Heidrun Vogt

In Focus: Spotted wing drosophila, Drosophila suzukii, across perspectives

In August 2008, the first detection of the spotted wing drosophila, Drosophila suzukii, to the North America mainland in California caused great concern, as the fly was found infesting a variety of commercial fruits. Subsequent detections followed in Oregon, Washington, Florida and British Columbia in 2009; in Utah, North Carolina, South Carolina, Michigan, and Louisiana in 2010; and in Virginia, Montana, Wisconsin, Pennsylvania, New Jersey, Maryland and Mexico in 2011. In Europe, it has been detected in Italy and Spain in 2009 and in France in 2010. Economic costs to the grower from D. suzukii include the increased cost of production (increased labor and materials for chemical inputs, monitoring and other management tools) and crop loss. An effective response to the invasion of D. suzukii requires proper taxonomic identification at the initial phase, understanding basic biology and phenology, developing management tools, transferring information and technology quickly to user groups, and evaluating the impact of the research and extension program on an economic, social, and environmental level. As D. suzukii continues to expand its range, steps must be initiated in each new region to educate and inform the public as well as formulate management tactics suitable for the crops and growing conditions in each.

Comparison of a Synthetic Chemical Lure and Standard Fermented Baits for Trapping Drosophila suzukii (Diptera: Drosophilidae)

ABSTRACT We determined the attractiveness of a new chemical lure compared with fermented food baits in use for trapping Drosophila suzukii Matsumura, spotted wing drosophila (Diptera: Drosophilidae), in Connecticut, New York, and Washington in the United States and at Dossenheim in Germany. The chemical lure (SWD lure) and food baits were compared in two types of traps: the dome trap and a cup trap. Regardless of trap type, numbers of male and female D. suzukii trapped were greater with the SWD lure compared with apple cider vinegar (ACV) baits at the Washington and New York sites, and were comparable with numbers of D. suzukii captured with a wine plus vinegar bait (W + V) at Germany site and a combination bait meant to mimic W + V at the Connecticut site. Averaged over both types of attractants, the numbers of D. suzukii captured were greater in dome traps than in cup traps in New York and Connecticut for both male and female D. suzukii and in Washington for male D. suzukii. No such differences were found between trap types at the Washington site for female and Germany for male and female D. suzukii. Assessments were also made of the number of large (>0.5cm) and small (<0.5cm) nontarget flies trapped. The SWD lure captured fewer nontarget small flies and more large flies compared with ACV bait in New York and fewer nontarget small flies compared with W + V in Germany, although no such differences were found in Washington for the SWD lure versus ACV bait and in Connecticut for the SWD lure versus the combination bait, indicating that these effects are likely influenced by the local nontarget insect community active at the time of trapping. In New York, Connecticut, and Germany, dome traps caught more nontarget flies compared with cup traps. Our results suggest that the four-component SWD chemical lure is an effective attractant for D. suzukii and could be used in place of fermented food-type baits.

An invader supported by a parasite: Mistletoe berries as a host for food and reproduction of Spotted Wing Drosophila in early spring

The Spotted Wing Drosophila (SWD), Drosophila suzukii Matsumura, an invasive pest species in Europe and the Americas, is able to feed and reproduce on numerous fruit crops and a wide range of wild host plants. SWD is thought to overwinter outside of agricultural fields in forests and hedges. To identify overwintering sites and early spring oviposition hosts, traps were installed in forests. In spring 2015, traps in the canopy of pine trees parasitized by mistletoe, Viscum album subsp. laxum, captured significantly more SWD than traps in pine trees without mistletoe. We found SWD females with ripe eggs coinciding with ripening and ripe mistletoe berries. We investigated whether mistletoe may serve as a host for SWD. Under laboratory conditions, SWD developed from egg to adult in mistletoe berries. More adults emerged from wounded berries. Females were observed to feed on berries and survived up to eight days without other food. A few adults emerged from wild mistletoe berries. To understand the attraction of SWD to parasitized trees, we analyzed the volatile organic compounds (VOCs) collected from the headspace of mistletoe berries by GC–MS and identified the main components. Thirty-two VOCs were found. Wounded and unwounded berries differed significantly in the quantity of 11 VOCs emitted. The odor spectrum showed many similarities to other typical berry odors. The combination of field surveys and laboratory assays identified a new reproduction host for SWD in spring. This host plant may help SWD to withstand the bottleneck period for survival in winter and spring.

