Explorative Data Analysis of Drosophila suzukii Trap Catches from a Seven-Year Monitoring Program in Southwest Germany

The contrasting role of climate variation on the population dynamics of a native and an invasive insect pest

Since 2008, spotted-wing drosophila, Drosophila suzukii, has become a major pest of soft, thin-skinned fruits in the USA, causing significant annual yield losses. Historically, the native blueberry maggot fly, Rhagoletis mendax, has been a key blueberry pest in eastern North America and a driver of insecticide usage. After its invasion in 2011 into New Jersey (USA), D. suzukii has supplanted R. mendax as the main target of insecticide applications in the state. However, the impact of D. suzukii on the native R. mendax has not been documented, particularly in relation to local climate. Historical monitoring data from New Jersey blueberry farms were used to assess the role of climate on R. mendax and D. suzukii populations. Seasonal trap captures of R. mendax adults have decreased after D. suzukii invasion, while D. suzukii trap captures have increased. Similarly, D. suzukii first captures have occurred earlier each year, while R. mendax has been captured later in the growing season. Winter freezing and summer growing degree days were found to significantly correlate with D. suzukii activity. Using downscaled climate simulations, we projected that D. suzukii will arrive in New Jersey blueberry fields up to 5 days earlier on average by 2030 and 2 weeks earlier by 2050 with warming temperatures, exacerbating yield losses and insecticide usage. As regional temperatures are projected to warm and the invasive range continues to expand, we predict the rate of phenological development of the invasive D. suzukii and its impact on native insects to change noticeably, bringing new challenges for pest management strategies.

Invasive Drosophila suzukii outnumbers native controphics and causes substantial damage to fruits of forest plants

Impacts of biological invasions are diverse and can have far-reaching consequences for ecosystems. The spotted wing drosophila, Drosophila suzukii, is a major invasive pest of fruits, which negatively affects fruit and wine production. However, little is known about the ecological impact of this fly species on more natural ecosystems it has invaded, such as forests. In this study, we investigated the use of potential host plants by D. suzukii at 64 sites in different forest communities in Switzerland from mid-June to mid-October 2020. We examined more than 12,000 fruits for egg deposits of D. suzukii to assess its direct impact on the plants. We recorded symptoms of fruit decay after egg deposition to determine if D. suzukii attacks trigger fruit decay. In addition, we monitored the drosophilid fauna with cup traps baited with apple cider vinegar, as we expected that D. suzukii would outnumber and potentially outcompete native controphics, especially other drosophilids. Egg deposits of D. suzukii were found on the fruits of 31 of the 39 potential host plant species studied, with 18 species showing an attack rate > 50%. Overall, fruits of Cotoneaster divaricatus (96%), Atropa bella-donna (91%), Rubus fruticosus corylifolius aggr. (91%), Frangula alnus (85%) and Sambucus nigra (83%) were attacked particularly frequently, resulting also in high predicted attack probabilities that varied among forest communities. Later and longer fruiting, black fruit colour, larger fruit size and higher pulp pH all positively affected attack rates. More than 50% of the plant species showed severe symptoms of decay after egg deposition, with higher pulp sugar content leading to more severe symptoms. The high fruit attack rate observed was reflected in a high abundance and dominance of D. suzukii in trap catches, independent of forest community and elevation. Drosophila suzukii was by far the most abundant species, accounting for 86% (81,395 individuals) of all drosophilids. The abundance of D. suzukii was negatively associated with the abundance of the native drosophilids. Our results indicate that the invasive D. suzukii competes strongly with other frugivorous species and that its presence might have far-reaching ecosystem-level consequences. The rapid decay of fruits attacked by D. suzukii leads to a loss of resources and may disrupt seed-dispersal mutualisms through the reduced consumption of fruits by dispersers such as birds.

