Effects of eight insecticides on Drosophila suzukii and its major pupal parasitoid Trichopria drosophilae

doi:10.21203/rs.3.rs-4439214/v1

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Effects of eight insecticides on Drosophila suzukii and its major pupal parasitoid Trichopria drosophilae

https://doi.org/10.21203/rs.3.rs-4439214/v1

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Drosophila suzukii is an important fruit pest of global significance. The combined application of insecticides and natural enemies can control D. suzukii efficiently and reduce chemical insecticide residues. The pupal parasitoid Trichopria drosophilae has been evaluated for the biological control of D. suzukii. However, little is known about the toxicity of common insecticides to T. drosophilae. In this study, the toxicity of eight common insecticides to D. suzukii as well as the effects of semi-lethal and sublethal doses on T. drosophilae were evaluated. The eight insecticides had higher toxicities to D. suzukii larvae with lower LC₅₀ values than those for adults. Adults and larvae were sensitive to emamectin benzoate, spinetoram, lambda-cyhalothrin, abamectin, and sophocarpidine. Based on the median lethal dose (LR₅₀) and safety factor (SF) of insecticides, lambda-cyhalothrin and imidacloprid were classified as medium risk and the other six insecticides were classified as low risk against T. drosophilae adults. Although these insecticides exhibited low toxicity to D. suzukii pupae, the exposure of pupae or larvae to chlorantraniliprole, emamectin benzoate, sophocarpidine, abamectin, azadirachtin, and spinetoram at semi-lethal and sublethal doses could decrease the parasitism or eclosion rate of T. drosophilae. These results improve our understanding of the effects of insecticide residues on T. drosophilae development and provide a basis for combined use of chemical and biological methods for the management of D. suzukii.

Drosophila suzukii

Trichopria drosophilae

insecticides

toxicity

safety evaluation

Drosophila suzukii (Diptera: Drosophilidae) is an important invasive pest of small soft-skinned fruits globally (Lee et al. 2011). More than 60 plant species have been identified as primary hosts, including strawberry cherry, blueberry, waxberry, and grape (Kenis et al. 2016; Lee et al. 2015). It was first reported to cause economic damage in Japan and is now widely distributed in America, Europe, Africa, and Oceania (Lee et al. 2015). D. suzukii can cause 40–80% losses in fruit yield (Mitsui et al. 2006). In recent years, it is estimated that D. suzukii caused US$511 million in losses annually in western production regions of North America (Yeh et al. 2020; Mermer et al. 2022).

Although several preventive and control methods for D. suzukii, such as physical trapping, have recently been reported (Clymans et al. 2022), insecticides are still commonly used in orchards. In previous studies, some common chemical insecticides, such as beta-cypermethrin, pleocidin, emamectin benzoate, azadirachtin, and lambda-cyhalothrin have higher toxicities on D. suzukii adults than those of imidacloprid, chlorantraniliprole, and avermectins (Lin et al. 2015; Lai et al. 2018). In commercial orchards, various insecticides, including spinosad, cyantraniliprole, and lambda-cyhalothrin, are effective for the control of D. suzukii for at least 2 weeks until harvest, with very few damaged fruits (Shaw et al. 2019). In addition to variation among pesticides, larvae and adults of D. suzukii differ substantially in physiological characteristics and ecological habits and therefore are expected to exhibit differences in sensitivity and resistance to pesticides. For example, susceptibility to insecticides, such as pleocidin, emamectin benzoate, chlorantraniliprole, and lambda-cyhalothrin, is higher in D. suzukii larvae than in adults (Lin et al. 2016). Spirotetramat could reduce the survival of adult flies but is lethal to larvae under low-humidity conditions (Yang et al. 2023).

Long-term and repeated pesticide exposure increases the risk of resistance and is a threat to human health and ecosystems (Guedes et al. 2017; Gress and Zalom, 2018). Compared with a sensitive D. suzukii population, In field populations of D. suzukii,resistance to five insecticides (imidacloprid, avermectins, pleocidin, emamectin benzoate, and lambda-cyhalothrin) did not increase in Shandong Province, China; however, the sensitivity to azadirachtin decrease slightly (Lin et al. 2015). Van Timmeren et al. (2018) also compared the insecticide susceptibility of D. suzukii populations collected from insecticide-free blueberry sites. After 3 years of monitoring, the resistance of D. suzukii to malathion and spinetoram increased slightly, with no changes in resistance to methomyl and zeta-cypermethrin. Therefore, changes in resistance to insecticides depend on the usage frequency and duration. It is necessary to monitor the sensitivity of D. suzukii to common insecticides regularly to identify safe, efficient control agents.