Multiple Wolbachia infections in Rhagoletis pomonella

Rhagoletis pomonella Walsh (Diptera: Tephritidae) is a model species for sympatric speciation through host race formation on apple and hawthorn. The bacterial endosymbiont Wolbachia, a manipulator of arthropod reproduction, has been considered to contribute to speciation in several species. A potential role of Wolbachia in sympatric speciation of R. pomonella remains to be tested despite an earlier detection by PCR. In this study, we isolated Wolbachia from R. pomonella individuals from both host species using multi‐locus sequence typing (MLST) and the surface protein wsp. By cloning and sequencing of 311 plasmids, we found sequence types of at least four wPom strains. A complete MLST profile was obtained only for wPom1, whereas MLST loci of the other putative strains were difficult to assign because of multiple infections and low sample numbers. wPom1 occurs in both host races, whereas different sequence types were found at low frequencies only in apple‐infesting R. pomonella. This warrants further investigation as it cannot be excluded that Wolbachia plays a part in this model of sympatric speciation.

Substantial progress made in the rearing of the European cherry fruit fly, Rhagoletis cerasi

Successful and reproducible rearing of Rhagoletis cerasi L. (Diptera: Tephritidae) is imperative for substantiated research on this main pest in cherries in Europe, for instance for the applied research on control of the pest, as well as for basic research on its physiology, behaviour, or reproduction. Temperate Rhagoletis species with an obligatory diapause are more difficult to rear in the laboratory than most of the tropical or subtropical fruit fly species (Boller, 1989). The host range of the monoor oligophagous Rhagoletis species comprises a few main host plants, for R. cerasi these are sweet cherry [Prunus avium L. (Rosaceae)], tart cherry (Prunus cerasus L.), and fly honeysuckle [Lonicera xylosteum L. (Caprifoliaceae); Boller & Bush, 1974]. Rearing of Rhagoletis spp. included an adequate oviposition device and a suitable larval diet. In previous studies, Boller and Haisch et al. worked intensively on both aspects for R. cerasi. Boller and Prokopy developed black-dyed ceresin wax domes for oviposition (Prokopy & Boller, 1970, 1971; Boller & Ramser, 1971; Katsoyannos et al., 1977; Boller, 1984). However, this wax is no longer available. Hence, a new wax with similar or improved features had to be found. Boller (1984) and Haisch (1975) described different larval diets based mainly on brewer’s yeast, wheat germ, sugar, organic acids, and vitamins. Both authors reported pupation rates between 40 and 46%, but no reports on repeated or continuous laboratory rearing were published until now. In the experiments reported here, we could not reproduce these pupation rates. Furthermore, a Rhagoletis diet is sold by Bio-Serv (Frenchtown, NJ, USA) without information on its efficacy. For decades, the lack of a reproducible rearing method has hampered progress in R. cerasi research. The urgent need of applied research on R. cerasi becomes increasingly obvious because of the ongoing re-evaluation of pesticide usage in the EU. New and environmentally sound control strategies of this pest rely on the knowledge ascertained from basic research, wherefore successful rearing is a prerequisite. Based on present rearing knowledge, a reproducible rearing method was developed with an alternative for the formerly used, but now unavailable, wax for egg devices, and with optimised larval diets. Adult emergence, oviposition, and larval development until pupation of R. cerasi were quantified in this study.

Invasion genetics of American cherry fruit fly in Europe and signals of hybridization with the European cherry fruit fly

The American cherry fruit fly is an invasive pest species in Europe, of serious concern in tart cherry production as well as for the potential to hybridize with the European cherry fruit fly, Rhagoletis cerasi L. (Diptera: Tephritidae), which might induce new pest dynamics. In the first European reports, the question arose whether only the eastern American cherry fruit fly, Rhagoletis cingulata (Loew) (Diptera: Tephritidae), is present, or also the closely related western American cherry fruit fly, Rhagoletis indifferens Curran. In this study, we investigate the species status of European populations by comparing these with populations of both American species from their native ranges, the invasion dynamics in German (first report in 1993) and Hungarian (first report in 2006) populations, and we test for signals of hybridization with the European cherry fruit fly. Although mtDNA sequence genealogy could not separate the two American species, cross‐species amplification of 14 microsatellite loci separated them with high probabilities (0.99–1.0) and provided evidence for R. cingulata in Europe. German and Hungarian R. cingulata populations differed significantly in microsatellite allele frequencies, mtDNA haplotype and wing pattern distributions, and both were genetically depauperate relative to North American populations. The diversity suggests independent founding events in Germany and Hungary. Within each country, R. cingulata displayed little or no structure in any trait, which agrees with rapid local range expansions. In cross‐species amplifications, signals of hybridization between R. cerasi and R. cingulata were found in 2% of R. cingulata individuals and in 3% of R. cerasi. All putative hybrids had R. cerasi mtDNA indicating that the original between‐species mating involved R. cerasi females and R. cingulata males.

Bait spray for control of European cherry fruit fly: an appraisal based on semi-field and field studies.

