marmorated stink bug, Halyomorpha halys (Stål, 1855) (2019: 1 specimen, SA₇; 2 specimens, SA₈; 2020: 2 specimens, SA₇; 12 specimens, SA₈; 4 specimens, SA₉; 1 specimen, SA₁₀) (Table S3), which for years now has entered the Italian territory.

4. Discussion

In recent years, the attraction of adult chironomids to light has been reported by many researchers [17,18,24,25]. The striking positive phototactic behaviour of some species results in massive swarms which often cause inconvenience and annoyance to waterfront residents, businesses, and tourists [21,26,27]. Their annoyance is mostly related to the chironomids’ phenomenal abundance during the summer season along the riparian areas, with a negative impact on all the various anthropic activities in the area.

A research investigation, carried out from 2007 to 2009 at Lake Trasimeno, revealed that the nuisance species, namely Chironomus plumosus (67%), Tanypus punctipennis (22%), and Procladius choreus (9%), expressed positive phototactic behaviour [24].

In recent studies dealing with both the littoral benthos and the central areas of this lake [28,29,30], it was discovered that Chironomus plumosus represented a remarkable component of the benthonic fauna (24.35% and 98.84% of the chironomid larval community in the littoral and central zones, respectively), reaching the highest density levels (1556.7 ind. m⁻², April 2021) in the central area.

Therefore, the positive phototactic behaviour of some adult insects (particularly chironomids) revealed by the use of the ChiroTraps installed at Passignano sul Trasimeno (PA) and Sant’Arcangelo (SA) are in agreement with expectations, emphasising that, in the years 2019–2020, the adult chironomid biomass prevailed at both sites (PA: 6476.94 g; SA: 8303.50 g), whereas the other biomass constituted only a small component (0.34% and 3.41%, respectively,

Table 2. Abundance and biomass of the zoological groups other than chironomids (other biomass) collected in the two ChiroTraps at Passignano sul Trasimeno (PA) and Sant’Arcangelo (SA). For each taxon, the number of individuals (N), weight (g), and the relative percentages in terms of abundance (N) and biomass (g) are reported for each year of collection.

Passignano sul Trasimeno (PA)		2019: 215 Days				2020: 171 Days
Other Biomass (Identified)		Total	Total	%	%	Total	Total	%	%
(Taxa)		(N)	(g)	(N)	(g)	(N)	(g)	(N)	(g)
Insecta	Blattodea	---	---	---	---	---	---	---	---
Insecta	Coleoptera	1	0.27	0.98	6.26	18	0.305	15.00	12.22
Insecta	Dermaptera	1	0.02	0.98	0.37	---	---	---	---
Insecta	Diptera	19	0.17	18.63	3.88	19	0.090	15.83	3.61
Insecta	Ephemeroptera	9	0.01	8.82	0.12	1	0.004	0.83	0.14
Insecta	Hemiptera	3	0.07	2.94	1.58	12	0.070	10.00	2.79
Insecta	Hymenoptera	6	0.13	5.88	2.95	13	0.174	10.83	6.99
Insecta	Lepidoptera	42	0.70	41.18	16.12	45	0.482	37.50	19.31
Insecta	Mantodea	---	---	---	---	---	---	---	---
Insecta	Neuroptera	1	0.00	0.98	0.07	1	0.002	0.83	0.09
Insecta	Odonata	2	0.01	1.96	0.31	2	0.009	1.67	0.34
Insecta	Orthoptera	---	---	---	---	---	---	---	---
Insecta	Trichoptera	3	0.00	2.94	0.10	---	---	---	---
Arachnida	Araneae	8	0.24	7.84	5.56	3	0.158	2.50	6.35
Gastropoda	Pulmonata	7	2.71	6.86	62.68	6	1.202	5.00	48.16
	Total	102	4.33	100	100	120	2.495	100	100
Sant’Arcangelo (SA)		2019: 206 days				2020: 170 days
Other biomass (identified)		Total	Total	%	%	Total	Total	%	%
(Taxa)		(N)	(g)	(N)	(g)	(N)	(g)	(N)	(g)
Insecta	Blattodea	---	---	---	---	1	0.01	0.01	0.01
Insecta	Coleoptera	79	8.04	1.53	8.06	924	10.63	6.14	7.79
Insecta	Dermaptera	---	---	---	---	1	0.01	0.01	0.00
Insecta	Diptera	120	0.68	2.32	0.68	338	1.76	2.25	1.29
Insecta	Ephemeroptera	76	0.03	1.47	0.03	244	0.14	1.62	0.10
Insecta	Hemiptera	45	1.07	0.87	1.07	139	3.00	0.92	2.20
Insecta	Hymenoptera	218	9.43	4.22	9.45	423	12.42	2.81	9.11
Insecta	Lepidoptera	4383	72.76	84.81	72.92	11,441	98.34	76.08	72.08
Insecta	Mantodea	1	0.17	0.02	0.17	1	0.03	0.01	0.02
Insecta	Neuroptera	31	0.13	0.60	0.13	91	0.25	0.61	0.19
Insecta	Odonata	63	4.80	1.22	4.81	194	7.31	1.29	5.36
Insecta	Orthoptera	2	0.10	0.04	0.10	1	0.01	0.01	0.00
Insecta	Trichoptera	131	0.28	2.53	0.28	1217	1.21	8.09	0.89
Arachnida	Araneae	16	0.55	0.31	0.55	22	0.59	0.15	0.43
Gastropoda	Pulmonata	3	1.74	0.06	1.74	1	0.73	0.01	0.53
	Total	5168	99.77	100	100	15,038	136.43	100	100

