Habitat manipulation to mitigate the impacts of invasive arthropod pests

Resident Hymenopteran Parasitoids with Potential Drosophilid Associations in Andean North Patagonia: Implications for the Biological Control of the Spotted Wing Drosophila

The spotted wing drosophila, Drosophila suzukii Matsumura, is native to Southeast Asia and has become a serious pest of soft-skinned fruits worldwide. Several control methods are being tested worldwide as part of an integrated pest management approach. Biological control is a promising alternative tactic that can be used to manage D. suzukii populations, especially in unmanaged areas. This study aimed at exploring the diversity of resident hymenopteran parasitoids of drosophilids in northwestern Patagonia, where D. suzukii is considered an important pest. The survey also aimed to investigate possible associations between parasitoids and D. suzukii in several crops and non-crop fruits, and to determine D. suzukii fruits infestation levels. Fourteen sites with mainly berry crops were sampled biweekly using cider vinegar traps and collecting fresh fruits from a variety of crop and non-crop fruit plants. We identified five species of hymenopteran parasitoids, obtained from the baited traps, that have the potential to associate with D. suzukii: Leptopilina heterotoma Thomson, Ganaspis brasiliensis Ihering, Hexacola hexatoma Hartig (Figitidae), Pachycrepoideus vindemmiae Rondani (Pteromalidae), and Spalangia endius Walker (Spalangiidae). Leptopilina heterotoma is reported for the first time in Argentina. High numbers of D. suzukii adults were recovered from baited traps and field-collected fruits. However, no parasitoid emerged from D. suzukii pupae recovered from fruits, nor were any dead parasitoids recorded inside D. suzukii pupae. Overall infestation levels of D. suzukii on field-collected fruits did not differ significantly between species/varieties. The results are discussed with emphasis on the possible functionality and perspectives of using these species as biological control agents.

A newly invasive species may promote dissimilarity of pest populations between organic and conventional farming systems

Insect taxa vary in their phenology across space creating dissimilar patterns of species abundance over time. The role of human disturbances and invasive species in these patterns of temporal dissimilarity (phenological differences) across space, however, remain largely unexplored. To dissect these patterns, we evaluated four common pests and one newly invasive species (Contarinia nasturtii; Swede midge) at 220 time points across 2 years on organic and conventional farms. We first summarized across time and evaluated differences in pest abundance between farm management (organic and conventional). We then used generalized additive models to describe temporal patterns of abundance, disentangling phenological differences across management systems. Last, we conducted a temporal beta diversity analysis to identify which species and management practices contribute most to dissimilarity. We found that aggregating pest abundance across time and species masked differences in pest phenology across management systems and concealed variation in pest abundance that was strongly driven by an invasive species, respectively. Overall, our results suggest that organic and conventional farms may be only superficially similar in pest abundance. Rather, by accounting for time, we demonstrate a more nuanced understanding of pest communities moving beyond abundance that may be particularly important for management of newly invasive species.

Effect of a cover crop on the aphid incidence is not explained by increased top-down regulation

Background

Cover crops can be used as a habitat management strategy to enhance the natural enemies and their temporal synchronization with a target pest. We examined the effect of winter oat intercropping within organic plum orchards on the natural enemy abundance and seasonal dynamics on the biological control of plum aphids in spring in Central Chile.

Methods

We compared the incidence and abundance of natural enemies and aphid pests from winter to the end of spring using two treatments: (1) plum trees with an oat cover crop (OCC) and (2) plum trees without a cover crop but with spontaneous vegetation (SV). We hypothesized that cover crops allow the development of winter cereal aphids, promoting the early arrival of natural enemies in spring, resulting in an earlier control of plum aphids.

Results

Winter cereal aphids developed well on the OCC, and as a result, a lower plum aphid incidence in spring was observed when compared to the SV. However, the abundance of natural enemies and the parasitism rates cannot explain the positive impacts of the oat cover crop on the aphid populations as there were no differences between treatments. A potential effect of the oat due to chemical and/or physical stimuli (bottom-up effects) could help to explain these results.