Economic development and increasing public awareness of safety have led to an emphasis of biological control methods using parasitoids (Wyckhuys et al. 2020). Several studies have reported that pesticide microdoses may have extended, trans-generational effects on predators. Passos et al. (2018) found that abamectin is highly toxic to nymphs of Macrolophus basicornis, a predator of Tuta absoluta, methoxyfenozide, teflubenzuron, and chlorantraniliprole caused low predator mortalities, and the adult survival rate was not affected by insecticides. Low concentrations of imidacloprid had acute toxicity against the coccinellid Harmonia axyridis and had weaker effects on the survival rates of ladybird adults and progeny and their fecundity (Dai et al. 2021). Parasitoids, such as Trichopria drosophilae, Pachycrepoideus vindemmiae, Ganaspis brasiliensis, Trichopria anastrephae, and Leptopilina japonica, have been used as potential biological control agents for D. suzukii (Herz et al. 2021; Rossi-Stacconi et al. 2022; Nair et al. 2023; Krüger et al. 2024). However, there is evidence for the toxicity of insecticides against these taxa. For example, Schlesener et al. (2019) evaluated the toxicity of eight insecticides to P. vindemmiae and T. anastrephae, revealing higher sensitivity in P. vindemmiae than T. anastrephae. Malathion could cause 100% mortality in adult T. anastrephae (Morais et al. 2022). Spinosyns (spinosad and spinetoram) and abamectin caused high P. vindemmiae mortality rates but did not affect T. anastrephae (Schlesener et al. 2019). The pupal parasitoid T. drosophilae has recently been investigated for the biological control of D. suzukii in Europe and other countries (Herz et al. 2021). Accordingly, it is important to select agents that are highly effective against D. suzukii with a low risk to T. drosophilae. However, the toxicities of common insecticides used in orchards to T. drosophilae are unclear.

Therefore, in this study, we determined the toxicity of eight common insecticides to D. suzukii at different developmental stages as well as acute toxicity to T. drosophilae adults. Concentration gradients of each insecticide were set according to the recommended doses in the field. The study results provide a theoretical basis for the scientific and rational use of pesticides as well as a reference for coordinating biological and chemical control.

Drosophila suzukii

D. suzukii adults were collected in June 2022 from a cherry orchard in Yantai (36.82°N, 120.69°E), Shandong Province, China. Flies were maintained in a climate-controlled growth chamber at 25 ± 0.5°C, 60 ± 0.5% relative humidity, and a 16 h:8 h photoperiod. One liter of artificial diet used for feeding D. suzukii was composed of 60 g of corn flour, 50 g of sucrose, 40 g of yeast extract, 10 g of agar powder, and water. Third-instar larvae, one-day pupae, and five-day-old adults were used for the following experiments.

Trichopria drosophilae

T. drosophilae adults were collected in June 2022 from a grape orchard in Jinan (36.55°N, 116.79°E), Shandong Province, China. Approximately 500–600 D. suzukii pupae were used for parasitism, and 100–120 pairs of wasps were placed in insect rearing cages with mesh covers (20 × 20 × 20cm). Additionally, 30% honey water was provided in the cage to supplement wasp nutrition. After 24 h of parasitism, the pupae were removed from the cage and new pupae were added for continued parasitism. Wasps that emerged after 17–20 days were used for subsequent experiments.

Insecticides

Eight insecticides were selected for this experiment, including six chemical pesticides (i.e., chlorantraniliprole, spinetoram, emamectin benzoate, lambda-cyhalothrin, imidacloprid, and abamectin) and two botanical insecticides (i.e., sophocarpidine and azadirachtin). The concentrations of active ingredients, production details, and maximum recommended field dose are shown in Table 1.

