Plasticity in Oviposition Site Selection Behavior in Drosophila suzukii (Diptera: Drosophilidae) in Relation to Adult Density and Host Distribution and Quality.

Flexibility in oviposition site selection under temporally shifting environmental conditions is an important trait that allows many polyphagous insects to flourish. Population density has been shown to affect egg-laying and offspring fitness throughout the animal kingdom. The effects of population density in insects have been suggested to be mutualistic at low densities, whereas intraspecific competition is exhibited at high densities. Here, we explore the effects of adult crowding and spatial resource variation on oviposition rate in the invasive pest Drosophila suzukii (Matsumura). In a series of laboratory experiments, we varied the density of adult males and females while holding oviposition substrate availability constant and measured per female oviposition rate using high and low-quality substrates. We found that oviposition behavior was affected more by substrate than adult density, though both variables had significant effects. When we varied the spatial arrangement of whole raspberries, we observed differences in oviposition rate and egg distribution between the grouped and solitary female treatments. Our results suggest that social interactions encourage oviposition, especially when exposed to unfamiliar or unnatural substrates. These results highlight the compensating effect of increased oviposition rate per female as adult populations decline. They will help researchers and crop managers better understand in-field population dynamics throughout the season as population densities change.

Susceptibility of North Carolina Chloridea (Heliothis) virescens (Lepidoptera: Noctuidae) Populations From Flue Cured Tobacco to Chlorantraniliprole.

The tobacco budworm, Chloridea (Heliothis) virescens (F.), has evolved resistance to numerous insecticides in the field. In tobacco, chlorantraniliprole can be applied as either a preventative systemic soil application at or near transplant, or a foliar application timed to current treatment thresholds. With a novel mode of action chlorantraniliprole provides an option for rotation with other insecticides to reduce the probability of insecticide resistance development. However, specific usage patterns in tobacco have the potential to increase the risk of resistance development to this insecticide. In particular, soil applied treatments may expose C. virescens to sublethal concentrations of the insecticide. We studied chlorantraniliprole susceptibility in nine field populations and one laboratory strain of C. virescens using a diet incorporation bioassay. Mortality was measured at 7, 10, and 14 d after exposure. Our results demonstrated that bioassays should be evaluated at 14 d after exposure to optimize interpretation of the dose-response due to the antifeeding properties of chlorantraniliprole. We observed low variation within field-collected populations. Field populations were as susceptible as the laboratory strain; the resistance ratio at the 14-d evaluation for field-collected populations ranged from 1.01 to 1.05. A discriminating dose of 0.117, 0.080, and 0.070 µg a.i./g diet could be used at 7, 10, and 14 d, respectively. Larval growth varied between field populations. Two field populations continued growing on diet containing chlorantraniliprole and differed in size from the laboratory and other populations. Further evaluation of growth inhibition will be necessary.

Social signals mediate oviposition site selection in Drosophila suzukii.

The information that female insects perceive and use during oviposition site selection is complex and varies by species and ecological niche. Even in relatively unexploited niches, females interact directly and indirectly with conspecifics at oviposition sites. These interactions can take the form of host marking and re-assessment of prior oviposition sites during the decision-making process. Considerable research has focused on the niche breadth and host preference of the polyphagous invasive pest Drosophila suzukii Matsumura (Diptera: Drosophilidae), but little information exists on how conspecific signals modulate oviposition behavior. We investigated three layers of social information that female D. suzukii may use in oviposition site selection-(1) pre-existing egg density, (2) pre-existing larval occupation, and (3) host marking by adults. We found that the presence of larvae and host marking, but not egg density, influenced oviposition behavior and that the two factors interacted over time. Adult marking appeared to deter oviposition only in the presence of an unmarked substrate. These results are the first behavioral evidence for a host marking pheromone in a species of Drosophila. These findings may also help elucidate D. suzukii infestation and preference patterns within crop fields and natural areas.

Detecting Specific Resource Use by Drosophila suzukii (Diptera: Drosophilidae) Using Gut Content Analysis.

Drosophila suzukii Matsumura (Diptera: Drosophilidae) is an invasive, highly polyphagous pest of soft-skinned fruits throughout much of the world. A better understanding of the ecology of adult flies, including their nutritional resources, is needed to advance ecologically based management approaches. In this study, we evaluate the capability of polymerase chain reaction-based gut content analysis to detect a known food resource from DNA extracted from laboratory-reared flies. Using strawberry as a focal host and available DNA primers, we validated that DNA from this host could be detected for up to 7 d post-consumption. With the development of specific primers for additional hosts, we expect that this technique will enable researchers to better understand how D. suzukii adults use, and move between, nutritional resources.

Integrated Pest Management Practices Reduce Insecticide Applications, Preserve Beneficial Insects, and Decrease Pesticide Residues in Flue-Cured Tobacco Production.

Integrated pest management (IPM) recommendations, including scouting and economic thresholds (ETs), are available for North Carolina flue-cured tobacco growers, although ETs for key pests have not been updated in several decades. Moreover, reported IPM adoption rates by flue-cured tobacco growers remain low, at < 40%, according to NC cooperative extension surveys conducted during the last four years. Previous research has suggested that timing insecticide treatments using currently available ETs can reduce the average number of applications to two or fewer per season. We conducted field-scale trials at nine commercial tobacco farms, three in 2104 and six in 2015, to quantify inputs associated with current scouting recommendations, to determine if current ETs were able to reduce insecticide applications as compared to grower standard practices, and to assess the impacts of reduced insecticide applications on end of season yield and pesticide residues. Two fields were identified at each farm and were scouted weekly for insects. One field was only treated with insecticides if pests reached ET (IPM), while the other field was managed per grower discretion (Grower Standard). IPM fields received an average of two fewer insecticide applications without compromising yield. More insecticide applications resulted in higher pesticide residues in cured leaf samples from Grower Standard fields than those from IPM fields. Reductions in insecticides and management intensity also resulted in larger beneficial insect populations in IPM fields.

Systemic Imidacloprid Affects Intraguild Parasitoids Differently.

Toxoneuron nigriceps (Viereck) (Hymenoptera, Braconidae) and Campoletis sonorensis (Cameron) (Hymenoptera, Ichneumonidae) are solitary endoparasitoids of the tobacco budworm, Heliothis virescens (Fabricius) (Lepidoptera, Noctuidae). They provide biological control of H. virescens populations in Southeastern US agricultural production systems. Field and greenhouse experiments conducted from 2011-2014 compared parasitism rates of parasitoids that developed inside H. virescens larvae fed on tobacco plants treated with and without imidacloprid. The parasitoids in our study did not have a similar response. Toxoneuron nigriceps had reduced parasitism rates, but parasitism rates of C. sonorensis were unaffected. Preliminary data indicate that adult female lifespans of T. nigriceps are also reduced. ELISA was used to measure concentrations of neonicotinoids, imidacloprid and imidacloprid metabolites in H. virescens larvae that fed on imidacloprid-treated plants and in the parasitoids that fed on these larvae. Concentrations were detectable in the whole bodies of parasitized H. virescens larvae, T. nigriceps larvae and T. nigriceps adults, but not in C. sonorensis larvae and adults. These findings suggest that there are effects of imidacloprid on multiple trophic levels, and that insecticide use may differentially affect natural enemies with similar feeding niches.