Impacts of Wildflower Interventions on Beneficial Insects in Fruit Crops: A Review

Integrated pest management (IPM) has been practiced by the fruit industry for at least 30 years. Naturally occurring beneficial insects have been encouraged to thrive alongside introduced predatory insects. However, Conservation Biological Control (CBC) and augmented biocontrol through the release of large numbers of natural enemies is normally only widely adopted when a pest has become resistant to available conventional pesticides and control has begun to break down. In addition, the incorporation of wild pollinator management, essential to fruit production, has, in the past, not been a priority but is now increasingly recognized through integrated pest and pollinator management (IPPM). This review focuses on the impacts on pest regulation and pollination services in fruit crops through the delivery of natural enemies and pollinating insects by provisioning areas of fruiting crops with floral resources. Most of the studies in this review highlighted beneficial or benign impacts of floral resource prevision to fruit crops. However, placement in the landscape and spill-over of beneficial arthropods into the crop can be influential and limiting. This review also highlights the need for longer-term ecological studies to understand the impacts of changing arthropod communities over time and the opportunity to tailor wildflower mixes to specific crops for increased pest control and pollination benefits, ultimately impacting fruit growers bottom-line with less reliance on pesticides.

Spotted Wing Drosophila in Sweet Cherry Orchards in Relation to Forest Characteristics, Bycatch, and Resource Availability

Forest vegetation is essential for the population development of the spotted wing drosophila (SWD). Yet, little is known of how the structure of surrounding forest areas influence the abundance of SWD within orchards. In this work, we use data from a field trial at five sites in Switzerland to analyse the relationship between the extent of forest area, its edge density, and its distance from the orchard with the occurrence of SWD in sweet cherry orchards in a Bayesian hierarchical model. Availability of cherries and bycatch were also included in the model to account for effects of resource availability and trap attractiveness, respectively. For all main effects and their interactions, we accounted for potential temporal changes by adding interactions with time. We found that the closer an orchard was to a forest, the more SWD were trapped within the orchard. However, the interaction of forest proximity with forest area caused a disproportionate decrease of SWD catches. Also, the within orchard variables, trap catches of other drosophilid flies and resource availability affected SWD trap catches, but their relation changed in the course of the experiment. The findings imply that reducing SWD occurrence in orchards and other crop fields requires not only the consideration of processes outside and within the host crop field, but also of temporally changing relationships between SWD and other factors.

Annual flower strips support pollinators and potentially enhance red clover seed yield

Ecological intensification provides opportunity to increase agricultural productivity while minimizing negative environmental impacts, by supporting ecosystem services such as crop pollination and biological pest control. For this we need to develop targeted management solutions that provide critical resources to service-providing organisms at the right time and place. We tested whether annual strips of early flowering phacelia Phacelia tanacetifolia support pollinators and natural enemies of seed weevils Protapion spp., by attracting and offering nectar and pollen before the crop flowers. This was expected to increase yield of red clover Trifolium pratense seed. We monitored insect pollinators, pests, natural enemies and seed yields in a total of 50 clover fields along a landscape heterogeneity gradient, over 2 years and across two regions in southern Sweden. About half of the fields were sown with flower strips of 125-2,000 m2. The clover fields were pollinated by 60% bumble bees Bombus spp. and 40% honey bees Apis mellifera. The clover seed yield was negatively associated with weevil density, but was unrelated to bee species richness and density. Flower strips enhanced bumble bees species richness in the clover fields, with the strongest influence in heterogeneous landscapes. There were few detectable differences between crop fields with and without flower strips. However, long-tongued bumble bees were redistributed toward field interiors and during phacelia bloom honey bees toward field edges. Clover seed yield also increased with increasing size of the flower strip. We conclude that annual flower strips of early flower resources can support bumble bee species richness and, if sufficiently large, possibly also increase crop yields. However, clover seed yield was mainly limited by weevil infestation, which was not influenced by the annual flower strips. A future goal should be to design targeted measures for pest control.