Table 1

Test pesticide and fungicide
Category	Active ingredient	Concentration of Active ingredient	Production enterprise	Max recommended field dose (mg/cm²)
Chemical pesticides	Chlorantraniliprole	5%	FMC Corporation Shanghai Agricultural Technology Co., Ltd.	3.9900E-04
	Spinetoram	60g/L	Delegate® WG, Dow AgroSciences S.r.l.	3.6000E-04
	Emamectin benzoate	2%	Syngenta (Suzhou) Crop Protection Co., Ltd.	1.3680E-04
	Lambda-cyhalothrin	3%	Syngenta (Nantong) Crop Protection Co., Ltd.	1.2000E-04
	Imidacloprid	70%	Bayer Crop Science (China) Co., Ltd.	6.3000E-04
	Abamectin	18g/L	Syngenta (Suzhou) Crop Protection Co., Ltd.	4.5000E-05
Botanical insecticides	Sophocarpidine	0.50%	Beijing Sanpu Baicao Green Plant Preparation Co., LTD	8.6250E-05
Botanical insecticides	Azadirachtin	0.30%	Delegate® WG, Dow AgroSciences S.r.l.	1.1250E-04

Toxicity of eight insecticides on D. suzukii at different stages

Eight insecticides were serially diluted to six concentrations with a geometrical ratio starting from the highest dose using distilled water. For the toxicity experiment of adults, the highest concentrations of eight insecticides (chlorantraniliprole, spinetoram, emamectin benzoate, lambda-cyhalothrin, imidacloprid, abamectin, sophocarpidine, and azadirachtin) were set as 640 ppm (mg/kg), 2 ppm, 16 ppm, 8 ppm, 80 ppm, 320 ppm, 320 ppm, and 320 ppm. The artificial diet was soaked for 2 min in rearing tubes (9.5 cm height and 1.5 cm diameter) with eight insecticides at six concentrations. The mortality of D. suzukii adults was assessed for each insecticide by exposing 20 adults in rearing tubes after 48 h. For the larval toxicity experiment, the highest concentrations of eight insecticides were set as 160 ppm (mg/kg), 2 ppm, 0.1 ppm, 4 ppm, 320 ppm, 2 ppm, 3.2 ppm, and 1280 ppm. Twenty second-instar larvae were added to a plastic Petri dish (3.5 cm diameter) containing an insecticide-contaminated diet. The mortality of D. suzukii larvae was assessed according to the emerging pupae in each treatment with eight insecticides. For the pupal toxicity experiment, the highest concentrations of eight insecticides were set as 320 ppm (mg/kg), 32 ppm, 32 ppm, 32 ppm, 320 ppm, 640 ppm, 640 ppm, and 640 ppm. Twenty pupae (1 day old) were soaked for 2 min in eight insecticides with six concentrations. The mortality of D. suzukii pupae was assessed according to the emerging adults in each treatment of eight insecticides. Three replicates were carried out for each concentration of eight insecticides. The concentration-mortality regression lines for eight insecticides at six concentrations for adults, larvae, and pupae were determined.

Acute virulence of eight insecticides on T. drosophilae

According to the maximum recommended field doses of eight insecticides in orchards (Table 1), six concentration gradients were set for each drug. The highest concentrations of insecticides in this experiment were 10 times the maximum recommended field doses. Additionally, 0.4 mL of liquid drug was added to a finger-shaped tube with a diameter of 1.9 cm and a length of 7.2 cm. Distilled water in a tube was used as a control. Using a hot-dog roller, the tubes were rolled until the liquid was evenly distributed on the wall of the tubes to form a dried drug film. Twenty wasps were put into the tube with the drug film for three repeats. The tube mouth was sealed with a cotton plug. After 1 h, the wasps were transferred into a clean finger-shaped tube without drugs and fed 10% honey water. The survival situation of the wasps was observed 24 h later. The dose-mortality relationships and 50% lethal concentrations (LR₅₀) of each insecticide for T. drosophilae adults were determined using a log-probit model at P > 0.05. The safety factor was calculated according to the following formula:

$\text{S}\text{a}\text{f}\text{e}\text{t}\text{y} \text{f}\text{a}\text{c}\text{t}\text{o}\text{r} =\frac{ \text{L}\text{R}50 }{\text{M}\text{a}\text{x} \text{r}\text{e}\text{c}\text{o}\text{m}\text{m}\text{e}\text{n}\text{d}\text{e}\text{d} \text{f}\text{i}\text{e}\text{l}\text{d} \text{d}\text{o}\text{s}\text{e}}$ Agents were assigned to four risk levels based on the safety factor. (1) Extremely high: Safety factor ≤ 0.05; (2) High: 0.05 < Safety factor ≤ 0.5; (3) Medium: 0.5 ≤ Safety factor ≤ 5; (4) Low: Safety factor > 5.