Molecular and behavioral studies reveal differences in olfaction between winter and summer morphs of Drosophila suzukii

Spotted-wing drosophila, Drosophila suzukii (Matsumura), is a major economic pest of several fruit crops in Europe, North and South America, and other parts of the world because it oviposits in ripening thin-skinned fruits. This vinegar fly exhibits two distinct morphotypes: a summer and a winter morph. Although adaptations associated with the winter morph enhance this invasive pest's capacity to survive in cold climates, winter is still a natural population bottleneck. Since monitoring early spring populations is important for accurate population forecasts, understanding the winter morph's response to olfactory cues may improve current D. suzukii management programs. In this study, a comparative transcriptome analysis was conducted to assess gene expression differences between the female heads of the two D. suzukii morphs, which showed significant differences in 738 genes (p ≤ 0.0001). Out of twelve genes related to olfaction determined to be differentially expressed in the transcriptome, i.e., those related to location of food sources, chemosensory abilities, and mating behavior, nine genes were upregulated in the winter morph while three were downregulated. Three candidate olfactory-related genes that were most upregulated or downregulated in the winter morph were further validated using RT-qPCR. In addition, behavioral assays were performed at a range of temperatures to confirm a differing behavioral response of the two morphs to food odors. Our behavioral assays showed that, although winter morphs were more active at lower temperatures, the summer morphs were generally more attracted to food odors. This study provides new insights into the molecular and behavioral differences in response to olfactory cues between the two D. suzukii morphs that will assist in formulating more effective monitoring and physiological-based control tools.

Field and greenhouse application of an attract-and-kill formulation based on the yeast Hanseniaspora uvarum and the insecticide spinosad to control Drosophila suzukii in grapes

BACKGROUND

The invasive insect Drosophila suzukii (Matsumura) is an important pest of several red grape varieties. The yeast Hanseniaspora uvarum (Niehaus), which is associated with D. suzukii, strongly attracts flies and stimulates them to feed on yeast-laden food. In the present study, a formulation based on H. uvarum culture with spinosad insecticide was applied to the foliage of vineyards and control of D. suzukii was compared to applying spinosad to the whole plant. After successful H. uvarum and insecticide application in the vineyard, we tested additional H. uvarum-based formulations with spinosad in a greenhouse to determine their capacity to control D. suzukii.

RESULTS

Application of the H. uvarum-spinosad formulation at 36.4 g of spinosad per hectare reduced the D. suzukii field infestation at the same rate as applying 120 g of spinosad per hectare and prevented spinosad residues on grapes. Leaves treated with H. uvarum and spinosad in the field and transferred to a laboratory assay caused high mortality to flies and reduced the number of eggs laid on fruits. Formulations with spinosad applied in the greenhouse showed that both H. uvarum culture and the yeast cell-free supernatant of a centrifuged culture increased fly mortality and reduced the number of eggs laid compared to the unsprayed control.

CONCLUSION

In comparison to typical spinosad spray applications, the use of H. uvarum in combination with spinosad as an attract-and-kill formulation against D. suzukii reduces pesticide residues on the fruits by targeting the treatment to the canopy and decreasing the amount of insecticide per hectare without compromising control efficacy.

Mass Trapping Drosophila suzukii, What Would It Take? A Two-Year Field Study on Trap Interference

Simple Summary

Drosophila suzukii is an invasive fruit fly that have became a key pest of soft-skinned fruits during the past decade. Today, the control of this pest relies strongly on broad-spectrum insecticides. Deploying attractive traps to control the pest population (mass trapping) could be part of the management strategy of D. suzukii. The present study analyses whether mass trapping with different attractants could be viable for D. suzukii control and how far traps should be maximally spaced in a grid. Traps in a grid compete for the same insects when they are spaced close enough and their radii of attraction overlap. Since the traps on the corners of a grid have fewer competing traps around than fully surrounded centre traps, the ratio of the catches in the corner traps and the centre traps indicates whether the traps are spaced close enough. By quantifying that trap interference in 4 × 4 trapping grids, it was found in this two-year field study that workable trap densities can be expected to control D. suzukii. From June onwards, synthetic lures in dry traps show equal or better results than the same traps with a reference liquid bait (apple cider vinegar).

Abstract

The invasion of Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) worldwide has disrupted existing or developing integrated pest management (IPM) programs in soft-skinned fruits. Currently, with a reliance on only broad-spectrum insecticides, there is a critical call for alternative control measures. Behavioural control is one of the pillars of IPM, and, in the present study, it is investigated whether mass trapping could be viable for D. suzukii management. By quantifying trap interference in 4 × 4 replicate trapping grids, an estimate of the attraction radius for a certain attractant and context can be obtained. Traps designed for dry trapping (no drowning solution, but a killing agent inside) and synthetic controlled released experimental lures were tested in a two-year field study. Apple cider vinegar (ACV) was included as a reference bait and trials were performed with 5, 10 and 15 m inter-trap spacings at different seasonal timings. Clear trap interference and, hence, overlapping attraction radii were observed both in spring and summer for both the synthetic lures and ACV. In early spring, ACV shows the most potential for mass trapping, however from June onwards, the experimental dry lures show equal or better results than ACV. Based on our findings, workable trap densities are deemed possible, encouraging further development of mass trapping strategies for the control of D. suzukii.