Substantial Mortality of Cabbage Looper (Lepidoptera: Noctuidae) From Predators in Urban Agriculture Is not Influenced by Scale of Production or Variation in Local and Landscape-Level Factors

As Midwestern (United States) cities experience population decline, there is growing interest in converting underutilized vacant spaces to agricultural production. Urban agriculture varies in area and scope, yet most growers use similar cultivation practices such as avoiding chemical control of crop pests. For community gardens and farms that sell produce commercially, effective pest suppression by natural enemies is important for both societal, economic, and marketing reasons. To gauge the amount of prey suppression at 28 urban food-production sites, we measured removal of sentinel eggs and larvae of the cabbage looper Trichoplusia ni (Hubner), a caterpillar pest that defoliates Brassica. We investigated how landscape and local factors, such as scale of production, influence cabbage looper mortality caused by predators. Predators removed 50% of eggs and 25% of larvae over a 3-d period. Landscape factors did not predict mortality rates, and the amount of loss and damage to sentinel prey were similar across sites that differed in scale (residential gardens, community gardens, and farms). To confirm that removal of sentinel items was likely caused by natural enemies, we set up a laboratory assay that measured predation of cabbage looper eggs and larvae by several predators occurring in urban gardens. Lady beetles caused the highest mortality rates, suggesting their potential value for biocontrol; spiders and pirate bugs also consumed both eggs and larvae at high rates. Our results suggest that urban growers benefit from high consumption rates of cabbage looper eggs and larvae by arthropod predators.

Spatiotemporal Dynamics of the Invasive Halyomorpha halys (Hemiptera: Pentatomidae) in and Between Adjacent Corn and Soybean Fields

Knowledge on movement and spatial patterns of insect pest populations among preferred hosts aids in the development of effective pest management strategies. In this study, we quantified the spatiotemporal dynamics of the invasive brown marmorated stink bug, Halyomorpha halys (Stål 1855), in relation to field corn, Zea mays L., and soybean, Glycine max (L.), crop phenology. We also examined the potential role of corn as a source of stink bugs in adjacent soybean. The highest density of stink bugs in each crop coincided with blister to milk-dough stages in corn (R2-R3/R4), and beginning seed to full seed (R5-R6) stages in soybean. In entire fields of adjacent corn and soybean, H. halys was found in very low density (<0.5/m(2)) or absent beyond 25 m from the field edge. Inverse distance weighted interpolations of H. halys densities suggest potential dispersal of H. halys, particularly adults and large nymphs, from corn into soybean, coinciding with the end of dough stage in corn and beginning of soybean seed development stage. These findings have important implications for managing H. halys through location and timing of scouting efforts, consideration of crop arrangement, and decisions on management interventions. Repeated scouting of field corn to assess H. halys densities, particularly from blister stage onwards, could inform decisions on management interventions for preventing or mitigating H. halys colonization into soybean. Where H. halys is an economic problem, reducing the extent of boundary shared between corn and soybean could reduce dispersal into soybean.

Economic value of biological control in integrated pest management of managed plant systems

Biological control is an underlying pillar of integrated pest management, yet little focus has been placed on assigning economic value to this key ecosystem service. Setting biological control on a firm economic foundation would help to broaden its utility and adoption for sustainable crop protection. Here we discuss approaches and methods available for valuation of biological control of arthropod pests by arthropod natural enemies and summarize economic evaluations in classical, augmentative, and conservation biological control. Emphasis is placed on valuation of conservation biological control, which has received little attention. We identify some of the challenges of and opportunities for applying economics to biological control to advance integrated pest management. Interaction among diverse scientists and stakeholders will be required to measure the direct and indirect costs and benefits of biological control that will allow farmers and others to internalize the benefits that incentivize and accelerate adoption for private and public good.