Effects of eight insecticides on the parasitism rate of T. drosophilae

LC₁₀ and LC₅₀ values of eight insecticides previously determined for D. suzukii larvae were used to investigate the toxicity on T. drosophilae offspring reared on contaminated hosts. Chemical exposure was performed by allowing T. drosophilae females to parasitize D. suzukii pupae (1 day old) exposed to insecticides in two ways. (1) For the treatment of host larvae, second-instar D. suzukii larvae were reared in artificial medium contaminated with the LC₁₀ and LC₅₀ for each insecticide; larvae developed until the pupal stage. An untreated control was included in which larvae were exposed to the clear artificial medium. (2) For the treatment of host pupae, 20 pupae (1 day old) were soaked for 1 min in the LC₁₀ and LC₅₀ for each insecticide. An untreated control was included in which pupae were soaked in distilled water. Air-dried pupae were used for this experiment.

Twenty contaminated fly pupae for each method were parasitized for 24 h by 10 T. drosophilae pairs (5 days old) in clear plastic tubes (9.5 cm height and 1.5 cm diameter). Tubes without wasps were used for the control treatment. Three replicates were carried out for each treatment. The parasitism rate and eclosion rate of T. drosophilae exposed to insecticides were calculated by dissecting the pupae and counting emerging offspring.

Data analysis

The dose-mortality relationships and the 10% and 50% lethal concentrations (LC₁₀ and LC₅₀) of each insecticide for larvae and adults of D. suzukii and T. drosophilae adults were determined using a log-probit model at P > 0.05. Raw data were analyzed for normality and homogeneity of variances using the Kolmogorov–Smirnov test.

One-way analysis of variance (ANOVA) was used to analyze survival rates of D. suzukii pupae exposed to insecticides at the LC₁₀ and LC₅₀ and the parasitism rate of T. drosophilae. Tukey’s multiple comparison tests were performed, with P < 0.05 indicating significance, using IBM SPSS Statistics for Windows version 20.00 (IBM Corp., Armonk, NY, USA).

Toxicity of eight insecticides on D. suzukii

Toxicity at the larval stage

The toxicological data for each insecticide for D. suzukii larvae evaluate using a probit analysis are shown in Table 2. Among the eight insecticides, emamectin benzoate resulted was the most toxic for D. suzukii, killing 50% and 10% of the tested population at 0.0213 mg/L and 0.0048 mg/L, respectively. The next most toxic insecticides to emamectin benzoate were spinetoram, lambda-cyhalothrin, abamectin, and sophocarpidine, followed by chlorantraniliprole and imidacloprid. Azadirachtin was the least toxic compound with a 50% lethal concentration of 533.3967 mg/L and 10% lethal concentration of 87.5383 mg/L. The 50% and 10% lethal concentrations of each insecticide in Table 2 were used for feeding D. suzukii larvae to pupae and for estimating the effects of insecticides on the parasitism rate of T. drosophilae.

Table 2

Probit anysis for the toxicity of eight insecticides on Drosophila suzukii larvae
Active ingredient	Probit model	SE(Slope)	SE(Intercept)	LC₅₀ (95% confidence interval)	LC₁₀ (95% confidence interval)	R²
Chlorantraniliprole	Y = 1.8211X-2.6701	0.1820	0.2800	29.256 (16.3529–51.5969)	5.7874 (0.9935–11.5132)	0.9572
Spinetoram	Y = 1.3941X + 0.8994	0.1591	0.1105	0.2264 (0.1722–0.2882)	0.0273 (0.0129–0.0447)	0.9304
Emamectin benzoate	Y = 1.9868X + 3.3223	0.1966	0.3403	0.0213 (0.0142–0.0320)	0.0048 (0.0017–0.0081)	0.9187
Lambda-cyhalothrin	Y = 1.5421X + 0.5023	0.1639	0.0835	0.4723 (0.3692-0.5900)	0.0697 (0.0373–0.1068)	0.9933
Imidacloprid	Y = 0.9624X-1.5239	0.1424	0.2567	38.3135 (25.8238–53.3766)	1.7856 (0.4200-4.0573)	0.9914
Abamectin	Y = 2.2978X + 1.0465	0.2413	0.1301	0.3499 (0.2889–0.4179)	0.0967 (0.0633-0.1300)	0.9783
Sophocarpidine	Y = 2.4017X + 0.8045	0.2281	0.1083	0.4624 (0.3874–0.5463)	0.1353 (0.0941–0.1764)	0.9839
Azadirachtin	Y = 1.6328X-4.4526	0.1715	0.4351	533.3967 (269.1816-2287.1624)	87.5383 (6.4973–186.3520)	0.8404

