Chemical Cues from Entomopathogenic Nematodes Vary Across Three Species with Different Foraging Strategies, Triggering Different Behavioral Responses in Prey and Competitors

Chemical cues play important roles in predator–prey interactions. Semiochemicals can aid predator foraging and alert prey organisms to the presence of predators. Previous work suggests that predator traits differentially influence prey behavior, however, empirical data on how prey organisms respond to chemical cues from predator species with different hunting strategies, and how foraging predators react to cues from potential competitors, is lacking. Furthermore, most research in this area has focused on aquatic and aboveground terrestrial systems, while interactions among belowground, soiling-dwelling organisms have received relatively little attention. Here, we assessed how chemical cues from three species of entomopathogenic nematodes (EPNs), each with a different foraging strategy, influenced herbivore (cucumber beetle) and natural enemy (EPN) foraging behavior. We predicted these cues could serve as chemical indicators of increased predation risk, prey availability, or competition. Our findings revealed that foraging cucumber beetle larvae avoided chemical cues from Heterorhabditis bacteriophora (active-foraging cruiser EPNs), but not Steinernema carpocapsae (ambusher EPNs) or Steinernema riobrave (intermediate-foraging EPNs). In contrast, foraging H. bacteriophora EPNs were attracted to cues produced by the two Steinernema species but not conspecific cues. Notably, the three EPN species produced distinct blends of olfactory cues, with only a few semi-conserved compounds across species. These results indicate that a belowground insect herbivore responds differently to chemical cues from different EPN species, with some EPN species avoiding prey detection. Moreover, the active-hunting EPNs were attracted to heterospecific cues, suggesting these cues indicate a greater probability of available prey, rather than strong interspecific competition.

Transcriptome Sequencing of the Striped Cucumber Beetle, Acalymma vittatum (F.), Reveals Numerous Sex-Specific Transcripts and Xenobiotic Detoxification Genes

Acalymma vittatum (F.), the striped cucumber beetle, is an important pest of cucurbit crops in the contintental United States, damaging plants through both direct feeding and vectoring of a bacterial wilt pathogen. Besides providing basic biological knowledge, biosequence data for A. vittatum would be useful towards the development of molecular biopesticides to complement existing population control methods. However, no such datasets currently exist. In this study, three biological replicates apiece of male and female adult insects were sequenced and assembled into a set of 630,139 transcripts (of which 232,899 exhibited hits to one or more sequences in NCBI NR). Quantitative analyses identified 2898 genes differentially expressed across the male–female divide, and qualitative analyses characterized the insect’s resistome, comprising the glutathione S-transferase, carboxylesterase, and cytochrome P450 monooxygenase families of xenobiotic detoxification genes. In summary, these data provide useful insights into genes associated with sex differentiation and this beetle’s innate genetic capacity to develop resistance to synthetic pesticides; furthermore, these genes may serve as useful targets for potential use in molecular-based biocontrol technologies.

Parasitoids, Nematodes, and Protists in Populations of Striped Cucumber Beetle (Coleoptera: Chrysomelidae)

The striped cucumber beetle, Acalymma vittatum (Fabricius), is an important pest of cucurbit production in the eastern United States, where most commercial producers rely on insecticides to control this pest species. Biological control provides an alternative to insecticide use, but for A. vittatum, top-down control has not been well developed. In the northeastern United States, two parasitoid species, Celatoria setosa (Coquillett) (Diptera: Tachinidae) and Centistes diabroticae (Gahan) (Hymenoptera: Braconidae) have been reported from A. vittatum, but their distribution is poorly known. To determine whether these parasitoid species are attacking A. vittatum in Pennsylvania and the amount of mortality they provide, we characterized the parasitoid dynamics in two distinct efforts. First, we reared parasitoids from beetles captured at two research farms. Second, we focused on one of these farms and dissected beetles to quantify both parasitoid and parasite species attacking A. vittatum. Both efforts confirmed Cl. setosa and Cn. diabroticae, and parasitism rates varied widely between locations and among years (4–60%). Unexpectedly, our dissections revealed that a potentially undescribed nematode species (Howardula sp.) as the most common parasite in the community. We also discovered gregarine protists. Despite being smaller than females, males were more commonly attacked by parasitic species, but we detected no relationship between the size of beetles and abundance of parasitic species in A. vittatum. This work provides a baseline understanding of the parasitoid and parasite community attacking A. vittatum and advances opportunities for conservation biological control using these natural-enemy species.