). The non-chironomid taxa collected with the PA ChiroTrap showed a limited biomass, together with a small number of specimens (2019: 4.33 g and 102 specimens; 2020: 2.50 g and 120 specimens). The biomass represented by Gastropoda: Pulmonata was the highest (2019: 2.71 g; 2020: 1.202 g), while Lepidoptera was the prevalent taxon and showed the highest number of specimens (2019: 42 specimens; 2020: 45 specimens).

In addition to some chironomid species, many nocturnal insects (other biomass in the present study) exhibit a positive phototaxis to artificial lights, thus making light traps a useful tool to monitor and manage insect populations (see review in Owens and Lewis, 2018 [1]).

The other biomass, which encompassed the taxa significantly trapped by the ChiroTraps and therefore limited to the site SA, are listed here in order of their biomass: Lepidoptera (2019: 72.92% and 2020: 72.08%), Hymenoptera (2019: 9.45% and 2020: 9.11%), Coleoptera (2019: 8.06% and 2020: 7.79%), Odonata (2019: 4.81% and 2020: 5.36%). Minor values are those of Hemiptera, Diptera (except for chironomids), and Trichoptera (Table 2).

It seems acceptable to hypothesize that Odonata can be attracted not so much to the lights as to the possible prey trapped in the ChiroTraps, likewise, the Arachnida: Araneae. Gastropoda: Pulmonata can be found only occasionally.

Particular attention must be given to Lepidoptera, which represented the largest component of SA other biomass in terms of both biomass and abundance (2019: 72.76 g, 4383 specimens; 2020: 98.34 g, 11,441 specimens). Indeed, among this taxon, Heterocera in particular (mainly Sphingidae, Noctuidae, Erebidae, and Geometridae) are well known for their positive phototaxis (Table S1) [4,5,6,7,8,31,32,33,34,35].

Several studies have shown that the habitat affects the taxonomic composition of the fauna attracted by the lights. In urban or peri-urban environments [31,32], the insect taxa most attracted by the lights are Diptera, Lepidoptera, and Hymenoptera; in rural environments characterised by cultivations, consistently, Diptera are the main taxa (and within them chironomids are the most represented) [6]; and while taking into account forest environments [4,33], Lepidoptera, Coleoptera, and Diptera are usually the most abundant taxa. Clearly, in environments close to freshwater, the proportion of aquatic insects such as Ephemeroptera, Trichoptera, and Plecoptera increases [33,36]. When comparing the other biomass between the two investigated sites, it becomes evident that at the site SA the biodiversity is greater, not only for the species richness point of view, but also in relation to the exclusive presence of some species of conservation interest, included in the “Habitat” Directive. In particular, Euplagia quadripunctaria, flying from June to September [37], is a priority species of the Directive. The Sphingidae Proserpinus proserpina, which feeds on the nectar of plant species [38], has been recorded for the first time in the Trasimeno area. The occurrence of Lucanus cervus is markedly linked to the presence of senescent oaks [39,40], which limits the spreading of this species. Among the species of European interest, Lindenia tetraphylla is the most abundant species: 22 specimens were found from 2019 to 2020. This species has been reported in very few Italian sites. The present study data confirm its steady population at Lake Trasimeno.