Toxicity at the pupal stage

The toxicological data for each insecticide for D. suzukii pupae are shown in Fig. 1. These eight insecticides showed low toxicity on pupae at the present concentrations. Therefore, probit analysis was difficult. The mortality rates of pupae exposed to chlorantraniliprole (P = 0.259), spinetoram (P = 0.705), lambda-cyhalothrin (P = 0.315), imidacloprid (P = 0.667), sophocarpidine (P = 0.101), and azadirachtin (P = 0.087) at different concentrations did not differ significantly from those of the control group. Though the mortality of pupae exposed to emamectin benzoate at 32 mg/L was significantly higher than that of control (P = 0.001), there were no significant differences in mortality between emamectin benzoate at concentrations from 0.125 mg/L to 8 mg/L and the control group. The mortality rates of pupae exposed to abamectin at 160 mg/L and 320 mg/L were significantly higher than that of the control group (P = 0.004); however, there were no significant difference in mortality between abamectin at concentrations of 2.5 mg/L to 40 mg/L and the control group.

Toxicity at the adult stage

The toxicological data for each insecticide for D. suzukii adults evaluated in a probit analysis are shown in Table 3. Among the eight insecticides, spinetoram and lambda-cyhalothrin were the most toxic for D. suzukii, killing 50% of the tested population at concentrations of 0.3191 mg/L and 0.7994 mg/L, respectively. Chlorantraniliprole, mectin, sophocarpidine, and azadirachtin were less toxic for D. suzukii with 50% lethal concentrations of 61.6784, 44.8573, 67.4941, and 56.7422, respectively.

Table 3

Probit anysis for the toxicity of eight insecticides on Drosophila suzukii adults
Active ingredient	Probit model	SE(Slope)	SE(Intercept)	LC₅₀	95% confidence interval	R²
Chlorantraniliprole	Y = 1.5170X-2.7157	0.1431	0.2829	61.6784	38.6239–90.1389	0.9951
Spinetoram	Y = 2.7553X + 1.3668	0.2292	0.1442	0.3191	0.2785–0.3666	0.992
Emamectin benzoate	Y = 2.0340X-0.7384	0.2035	0.1311	2.3069	1.8796–2.7511	0.9918
Lambda-cyhalothrin	Y = 2.9742X + 0.2892	0.2483	0.0837	0.7994	0.7051–0.90481	0.9843
Imidacloprid	Y = 3.9868X-3.6234	0.3692	0.3596	8.1065	7.1694–9.1329	0.9937
Abamectin	Y = 2.2653X-3.7420	0.1788	0.3084	44.8573	27.7961–70.8038	0.9104
Sophocarpidine	Y = 1.7664X-3.2312	0.1644	0.3073	67.4941	55.4569–82.3719	0.9642
Azadirachtin	Y = 2.3776X-4.1701	0.1895	0.3389	56.7422	48.9895–65.8122	0.9838

Acute virulence of eight insecticides on T. drosophilae

The acute toxicity of eight insecticides on T. drosophilae is shown in Fig. 2. Compared with those of the control, the survival rates of T. drosophilae adults exposed to chlorantraniliprole (P = 0.566), emamectin benzoate (P = 0.136), abamectin (P = 0.563), and sophocarpidine (P = 0.463) did not differ significantly. Six insecticides show low acute toxicities to T. drosophilae adults. High toxicity to T. drosophilae adults was found for high concentrations of spinetoram (P < 0.001), lambda-cyhalothrin (P < 0.001), imidacloprid (P < 0.001), and azadirachtin (P = < 0.001). The mortality rates of T. drosophilae adults after 1 h of treatment with field concentrations of spinetoram, lambda-cyhalothrin, and imidacloprid were approximately 15%, 8.5%, and 40%, respectively. According to the log-probit analysis at the P > 0.05 level of dose-mortality relationships, 50% lethal concentrations (LR₅₀) of four insecticides for T. drosophilae adults are shown in Table 4. The safety factors of lambda-cyhalothrin and imidacloprid were 4.3975 and 1.3947, respectively, indicating a medium risk level to T. drosophilae adults. Spinetoram and azadirachtin were classified as low risk with safety factors of 6.4778 and 5.0024.