Tissue-specific volatile-mediated defense regulation in maize leaves and roots

SUMMARYPlant leaves that are exposed to herbivore induced plant volatiles (HIPVs) respond by increasing their defenses. Whether this phenomenon also occurs in the roots is unknown.Using maize (Zea mays), whose leaves respond strongly to leaf HIPVs, we measured the impact of root HIPVs, emanating from plants infested by the banded cucumber beetle (Diabrotica balteata), on constitutive and herbivore-induced levels of root soluble sugars, starch, total soluble proteins, free amino acids, volatile and non-volatile secondary metabolites, defense gene expression, growth and root herbivore resistance of neighboring plants.HIPV exposure did not alter constitutive or induced levels of any of the measured root traits. Furthermore, HIPV exposure did not reduce the performance and survival of banded cucumber beetle larvae on maize or teosinte. Cross-exposure experiments revealed that maize roots, in contrast to maize leaves, neither emit nor respond strongly to defense-regulating HIPVs.Together, these results demonstrate that volatile-mediated defense regulation is restricted to the leaves of maize and teosinte, a finding which is in line with the lower diffusibility of volatiles in the soil and the availability of other, potentially more efficient information conduits below ground.

Comparing Prophylactic Versus Threshold-Based Insecticide Programs for Striped Cucumber Beetle (Coleoptera: Chrysomelidae) Management in Watermelon

In cucurbit crops such as watermelon, implementation of integrated pest management (IPM) is important due to the high reliance on bees for fruit set, along with mounting evidence of the risks of insecticide use associated with pollinator health. Yet, IPM adoption, on-farm pesticide use behaviors, their costs, and impacts on the primary insect pest (striped cucumber beetle, Acalymma vittatum F.) are poorly known in one of the key watermelon-growing regions, the Midwestern United States. To better understand how to implement IPM into watermelon production, we assessed pest management practices on commercial watermelon farms using 30 field sites in Indiana and Illinois over 2 yr in 2017 and 2018. Across all sampling dates, beetles never crossed the economic threshold of five beetles/plant at any farm and most were maintained at densities far below this level (i.e., <1 beetle/plant). Moreover, we documented a wide range of insecticide inputs (mean ca. 5 applications per field per season; max. 10 applications) that were largely dominated by inexpensive foliar pyrethroid sprays; however, insecticide application frequency was poorly correlated with pest counts, suggesting that most of these applications were unnecessary. We calculated that the cost of the average insecticide program far exceeds the cost of scouting, and thus IPM is estimated to save growers ca. $1,000 per field under average conditions (i.e., field size, insecticide cost). These data strongly indicate that current management practices on commercial farms in the Midwest would benefit from implementing more threshold-based IPM programs with potential increases in both farm profitability and pollination services.

Bacterial communities of herbivores and pollinators that have co-evolved Cucurbita spp

Insects, like all animals, are exposed to diverse environmental microbes throughout their life cycle. Yet, we know little about variation in the microbial communities associated with the majority of wild, unmanaged insect species. Here, we use a 16S rRNA gene metabarcoding approach to characterize temporal and geographic variation in the gut bacterial communities of herbivores (Acalymma vittatum and A. trivittatum) and pollinators (Eucera (Peponapis) pruinosa) that have co-evolved with the plant genus Cucurbita (pumpkin, squash, zucchini and gourds). Overall, we find high variability in the composition of bacterial communities in squash bees and beetles collected from different geographic locations and different time points throughout a growing season. Still, many of the most common OTUs are shared in E. (P.) pruinosa, A. vittatum and A. trivittatum. This suggests these insects may be exposed to similar environmental microbial sources while foraging on the same genus of host plants, and that similar microbial taxa may aid in digestion of Cucurbita plant material. The striped cucumber beetle A. vittatum can also transmit Erwinia tracheiphila, the causal agent of bacterial wilt of cucurbits. We find that few field-collected A. vittatum individuals have detectable E. tracheiphila, and when this plant pathogen is detected, it comprises less than 1% of the gut bacterial community. Together, these results are consistent with previous studies showing that plant feeding insects have highly variable gut bacterial communities, and provides a first step towards understanding the spatiotemporal variation in the microbial communities associated with herbivores and pollinators that depend on Cucurbita host plants.