Drosophila suzukii (Diptera: Drosophilidae): A Decade of Research Towards a Sustainable Integrated Pest Management Program

Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) also known as spotted-wing drosophila (SWD), is a pest native to Southeast Asia. In the last few decades, the pest has expanded its range to affect all major European and American fruit production regions. SWD is a highly adaptive insect that is able to disperse, survive, and flourish under a range of environmental conditions. Infestation by SWD generates both direct and indirect economic impacts through yield losses, shorter shelf life of infested fruit, and increased production costs. Fresh markets, frozen berries, and fruit export programs have been impacted by the pest due to zero tolerance for fruit infestation. As SWD control programs rely heavily on insecticides, exceedance of maximum residue levels (MRLs) has also resulted in crop rejections. The economic impact of SWD has been particularly severe for organic operations, mainly due to the limited availability of effective insecticides. Integrated pest management (IPM) of SWD could significantly reduce chemical inputs but would require substantial changes to horticultural management practices. This review evaluates the most promising methods studied as part of an IPM strategy against SWD across the world. For each of the considered techniques, the effectiveness, impact, sustainability, and stage of development are discussed.

Global analysis of the seasonal abundance of the invasive pest Drosophila suzukii reveal temperature extremes determine population activity potential

BACKGROUND

The global pest spotted winged drosophila (Drosophila suzukii) continues to have a significant economic impact on fruit production in areas where it is established, in addition to newly invaded ranges. Management activities spanning national biosecurity responses to farm-scale pest control are limited by the inability to predict the timing and severity of seasonal outbreaks of D. suzukii and its climatic drivers.

RESULTS

Here, we compiled and analysed data on international seasonal abundances for D. suzukii under different climates, crop types and management contexts to improve the predictability of seasonal population dynamics. In relating seasonal abundances to environmental predictors, specifically temperature, we found strong negative effects of exposure to high and low temperatures during the preceding month. Unlike most regional studies on D. suzukii phenology that focus on temperature in the physiological development range, we show that thermal extremes better explain seasonal population fluctuations.

CONCLUSION

Although trap catches remain an indirect measure of infestations and must be interpreted carefully in terms of crop risk, our results should support monitoring programmes through enhanced knowledge of the climatic factors affecting D. suzukii population activity. The negative impact of high temperatures suggests that late-season management strategies focusing on manipulating crop microclimates to temperatures above 25 °C can reduce D. suzukii abundance. We show that early season abundance is modulated by climate, particularly the depth of cold extremes experienced in the preceding time interval. These associations may be further developed into early-season crop risk forecasts to support monitoring programs. © 2021 Society of Chemical Industry.

Optimization of a Larval Sampling Method for Monitoring Drosophila suzukii (Diptera: Drosophilidae) in Blueberries

Managing spotted-wing drosophila, Drosophila suzukii (Matsumura), in fruit crops is complicated by the unreliability of currently available traps for monitoring adult flies, combined with the difficulty of detecting larval infestation before fruit damage is apparent. A simple method to extract larvae from fruit in liquid, strain the solution, then count them in a coffee filter was developed recently for use in integrated pest management programs. Here, we present a series of experiments conducted to improve fruit sampling by making it faster, less expensive, and more accurate. The volume of blueberries sampled (59-473 ml) did not significantly affect the detection of second and third instars, but we found that 118-ml samples were best for detecting the smallest larvae. These small instars were more detectable when berries were lightly squeezed before immersion, whereas larger instars were similarly detectable without using this step. We also found that immersing fruit for 30 min was sufficient before counting larvae, and similar numbers of larvae were found in the filter using room temperature water rather than a salt solution. The process of filtering, detection, and counting larvae took only 2-4 min per sample to process, depending on larval density. Using a microscope to count the larvae was consistently the best approach for detecting D. suzukii larvae. Based on these results, we discuss how fruit sampling can be streamlined within IPM programs, so growers and their advisors can improve control and reduce the cost of monitoring this invasive pest.