Table 4

Acute toxicity of four drugs on Trichopria drosophilae adults
Active ingredient	Probit model	SE(Slope)	SE(Intercept)	LR₅₀	95% confidence interval	R²	Safety factor	Risk level
Spinetoram	Y = 1.3395X + 3.5259	0.1875	0.5648	2.3320E-03	0.001709–0.003626	0.9912	6.4778	Low
Lambda-cyhalothrin	Y = 2.8135X + 9.2216	0.2948	0.9874	5.2800E-04	0.000454–0.006211	0.9933	4.3975	Medium
Imidacloprid	Y = 1.4006X + 4.2805	0.1336	0.4323	8.7900E-04	0.000657–0.001188	0.9989	1.3947	Medium
Azadirachtin	Y = 3.3719X + 10.9574	0.4679	1.5247	5.6300E-04	0.000483–0.000656	0.9705	5.0024	Low
Risk lever: Sky-high, Safety factor《0.05; Risk lever: High, 0.05《Safety factor《0.5; Risk lever: Medium, 0.05《Safety factor《0.05; Risk lever: Low, Safety factor > 5.

Effects of eight insecticides on parasitism and eclosion rates of T. drosophilae

LC₁₀ and LC₅₀ values of eight insecticides previously determined for D. suzukii larvae (Table 2) were used to estimate the effects on parasitism and eclosion rates of T. drosophilae. Two methods were used for this experiment. First, host larvae treated with eight insecticides at the LC10 and LC50 concentrations were reared to the pupal stage (Fig. 3). Compared with that of the control (98.33 ± 1.67%), the parasitism rate of T. drosophilae was significantly lower after treatment with sophocarpidine at the LC₅₀ concentration (66.66 ± 6.66%; P = 0.010). However, there were no significant difference in parasitism rates for the other seven insecticides. The eclosion rates of T. drosophilae exposed to chlorantraniliprole, imidacloprid, sophocarpidine, and azadirachtin at the LC₅₀ concentration were reduced to 36.67 ± 3.33%, 41.48 ± 5.96%, 33.33 ± 3.33%, and 35.00 ± 2.89%, respectively, compared with that of the control (63.33 ± 1.67%; P < 0.001). The parasitism rate of T. drosophilae was not affected by exposure to eight insecticides at the LC₁₀ concentration (P = 0.053). Only sophocarpidine at the LC₁₀ concentration reduced the eclosion rate of T. drosophilae (40.00 ± 5.77%) significantly (P = 0.005).

Second, the pupae were treated directly with eight insecticides at the LC₁₀ and LC₅₀ concentrations. The parasitism rates and eclosion rates of T. drosophilae were lower than those for after the treatment of larvae (Fig. 4). Compared with that of the control, parasitism rates of T. drosophilae exposed to spinetoram, imidacloprid, and azadirachtin at the LC₅₀ concentration were reduced to 73.33 ± 7.26%, 40.00 ± 0.00%, and 45.00 ± 5.77%, respectively. Only azadirachtin at the LC₁₀ concentration reduced the parasitism rate of T. drosophilae (75.00 ± 8.66%) significantly (P = 0.001). All eight insecticides at the LC₅₀ and LC₁₀ concentrations reduced the eclosion rate significantly (LC₅₀: P < 0.001; LC₁₀: P < 0.001). Imidacloprid at the LC₅₀ concentration applied to pupae directly inhibited the next oviposition of wasps. Therefore, treating D. suzukii pupae directly had greater effects on parasitism and eclosion rates of T. drosophilae than those of treating D. suzukii larvae.