Biodiversity richness at the site SA also includes the invasive alien brown marmorated stink bug, Halyomorpha halys, the presence of which gradually changed from 2019 to 2020, increasing from 3 to 19 specimens. In Europe, the first records of this species can be traced back to 2007, in the area around the city of Zurich, Switzerland [41], and to 2012, near Modena in Italy [42]. This invasive bug is endemic to China, Korea, Taiwan, and Japan [43]. These insects are phytophagous, feeding on several vegetables of agricultural, ornamental, and horticultural interest, thus causing relevant damage to crops [41,43]. In addition, this species also represents a source of intense annoyance in residential areas because the individuals can aggregate in large numbers before overwintering. With the aim to reduce the use of pesticides as management methods, experimental investigations using light trapping proved that this species shows positive phototactic attraction to white light [12], and specifically to short wavelength lights (368–455 nm) [44].

Since even the scientific literature in the field has very contrasting results regarding the differences in the attractiveness of the various types of light sources [6,7,8], we believe that the remarkable difference in both biomass and other biomass at the sites PA and SA could be linked not so much to the different lighting of the ChiroTraps (PA: LED lamp, 100 W with white light, 6000 K; SA: gas-discharge lamp, 250 W), but to the unique environments where they were located. PA is a densely populated area, the artificial littoral zone of which is linked to Passignano sul Trasimeno; SA shows a low environmental impact, is located 1 km from Sant’Arcangelo, and is characterised by a littoral floristic component constituted primarily of common reeds, poplars, and willows.

In fact, previous overnight experimental tests—carried out in parallel using a gas-discharge lamp (250 W mercury vapour lamp) and an LED lamp (20 W, 5000 K), which projected light onto a white canvas [45]—revealed low levels of other biomass at the site Passignano sul Trasimeno in comparison to the high levels detected at Sant’Arcangelo, mainly due to Lepidoptera.

In fact, notably relevant was the testing of a chironomid light attraction system installed far from the littoral zone of Lake Trasimeno (at 300 m from the coastal line of Castiglione del Lago). This light system was located on a raft (dimensions 3 × 2 m) equipped with two chambers (1.5 × 2 in m), each provided with a light attraction (LED lamps, 200 W, 6000 K) and aspiration system (ChiroBoat, Laika s.r.l. project; Figure S4), and revealed a remarkable ability to trap chironomids. About 780 g dry weight was captured on a single day (29 August 2018), including only a negligible amount of non-chironomid biomass [30]. The chironomid biomass captured in a single day corresponded to 1/8 and 1/11 of the catches in the ChiroTraps during the two-year period at PA and SA. At both sites, the ChiroTraps showed an illuminated surface of 4 m², whereas in the two-chamber ChiroBoat, this area was about 40 m². Indeed, the support surfaces of the inner walls of the traps seem to be related to attracting and capturing chironomids as well. The ChiroBoat test has also shown that the other biomass (except for chironomids) is negligible when the captures are carried out in the pelagic area of the lake, far from the coasts.

Based on these results, the best approach for controlling the pestiferous chironomids could be their trapping at the emergence location from water (areas far from the coasts), because this method would enhance capture capability and selectivity, with minor impact on biodiversity. Therefore, in the coming years, floating capture systems will be used (a Laika s.r.l. project) in the middle area of Lake Trasimeno. This strategy should attract adult chironomids from all sides by means of LED lamps’ 6000 K cold white light, with a much greater illuminated support surface for trapping these insects.

Avoiding the use of insecticides, through the extensive use of the attraction/aspiration technique, could allow the containment of swarms of pestiferous chironomid populations without compromising the environmental quality of the lake ecosystem, thereby leading to an increase in its biodiversity. Finally, the collected biomass of adult chironomids could be used for the production of feed to enhance the breeding of native fish species aimed at the repopulation of the lake, thereby triggering a sustainable circular economy process.

5. Conclusions

The use of a trapping system for adult insects, based on their positive phototaxis coupled with a suction mechanism, proved to be a successful tool to trap swarms of the pestiferous non-biting chironomid midges, both those dwelling in urban centres (PA) and those with a more natural environment (SA). This approach could reduce or eliminate the use of synthetic insecticides, which may contaminate the lacustrine environment.

In the PA and SA sites, both facing Lake Trasimeno and subjected to the same seasonal effects, the light attraction had a different impact on the local biodiversity, which could be linked not so much to the kind of light source as to the naturalness of the surrounding environment. Therefore, for more efficient chironomid control, the ChiroTraps should be placed in urban centres—or floating on the lake surface, as the ChiroBoat was—to produce minor effects on the local biodiversity.

Finally, the present study considered the various scientific approaches implemented for controlling the swarms of insects (mostly chironomid midges) at Lake Trasimeno, thus suggesting innovative strategies for safeguarding this lacustrine ecosystem. Reconciling environmental quality with tourist-economic enjoyment is not an option.