Having it all: hybridizing conventional and community science monitoring for enhanced data quality and cost savings

Large-scale monitoring is used to track population trends for many ecologically and economically important wildlife species. Often, population monitoring involves professional staff travelling to collect data (i.e., conventional monitoring) or in efforts to reduce monitoring costs, by engaging volunteers (i.e., community science). Although many studies have discussed the advantages and disadvantages of conventional vs. community science monitoring, few have made direct, quantitative comparisons between these two approaches. We compared data quality and financial costs between contemporaneous and overlapping conventional and community science programs for monitoring a major forest pest, the spruce budworm ( Choristoneura fumiferanae Clem.). Although community science trapping sites were clumped around urban areas, abundance estimates from the programs were strongly spatially correlated. However, annual program expenditures were nearly four times lower in the community science versus the conventional program. We modelled a hypothetical hybrid model of the two programs, which provided full spatial coverage and potentially the same data, but at half the cost of the conventional program and with the added opportunity for public engagement. Our study provides a unique quantitative analysis of merits and costs of conventional versus community science monitoring. Our study offers insights on how to assess wildlife monitoring programs where multiple approaches exist.

Search for Alternative Control Strategies of Drosophila suzukii (Diptera: Drosophilidae): Laboratory Assays Using Volatile Natural Plant Compounds

Simple Summary

Adult flies of the invasive fruit pest, Drosophila suzukii, commonly known as spotted wing drosophila, showed susceptibility towards several natural plant products tested in bioassays under laboratory conditions. Depending on the compound tested, contact toxicity, modified food uptake, or reduction in numbers of eggs deposited and hatched was found. The natural plant substances thereby identified will be further assessed under field conditions and can be used to develop innovative pest control strategies.

Abstract

Drosophila suzukii (Diptera: Drosophilidae), is native to southeastern Asia and invaded Europe during the past decade. It causes serious economic damage in cherries and soft fruits. Control strategies rely on few insecticides with varying success. Due to environmental concern, the use of synthetic chemicals is restricted. Therefore, research effort is put into the quest for alternative substances applicable in chemical pest control. In laboratory assays, we tested 17 volatile plant compounds from different chemical classes for their contact toxicity, feeding modification, and oviposition repellency. Toxicity through contact with treated surfaces was evaluated after 1 h, 4 h, and 24 h; effects on food uptake were observed with capillary feeding (CAFE)—tests and oviposition trials compared egg numbers laid in raspberry medium with or without treated filter paper. Cinnamon oil and its components had the highest contact toxicity with an LC₉₀ = 2–3%, whereas lemongrass oil, its main components, and farnesol were less toxic (LC₉₀ = 7–9%), and geraniol was the least toxic. In CAFE tests, feeding stimulation was observed through 0.1% and 1% solutions of citronellol, lemongrass oil and farnesol. Cinnamon oil, cinnamaldhyde, and ethyl cinnamate were not consumed at a concentration of 1%. In the presence of citral, eugenol, and lemongrass oil, oviposition was reduced, and in the presence of limonene, no eggs were deposited. The natural products found most efficient in either bioassay will be further tested under field conditions.

Tunnel entries and a killing agent uncover the importance of fly retention in Drosophila suzukii traps

BACKGROUND

Spotted wing drosophila (SWD), Drosophila suzukii (Matsumura) (Diptera: Drosophilidae), is a pest of stone and small fruits causing considerable economic losses. Current management strategies rely primarily on calendar-based spraying, owing to the poor relationship between monitoring data and damage levels, and the lack of success of mass-trapping tools. The aim of this study was to evaluate different trap models for SWD, with an emphasis on their fly-retention capacity. To this end, we examined and quantified the added value of two fly-retaining trap features; tunnel entries to impede escape and an insecticide-coated inner surface as a killing agent.

RESULTS

An insecticide-coated inner surface resulted in significantly higher trap retention after 24 h in the laboratory (4.9- to 7.4-fold greater, depending on trap type) compared to a noncoated trap. Trapping efficacy was significantly improved in field trials by such a killing agent in the trap (1.2- to 4.5-fold greater). Tunnel entries significantly improved trap retention in the laboratory and field (by 1.5-fold).