Floral diversity increases beneficial arthropod richness and decreases variability in arthropod community composition

Declines in species diversity resulting from anthropogenic alterations of the environment heighten the need to develop management strategies that conserve species and ecosystem services. This study examined how native plant species and their diversity influence the abundance and richness of beneficial arthropods, a functionally important group that provides ecosystem services such as pollination and natural pest suppression. Beneficial arthropods were sampled in replicated study plots containing native perennials planted in one-, two-, and seven-species mixtures. We found plant diversity had a positive impact on arthropod richness but not on arthropod abundance. An analysis of arthropod community composition revealed that each flower species attracted a different assemblage of beneficial arthropods. In addition, the full seven-species mixture also attracted a distinct arthropod community compared to single-species monocultures. Using a multivariate approach, we determined whether arthropod assemblages in two- and seven-species plots were additive and could be predicted based on assemblages from their component single-species plots. On average, assemblages in diverse plots were nonadditive when compared to assemblages predicted using single-species plots. Arthropod assemblages in two-species plots most closely resembled those of only one of the flower species in the mixture. However, the arthropod assemblages in seven-species plots, although statistically deviating from the expectation of an additive model, more closely resembled predicted communities compared to the assemblages found in two-species plots, suggesting that variability in arthropod community composition decreased as planting diversity increased. Our study demonstrates that careful selection of plants in managed landscapes can augment beneficial arthropod richness and support a more predictable arthropod community, suggesting that planning and design efforts could shape arthropod assemblages in natural as well as managed landscapes to meet targeted conservation or management goals.

Effects of local and landscape factors on population dynamics of a cotton pest

BACKGROUND

Many polyphagous pests sequentially use crops and uncultivated habitats in landscapes dominated by annual crops. As these habitats may contribute in increasing or decreasing pest density in fields of a specific crop, understanding the scale and temporal variability of source and sink effects is critical for managing landscapes to enhance pest control.

METHODOLOGY/PRINCIPAL FINDINGS

We evaluated how local and landscape characteristics affect population density of the western tarnished plant bug, Lygus hesperus (Knight), in cotton fields of the San Joaquin Valley in California. During two periods covering the main window of cotton vulnerability to Lygus attack over three years, we examined the associations between abundance of six common Lygus crops, uncultivated habitats and Lygus population density in these cotton fields. We also investigated impacts of insecticide applications in cotton fields and cotton flowering date. Consistent associations observed across periods and years involved abundances of cotton and uncultivated habitats that were negatively associated with Lygus density, and abundance of seed alfalfa and cotton flowering date that were positively associated with Lygus density. Safflower and forage alfalfa had variable effects, possibly reflecting among-year variation in crop management practices, and tomato, sugar beet and insecticide applications were rarely associated with Lygus density. Using data from the first two years, a multiple regression model including the four consistent factors successfully predicted Lygus density across cotton fields in the last year of the study.

CONCLUSIONS/SIGNIFICANCE

Our results show that the approach developed here is appropriate to characterize and test the source and sink effects of various habitats on pest dynamics and improve the design of landscape-level pest management strategies.

Exploiting Allee effects for managing biological invasions

Biological invasions are a global and increasing threat to the function and diversity of ecosystems. Allee effects (positive density dependence) have been shown to play an important role in the establishment and spread of non-native species. Although Allee effects can be considered a bane in conservation efforts, they can be a benefit in attempts to manage non-native species. Many biological invaders are subject to some form of an Allee effect, whether due to a need to locate mates, cooperatively feed or reproduce or avoid becoming a meal, yet attempts to highlight the specific exploitation of Allee effects in biological invasions are surprisingly unprecedented. In this review, we highlight current strategies that effectively exploit an Allee effect, and propose novel means by which Allee effects can be manipulated to the detriment of biological invaders. We also illustrate how the concept of Allee effects can be integral in risk assessments and in the prioritization of resources allocated to manage non-native species, as some species beset by strong Allee effects could be less successful as invaders. We describe how tactics that strengthen an existing Allee effect or create new ones could be used to manage biological invasions more effectively.