Insect Exclusion Screens Reduce Cucumber Beetle Infestations in High Tunnels, Increasing Cucurbit Yield

As high tunnel vegetable production acreage increases in the United States, so does the need for management strategies tailored to their unique growing environment. Cucumbers are an ideal crop in these systems; they can be vertically trellised to maximize the production area and provide high yields to balance the increased costs associated with high tunnel construction. One of the most limiting factors in cucurbit production in general is the cucumber beetle complex and the bacterial pathogen they transmit. In this study, we investigated the optimal size of netting installed on high tunnels to prevent cucumber beetle colonization while maintaining ventilation to reduce heat stress. Of the three mesh sizes investigated across 4 yr, the intermediate mesh with a pore size of 0.72 × 0.97 mm was optimal to exclude cucumber beetles, maintain ventilation, and produce the highest yields for both cucumber and melon plants. The smallest (0.16 mm2) and intermediate mesh sizes resulted in secondary pest outbreaks (e.g., aphids), which did not occur in open tunnels and to a lesser extent in tunnels covered with the largest (1.00 × 4.00 mm) mesh. Despite these secondary pests, yield was higher in small- and intermediate-sized mesh treatments due to relief from cucumber beetle infestations, including striped (Acalymma vittatum Fabr. (Coleoptera: Chrysomelidae)) and spotted (Diabrotica undecimpunctata howardi Barber (Coleoptera: Chrysomelidae)) beetles. Overall, we conclude that insect exclusion netting is an effective method to exclude cucumber beetles from high tunnels, but mesh size should be carefully considered when weighing the collective effects on yield and primary/secondary pest abundance.

Striped Cucumber Beetle Acalymma vittatum F. (Insecta: Coleoptera: Chrysomelidae)

The striped cucumber beetle, Acalymma vittatum F. (Figure 1) is a serious agricultural pest of plants in the family Cucurbitaceae in eastern North America. Crops affected by larval and adult feeding include cucumber, Cucumis sativus L., cantaloupe, Cucumis melo L., pumpkin, Cucurbita pepo L., and other Cucurbita spp. (Dill and Kirby 2016). The striped cucumber beetle is a vector of the plant disease bacterial wilt (Eaton 2016). Though the striped cucumber beetle occurs throughout Florida, it is the least commonly reported among three chrysomelid species on cucurbit crops in the state. The spotted cucumber beetle, Diabrotica undecimpunctata howardi Barber, and banded cucumber beetle, Diabrotica balteata LeConte, are more common in Florida, causing damage symptoms that are similar to striped cucumber feeding damage (Webb 2010). Includes: Introduction - Distribution - Description and Life Cycle - Damage - Monitoring - Management - Selected References.https://edis.ifas.ufl.edu/in1215 Also published at http://entnemdept.ufl.edu/creatures/VEG/BEAN/striped_cucumber_beetle.html

Muskmelon Cultivar Attractiveness to Striped Cucumber Beetle and Susceptibility to Bacterial Wilt

Bacterial wilt of cucurbits, incited by Erwinia tracheiphila (E. F. Smith) and vectored by the striped cucumber beetle [Acalymma vittatum (F.)] (SCB), is a serious disease of muskmelon (Cucumis melo L.). Cultivars differ in attractiveness to SCB and susceptibility to bacterial wilt, but no cultivar resistant to bacterial wilt has been introduced. In 2015 and 2016, replicated field plots of eight cultivars were grown at Lafayette, Wanatah, and Vincennes, IN, to identify differences in attractiveness to SCB and susceptibility to bacterial wilt. ‘Savor’ had significantly more beetle activity than ‘Hales Best’, ‘Superstar’, and ‘Aphrodite’ in three of six site-years, and more than ‘Diplomat’, ‘Dream Dew’, ‘Athena’, and ‘Wrangler’ in two site-years. Beetle activity for ‘Athena’, ‘Superstar’, and ‘Wrangler’ did not differ significantly from ‘Aphrodite’ for any site-year. Bacterial wilt severity was significantly greater for ‘Diplomat’ and ‘Dream Dew’ than for other cultivars in four site-years. ‘Superstar’ had the least disease in five site-years, but significantly less than ‘Aphrodite’, ‘Athena’, and ‘Hales Best’ in only one site-year. At one site, additional plots of each cultivar were populated with five SCBs per plant, and rowcovers were applied to keep the SCBs near the plants for 3 weeks. This resulted in similar beetle activity on all cultivars, but most disease in ‘Dream Dew’ and least in ‘Superstar’ and ‘Athena’. Marketable yield was generally highest for ‘Aphrodite’, ‘Superstar’, and ‘Athena’ when plants were exposed to natural beetle populations. Overall, ‘Savor’ and ‘Diplomat’ were the most attractive to beetles, and ‘Diplomat’ and ‘Dream Dew’ were the most susceptible to bacterial wilt. ‘Aphrodite’, ‘Athena’, and ‘Superstar’ were less attractive to beetles and showed more tolerance to bacterial wilt in both 2015 and 2016.