BACKGROUND The European cherry fruit fly, Rhagoletis cerasi, is the major insect pest of sweet and tart cherries. Its management is becoming increasingly difficult in many countries as formerly effective but broad-spectrum insecticides are removed from the market. With the objective of identifying suitable and environmentally safe alternatives, we investigated bait sprays containing two families of plant-derived insecticides: azadirachtins (NeemAzal-T(®) and NeemAzal-T/S(®) ) and pyrethrins (Spruzit Neu(®) ). RESULTS In 12 semi-field trials conducted within cages, weekly applications of 0.0001 or 0.0005% neem in a bait formulation effectively reduced fruit infestation. However, addition of 0.000125-0.001% pyrethrins did not improve the efficacy of the neem formulations, and when used alone pyrethrins were less effective than neem alone. Two years of field trials were also conducted within orchards wherein an insecticidal barrier of treated trees excluded immigration of fertile R. cerasi from elsewhere. In blocks treated with 0.0005% neem in a bait formulation, we observed 94% (2011) or 86% (2012) reduction of fruit infestation over control blocks. CONCLUSION Bait sprays containing neem are a promising alternative for the management of R. cerasi, especially where the risk of immigration of fertilized females is low, as in isolated orchards or as part of area-wide treatments.

Response of Drosophila suzukii (Diptera: Drosophilidae) to extreme heat and dryness

Drosophila suzukii Matsumura (Diptera) is a polyphagous herbivore native to East Asia that develops in cultivated and wild fruits. In 2011, it appeared in Germany. In 2012, economic damage was recorded and, in 2014, the harvest of stone and soft fruits was lost in some regions. By contrast, during 2015, populations remained lower. Record temperatures and dryness might have impeded population growth during that year. To test this hypothesis, flies were exposed to a 4‐day simulation. We evaluated the effect of fluctuating temperature and humidity on mortality and reproduction of D. suzukii from three age classes (average age: class 1: 3 days; class 2: 11 days; class 3: 20 days). Maximum temperatures were 27, 33 and 39 °C. Relative humidity oscillated between 18% and 85%. Fly mortality through heat stress ranged from 50% to 80%. Higher rates died of oldest flies and females. Offspring per female did not differ between heat stressed and control groups. Flies of both sexes were not sterilized through heat and dryness. Prior acclimation reduced any negative effects. We concluded that heat waves and extreme dryness during 2015 were one cause of the low population densities of D. suzukii observed under field conditions.

Validation of Individual-Based Markov-Like Stochastic Process Model of Insect Behavior and a “Virtual Farm” Concept for Enhancement of Site-Specific IPM

The paper reports application of a Markov-like stochastic process agent-based model and a “virtual farm” concept for enhancement of site-specific Integrated Pest Management. Conceptually, the model represents a “bottom-up ethological” approach and emulates behavior of the “primary IPM actors”—large cohorts of individual insects—within seasonally changing mosaics of spatiotemporally complex faming landscape, under the challenge of the local IPM actions. Algorithms of the proprietary PESTonFARM model were adjusted to reflect behavior and ecology of R. cerasi. Model parametrization was based on compiled published information about R. cerasi and the results of auxiliary on-farm experiments. The experiments were conducted on sweet cherry farms located in Austria, Germany, and Belgium. For each farm, a customized model-module was prepared, reflecting its spatiotemporal features. Historical data about pest monitoring, IPM treatments and fruit infestation were used to specify the model assumptions and calibrate it further. Finally, for each of the farms, virtual IPM experiments were simulated and the model-generated results were compared with the results of the real experiments conducted on the same farms. Implications of the findings for broader applicability of the model and the “virtual farm” approach—were discussed.

Wolbachia in Parasitoids Attacking Native European and Introduced Eastern Cherry Fruit Flies in Europe

Abstract The eastern cherry fruit fly, Rhagoletis cingulata Loew (Diptera: Tephritidae), is an economically important pest of cherries in North America. In 1983 it was first reported in Europe where it shares its ecological niche with the native European cherry fruit fly, Rhagoletis cerasi L. (Diptera: Tephritidae). Their coexistence in Europe led to the recent horizontal transmission of the Wolbachia strain wCer1 from R. cerasi to R. cingulata. Horizontal Wolbachia transmission is mediated by either sharing of ecological niches or by interacting species such as parasitoids. Here we describe for the first time that two braconid wasps, Psyttalia rhagoleticola Sachtleben (Hymenoptera: Braconidae) and Utetes magnus Fischer (Hymenoptera: Braconidae), naturally parasitizing R. cerasi, use the invasive R. cingulata in Europe as a new host. In contrast, no parasitoids that parasitize R. cingulata in its native American range were detected in the introduced European range. Diagnostic Wolbachia PCR screening and sequence analyses demonstrated that all P. rhagoleticola individuals were infected with the newly described Wolbachia strain wRha while all U. magnus individuals were uninfected. wRha is different from wCer1 but had an Wolbachia surface protein (wsp) gene sequence that was identical to wCer2 of R. cerasi and wCin2 of R. cingulata. However, multi locus sequence typing revealed differences in all loci between wRha and the tephritids strains. The horizontal transmission of wCer1 between the two tephritid species did not result in fixed heritable infections in the parasitoids. However, the parasitoids may have acted as a transient wCer1 vector.