Eight insecticides evaluated in this study had higher toxicities to D. suzukii larvae than adults according to the 50% lethal concentration. These results were consistent with those of previous studies; for example, the susceptibilities of D. suzukii larvae to emamectin benzoate, chlorantraniliprole, and lambda-cyhalothrin were higher than those of adults (Lin et al. 2016). Spirotetramat reduced the survival rate of D. suzukii adults but was lethal to larvae (Yang et al. 2023). Lambda-cyhalothrin, spinosad, and spinetoram showed inhibitory effects on the development of D. suzukii larvae (Lisi e al. 2023). In addition, the response of insects to insecticides are related to insect sex, age evaluated at the adult stage, and the duration of exposure to the active ingredient (Blouquy et al. 2021). Therefore, insecticides should be applied at a reasonable concentration at the most sensitive stage of pests.

Among eight insecticides in our study, emamectin benzoate, spinetoram, lambda-cyhalothrin, abamectin, and sophocarpidine had high toxicity on D. suzukii larvae. Adults were more sensitive to emamectin benzoate, spinetoram, lambda-cyhalothrin than to the other five insecticides. Therefore, three insecticides (i.e., emamectin benzoate, spinetoram, and lambda-cyhalothrin) could be used as effective agents for controlling D. suzukii. As a polycidin insecticide, spinetoram is efficient and safe, with highly effective insecticidal activity against target pests and low toxicity against non-target insects (Sparks et al. 2008). Lambda-cyhalothrin is one of the most common pyrethroids used for agricultural and household pest control, acting through contact and the nervous system (He et al., 2008). Spinetoram and lambda-cyhalothrin are widely used for controlling D. suzukii (Shaw et al. 2019; Van Timmeren et al. 2019; Van Timmeren et al. 2019; Mermer et al. 2023; Shawer et al. 2022). Moreover, as a new type of highly efficient antibiotic insecticide based on abamectin B₁, emamectin benzoate is widely used for pest control (Xu et al. 2021). However, it is rarely used for controlling D. suzukii, except for several reports in China (Lin et al. 2016; Wu et al. 2021). Further studies are needed to determine whether emamectin benzoate can be used for controlling D. suzukii and particularly to evaluate its safety against parasitic wasps.

An important method for evaluating drug safety in parasitic wasps is the acute virulence test in which parasitic wasps are exposed to drugs indirectly for short time. Since Trichogramma spp. has been widely released for the control of a variety of agricultural and forestry pests in China, the toxicity of many commonly used pesticides to Trichogramma spp. has been evaluated using acute virulence tests (Li et al. 2018; Li et al. 2019; Jin et al. 2021). T. drosophilae is an important pupal parasitoid of D. melanogaster and D. suzukii, laying a single egg per oviposition inside pupae. In this study, based on the median lethal rate (LR₅₀) and SF, lambda-cyhalothrin and imidacloprid were identified as medium risk and the other six insecticides were identified as low-risk against T. drosophilae adults. In particular, the mortality rate of T. drosophilae adults after 1 h of treatment with field concentrations of imidacloprid was about 40%. Therefore, lambda-cyhalothrin and imidacloprid should be used with caution in integrated pest management programs against D. suzukii.

In addition to the increased mortality caused by indirect contact with drugs, the long-term, trans-generation effects of sublethal doses of pesticides on parasitic wasps should also be considered when assessing the safety of pesticides (Costa et al. 2014; Lisi et al. 2023). Treating host larvae or pupae with insecticides can affect the parasitic behavior of parasitoids and the emergence of the next generation. For example, spinosad applied to host insects at the late larval and pupal stages reduced adult emergence for two species of Trichogramma significantly (Liu et al. 2012). Lambda-cyhalothrin applied at the pre-pupal and pupal stages and spinosad applied to pre-pupae significantly reduced the adult emergence of Trichogramma galloi (Costa et al. 2014). For two pupal parasitoids of D. suzukii, the sensitivity to dry residues of eight commercial insecticides was higher for P. vindemmiae than for T. anastrephae, resulting in a significant reduction in parasitism for the former species (Schlesener et al. 2019). Sublethal doses of Cyazypyr and dimethoate negatively affected the success of parasitism and the number of progeny of T. drosophilae (Lisi et al. 2023). The results of our study were consistent with those of these previous reports. In our study, although the insecticides showed no toxicity against D. suzukii pupae at the sublethal dose (LC₁₀) and LC₅₀, the direct exposure of larvae and pupae to some insecticides could decrease the parasitism and eclosion rates of T. drosophilae. When D. suzukii larvae were exposed to sophocarpidine at the LC₅₀ concentration, the parasitism rate of T. drosophilae adults decreased by 32.21%. The eclosion rates of T. drosophilae in the next generation were reduced by chlorantraniliprole, imidacloprid, sophocarpidine, and azadirachtin at the LC₅₀ concentration and sophocarpidine at the sublethal dose. When D. suzukii pupae were exposed to insecticides, the parasitism rates of T. drosophilae adults were reduced by spinetoram, imidacloprid, and azadirachtin at the LC₅₀ concentration. The sublethal doses of all of the insecticides decreased the eclosion rate of the next generation but did not affect the parasitism rates of T. drosophilae adults. These results also suggested that the semi-lethal and sublethal doses of insecticides had greater effects on the eclosion rate of T. drosophilae in the next generation than on parasitism rates of the F₀ generation. Eggs successfully entering D. suzukii pupae might not develop into adults.