CONCLUSION

The outcomes of this study clearly reveal the substantial impact of the fly-retention capacity of SWD traps on their overall capture performances. It was demonstrated for the first time that an insecticide-coated inner surface as a killing agent significantly improves trap efficacy for SWD. This finding can readily be implemented in any trap model to improve monitoring and mass trapping of SWD. Also tunnel entries were shown to have a significant influence on the fly retention and, hence, substantially enhance trapping efficacy.

Population Dynamics of Drosophila suzukii in Coastal and Mainland Sweet Cherry Orchards of Greece

Simple Summary

Drosophila suzukii or spotted-wing drosophila is a fruit fly of the same family as the vinegar fly. The majority of Drosophila fruit flies are not considered agricultural pests. However, D. suzukii is a pest of several high-value soft and thin skin fruits such as strawberries and sweet cherries. Drosophila suzukii is also considered worldwide as one of the most important invasive insect pests. We monitored the insect’s flight and we assessed the infestation levels of different sweet-cherry cultivars, in coastal and mainland cherry orchards of Greece, for two consecutive years (2018–2020). Adults were captured throughout the year in the coastal area with two peaks in spring and late-autumn. Captures were nearly zero during the hot summer months. Trap captures exhibited only one peak in autumn at the mainland area, and ceased during winter and spring. Higher sweet-cherry infestation levels were recorded in the coastal than in the mainland area and in unmanaged than in commercial orchards. Both early and late-ripening cultivars were highly susceptible to D. suzukii infestation in the coastal area. Infestation rates were higher in late-ripening cultivars than in early-ripening ones in the mainland area. We conclude that D. suzukii has adapted well to the Mediterranean climate of Greece, and is able to progressively exploit several crops and wild hosts of mainland and coastal areas.

Abstract

Despite the recent invasion and wide spread of Drosophila suzukii Matsumura (Diptera: Drosophilidae) in Europe, little is known regarding its population trends in coastal areas of the southern Mediterranean countries. Using adult trapping and fruit sampling, we studied the population dynamics of D. suzukii in coastal and mainland (semi-highland) cherry orchards of Greece, from 2018 to 2020. Adults were captured in traps baited with apple cider vinegar, placed in conventional and unmanaged sweet-cherry orchards, and in neighbouring wild growing hosts. Sampling of sweet-cherry fruit to assess infestation levels was conducted from early and late-ripening cherry cultivars in both areas. Adults were captured throughout the year in the coastal area with two peaks registered in spring and late-autumn. Captures were nearly zero during the hot summer months. Flight activity exhibited only one peak in autumn at the mainland area, and ceased during winter and spring. Captures in wild hosts were lower during the sweet-cherry ripening period than later in the season. Higher sweet-cherry infestation levels were recorded in the coastal than in the mainland area and in unmanaged than in commercial orchards. Both early and late-ripening cultivars were highly susceptible to D. suzukii infestation in the coastal area. Infestation rates were higher in late-ripening cultivars than in early-ripening ones in the mainland area. We conclude that D. suzukii has well adapted to the Mediterranean climate of Greece, and is able to progressively exploit several crops and wild hosts of mainland and coastal areas.

Agricultural-Grade Apple Cider Vinegar Is Remarkably Attractive to Drosophila suzukii (Diptera: Drosophiliadae) in Mexico

Due to its availability and low cost, apple cider vinegar (ACV) is a frequently used as an attractant for monitoring the invasive spotted wing drosophila, Drosophila suzukii. In laboratory cage experiments, the attraction of ACV alone was compared with ACV in mixtures with different concentrations of acetic acid, propionic acid, different hydrolyzed proteins, synthetic fruit flavors (strawberry, blackberry and apple) and the addition of fruit nectars (grape, pineapple and apple). The addition of 5% apple nectar to ACV significantly increased fly captures, whereas other combinations were similar to or less attractive than ACV alone. Apple flavored vinegar was not attractive to flies. Captures did not vary significantly among the brands of ACV commonly sold in Mexico, except for one poorly-performing brand, but cup traps baited with an agricultural-grade ACV unfit for human consumption captured approximately two-fold more flies than the commercial attractants Suzukii Trap, Suzukii Trap Max Captures or ACV alone in cage experiments. Field trials performed in polytunnels planted with raspberry crops in Mexico resulted in two-fold to ten-fold higher numbers of D. suzukii captured by the agricultural-grade ACV compared to Droskidrink (a mixture of ACV, red wine and sugar), Suzukii Trap, Suzukii Trap Max Captures or edible grade ACV alone. The species selectivity of the agricultural grade ACV was similar to that of other attractants tested. Agricultural-grade ACV also captured higher numbers of female than male flies in field trials. We conclude that the remarkably high attractiveness and low cost of agricultural-grade ACV makes it a useful tool for monitoring D. suzukii populations in berry crops.