To improve integrated pest management strategies for D. suzukii, extensive research has evaluated the control effects of insecticides. For example, Mermer et al. (2022) indicated that micro-sprinkler systems provide a valuable alternative and rapid technique to suppress D. suzukii pest populations during high-pressure periods in highbush blueberry production. In comparison with typical spinosad spray applications, the use of Hanseniaspora uvarum in combination with spinosad as an attract-and-kill formulation against D. suzukii reduces pesticide residues on fruits by targeting the canopy and decreasing the amount of insecticide per hectare, without compromising control efficacy (Rehermann et al. 2022; Spitaler et al. 2022).

Moreover, monitoring the susceptibility and resistance of D. suzukii to insecticides is critical for the screening of highly effective insecticides. Mertz et al. (2022) examined the toxicity of 19 alternative insecticides to a susceptible D. melanogaster strain and cross-resistance using a field-collected population. There were high levels of resistance to zeta-cypermethrin, malathion, and acetamiprid in all populations sampled over 33 months. Resistance to spinetoram started to evolve in two vineyards. Moreover, Gress et al. (2022) presented a simple, cost-effective tool for assaying the resistance of D. suzukii in commercial caneberry fields near Watsonville, CA to three commonly used insecticides (malathion, spinosad, and zeta-cypermethrin). Therefore, monitoring the resistance of D. suzukii on susceptible insecticides and selecting resistant insect lines is important for resistance management research and will be a focus of our future study.

In summary, the eight insecticides had higher toxicities to D. suzukii larvae with lower LC₅₀ values than those for adults. Lambda-cyhalothrin and imidacloprid caused mortality in T. drosophilae adults. For some insecticides that were not lethal to T. drosophilae adults at the recommended dose in the field (e.g., chlorantraniliprole, emamectin benzoate, sophocarpidine, abamectin, azadirachtin, and spinetoram), the semi-lethal and sublethal doses may affect the parasitism rate of the F₀ generation and the eclosion rate of the next generation. The results of this study improve our understanding of the effects of insecticide residues on T. drosophilae populations and provide a basis for the development of scientific and efficient measures to manage D. suzukii and protect T. drosophilae.

Author Contribution Statement

HHG and XHZ conceived and designed research. HHG and PC conducted experiments. QYZ contributed new reagents and/or analytical tools. HHG and ASZ analyzed data. HHG wrote the manuscript. All authors read and approved the manuscript.

Ethical Approval

It is not applicable.

Author Contribution

Acknowledgements

This work was supported financially through a grant from the National Natural Science Foundation of China (31801750). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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Effects of eight insecticides on Drosophila suzukii and its major pupal parasitoid Trichopria drosophilae

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Abstract

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Introduction

Materials and Methods

Drosophila suzukii

Trichopria drosophilae

Insecticides

Toxicity of eight insecticides on D. suzukii at different stages

Acute virulence of eight insecticides on T. drosophilae

Effects of eight insecticides on the parasitism rate of T. drosophilae

Data analysis

Results

Toxicity of eight insecticides on D. suzukii

Toxicity at the larval stage

Toxicity at the pupal stage

Acute virulence of eight insecticides on T. drosophilae

Effects of eight insecticides on parasitism and eclosion rates of T. drosophilae

Discussion

Conclusion

Declarations

Author Contribution Statement

Ethical Approval

Author Contribution

Acknowledgements

References

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