Marking Drosophila suzukii (Diptera: Drosophilidae) with Fluorescent Dusts

The marking of Drosophila suzukii can be an important instrument for studying the ecology and behaviour of this economically important fruit pest, aiding the development of new Integrated Pest Management (IPM) tools or strategies. There is, however, a need for a cost-effective methodology that provides an easily detectable and stable mark. Whereas fluorescent pigment powders are often used in entomological research, the pigments (series, dyes), application techniques, or doses need to be evaluated for each studied species in terms of their efficacy and possible adverse effects on the performance of the insect. The effectiveness of different application techniques and dyes (RadGlo® TP-series) and their effect on the survival of adult D. suzukii were investigated in the laboratory. Furthermore, the influence of the marking on the behaviour of the flies was examined in laboratory trap assays (olfaction) and a field recapture study (general orientation). The persistence and detectability of the marks was evaluated both on living flies (for different application techniques) and dead flies under trapping/storage conditions. The use of fluorescent powders to mark D. suzukii flies yielded a clearly detectable and highly persistent mark, without any adverse effects on the survival and behaviour of the flies.

Morphometric criteria to differentiate Drosophila suzukii (Diptera: Drosophilidae) seasonal morphs

Temperate insect species often enter diapause in preparation for overwintering. One such species is the invasive vinegar fly, Drosophila suzukii (Matsumura), which has seasonal polymorphisms, considered winter and summer morphs. To date, the morphs have been differentiated by color and size with winter morphs typically being darker and larger compared to summer morphs. ‘Dark’ and ‘large’ are subjective, however, and standardizing an identification process can ensure that the morph of interest is being accurately characterized. The goal of our research was to investigate a quantitative method to distinguish between D. suzukii morphs based on body and wing size. We reared winter and summer morph D. suzukii in the laboratory using standard procedures, and measured wing length, wing width, and hind tibia length. Additionally, we collected field D. suzukii to document the seasonal phenology of the morphs in Minnesota based on our model’s cutoff criteria. A classification and regression tree analysis were used to determine which metrics would be best for predicting field-caught D. suzukii morphs. Using laboratory-reared flies as our known morphs for the training data in the classification model we developed classification trees based on wing length and the ratio of wing length to hind tibia length. The frequency of winter and summer morphs present in the field varied based on which classification tree was used. Nevertheless, we suggest ratio of wing length to hind tibia length as the most robust criteria for differentiating D. suzukii morphs because the ratio accounts for the size variability between laboratory-reared and field-caught flies and the error rate of misclassification is reduced to 0.01 for males. The results from this work can aid in future D. suzukii research by allowing scientists to objectively differentiate the morphs, and thereby improve our understanding of the biology and phenology of seasonal morph dynamics.

Mass-Rearing of Drosophila suzukii for Sterile Insect Technique Application: Evaluation of Two Oviposition Systems

Drosophila suzukii (Diptera: Drosophilidae) is an invasive pest of a wide range of commercial soft-skinned fruits. To date, most management tactics are based on spraying of conventional and/or organic insecticides, baited traps, and netting exclusion. Interest has been expressed in using the sterile insect technique (SIT) as part of area-wide integrated pest management (AW-IPM) programs to control D. suzukii infestations. Mass-rearing protocols are one of the prerequisites for successful implementation of the SIT. To establish mass-rearing methods for this species, two different egg-collection systems were developed and compared with respect to the number of eggs produced, egg viability, pupa and adult recovery, adult emergence rate, and flight ability. Female flies kept in cages equipped with a wax panel produced significantly more eggs with higher viability and adult emergence rate, as compared to the netted oviposition system. The wax panel system was also more practical and less laborious regarding the collection of eggs. Furthermore, the wax panel oviposition system can be adapted to any size or design of an adult cage. In conclusion, this system bears great promise as an effective system for the mass production of D. suzukii for SIT.

Temporal Dynamics of Host Use by Drosophila suzukii in California's San Joaquin Valley: Implications for Area-Wide Pest Management

A major challenge to the area-wide management of Drosophila suzukii is understanding the fly’s host use and temporal dynamics, which may dictate local movement patterns. We determined D. suzukii’s seasonal host use in California’s San Joaquin Valley by sampling common crop and non-crop fruits in a temporal sequence of fruit ripening. We then evaluated the suitability of selected fruits as hosts. Drosophila suzukii emerged from both intact and damaged cherries during the cooler, early season period. Fly density remained low through the hot spring–summer period and re-surged as temperatures lowered in fall when the fly did not cause damage to intact peach, nectarine, plum, pear, grape, pomegranate, apple, persimmon and citrus (in order of ripening) but did emerge from the damaged fruits of these crops. The fly also emerged from two ornamental fruits (loquats and cactus) but was not found on wild plum and two endemic wild fruits (buckthorn and bitter berry). Drosophila suzukii completed development (egg to adult) on cactus, mandarin carpel, pomegranate seed, wild plum and buckthorn at survival rates similar to cherry (51.2–68.8%), whereas it had a lower survival rate on bitter cherry (33.2%), table grape (31.5%), raisin grape (26.5%), and wine grape (4.5%). The high acidity levels of grapes negatively affected the fly’s fitness. Among 10 cherry cultivars, survival rate was not affected by sugar content, but it decreased with increasing egg density per gram of fruit. Results suggest that in California’s San Joaquin Valley, the early season crops are most vulnerable, summer fruits ripen during a period of low pest pressure, and late season fruits, when damaged, serve to sustain D. suzukii’s populations in this region.

Population Dynamics of Spotted Wing Drosophila (Drosophila suzukii (Matsumura)) in Maine Wild Blueberry (Vaccinium angustifolium Aiton)

A long-term investigation of D. suzukii dynamics in wild blueberry fields from 2012–2018 demonstrates relative abundance is either still increasing or exhibiting periodicity seven years after the initial invasion. Relative abundance is determined by physiological date of first detection and air temperatures the previous winter. Date of first detection of flies does not determine date of fruit infestation. The level of fruit infestation is determined by year, fly pressure, and insecticide application frequency. Frequency of insecticide application is determined by production system. Non-crop wild fruit and predation influences fly pressure; increased wild fruit abundance results in increased fly pressure. Increased predation rate reduces fly pressure, but only at high abundance of flies, or when high levels of wild fruit are present along field edges. Male sex ratio might be declining over the seven years. Action thresholds were developed from samples of 92 fields from 2012–2017 that related cumulative adult male trap capture to the following week likelihood of fruit infestation. A two-parameter gamma density function describing this probability was used to develop a risk-based gradient action threshold system. The action thresholds were validated from 2016–2018 in 35 fields and were shown to work well in two of three years (2016 and 2017).

Olfactory Preference of Drosophila suzukii Shifts between Fruit and Fermentation Cues over the Season: Effects of Physiological Status

Worldwide monitoring programs of the invasive fruit pest Drosophila suzukii Matsumura (Diptera: Drosophilidae), using fermentation baits like apple cider vinegar (ACV), revealed a counterintuitive period of low trap catches during summer, followed by an autumn peak. In this study, we demonstrate that ACV baited traps indeed provide a distorted image of the D. suzukii population dynamics as it is possible to capture higher numbers during this "low capture period" with synthetic lures. It was hypothesised that the preference of D. suzukii populations for fermentation cues like ACV is most pronounced during autumn, winter and spring, while the flies prefer fresh fruit cues during summer and that this seasonal preference is related to the changing physiology of the flies over the season. To test this hypothesis, the preference between fermentation cues (ACV) and host fruits (strawberries) and the effect of physiology (sex, seasonal morphology and feeding, mating and reproductive status) was investigated both in olfactometer laboratory experiments and a year-round field preference experiment. In olfactometer experiments we demonstrated that protein deprived females, virgin females with a full complement of unfertilised eggs and males show a strong preference for fermentation cues while fully fed reproductive summer morph females generally prefer fruit cues. These findings indicate that D. suzukii is attracted to fermentation volatiles in search of (protein-rich) food and to fruit volatiles in search of oviposition substrates. Winter morph and starved females displayed indiscriminating olfactory behaviour. In the field preference experiment, the hypothesised seasonal shift between fermentation and fruit cues was confirmed. This shift appeared to be highly temperature-related and was similarly observed for summer and winter morphs.