Contributions to pest suppression through predator phenology and functional diversity

Contributions to pest suppression through predator phenology and functional diversity

Summary

The goal of this research is to understand how changes in predatory insect phenology and diversity affect natural pest suppression in alfalfa. Specifically, we are using field surveys, feeding trials and outdoor cage experiments to examine the contribution of predator life stage diversity and species diversity to pea aphid suppression, a common secondary pest in alfalfa. Results from this study will also provide information about the impacts of broad spectrum pesticides on naturally occurring beneficial insect communities, as well as ways to enhance natural pest control in alfalfa fields through the judicious use of pesticides.

Objectives/Performance Targets

1. Quantify alfalfa yield, pest and beneficial insect species, and life stages present in commercial alfalfa fields treated and untreated for alfalfa weevil Determine the feeding capacity of adults and juveniles of four common insect predators in alfalfa. Determine the impact of predator life-stage phenology in diverse predator communities on prey suppression and yield

Accomplishments/Milestones

Objective 1. Quantify alfalfa yield, pest and beneficial insect species, and life stages present in commercial alfalfa fields treated and untreated for alfalfa weevil.

Starting in June 2011, we conducted insect surveys of four local alfalfa fields from four different producers located within Cache County, Utah. Original plans were to divide each field into two sections, one of which would be sprayed with pesticides to control alfalfa weevils and the other left unsprayed. However, due to extended spring weather conditions, alfalfa weevils appeared to have a diminished impact on alfalfa stands. Cold and wet conditions continued into June, prompting two of the four growers to deem pesticide applications unnecessary for their fields. We continued sampling all fields throughout the growing season, one to two times between each cutting using sweep nets and a D-vac suction sampler. We have since sorted, identified and counted important predators and pests from these 2011 based collections.

Insect sampling resumed in May 2012 with sweep nets and a suction sampler. During the 2012 growing season we involved four additional growers in the study and sampled a total of 10 different fields during the months of May through September. Two collection events took place between each cutting (approximately once every two weeks). Six of these fields were sprayed with broad spectrum pesticides to control alfalfa weevil and four were left untreated. Most sweep net samples from 2012 have been sorted, identified and counted. Thus far, a total of 272,857 insects from the 2012 sweep net collections have been counted and recorded. Suction samples are currently being processed. Data from both years are being entered into an Excel database where, once completed, it will be ready for insect community analysis. A preliminary graph of common predatory insects found during the first half of the 2012 field season can be viewed in Figure 1.

Alfalfa yield was also monitored from each of these fields by taking a standardized set of alfalfa stem clippings before and after the first and second cuts. Alfalfa samples from both years have been dried, weighed and recorded into Excel.

Objective 2. Determine the feeding capacity of adults and juveniles of four common insect predators in alfalfa.

With this objective, I am testing for differences in the abilities of adult and juvenile predators to capture and consume large or small prey items using four common predatory insects (Nabis sp., Geocoris sp., Hippodamia convergens and Coccinella septempunctata) found during our field surveys. This will highlight differences in foraging capacity of adult and juvenile predators and may indicate a mechanism by which adult and juvenile predators partition their food resources, allowing for enhanced pest suppression. Thus far, we have been able to perform preliminary feeding trials with Hippodamia convergens adults and larvae using large and small pea aphids. Unfortunately, rearing and maintaining colonies of several predatory insect species along with their food resources throughout the winter has proved remarkably difficult. Feeding trials are expected to resume once outdoor insect activity begins in spring 2013.

Objective 3. Determine the impact of predator life-stage phenology in diverse predator communities on prey suppression and yield.

Preliminary results from field survey collections suggest predatory insect assemblages are often comprised of varying ratios of adult and juvenile predators throughout the growing season (Figure 1). This highlights the importance of studying natural pest suppression activities within a dynamic, phenology driven format. When considering different predator life stages, adults and juveniles can be markedly different from each other, varying in size, form or function. This can lead to key differences in the ways they move, capture prey, choose habitats or allocate resources. It is hypothesized that these functional differences may allow juvenile predatory insects to contribute to the overall diversity of a system by increasing the number of functional roles, and thereby enhance pest suppression.

To examine the impact of predator life stages and diversity on pea aphid suppression, we performed a field cage experiment during summer 2011 and summer 2012. Field cages were setup in a plot of alfalfa at Utah State University’s Greenville Research Farm. Pea aphid populations were established in roughly equal amounts across all cages before adding predators. Predator communities were created using four common predators found in alfalfa (Nabis sp., Geocoris sp., Hippodamia convergens and Coccinella septempunctata) in a substitutive, factorial design. Each cage received a total of 12 individual predators. Treatments were: juveniles only (one specie and three species), adults only (one specie and three species), juveniles plus adults (one specie and three species) and an aphid control (four replicates with aphids and no predators). A plant control was also included (four replicates of plants with no aphids and no predators) to account for baseline alfalfa yield without the effects of aphids. There was a total of 32 cages (experimental units; 24 with predators + 4 aphid controls + 4 plant controls). All possible species compositions of three species from our pool of four predators were represented in treatments containing more than one species. All juvenile insects were added during the first instar life stage. After two weeks, cages were destructively sampled. Pea aphids were counted and the number of retrieved predators recorded. Dry weights of alfalfa were obtained to record predator and pea aphid effects on yield.

Juvenile insects used in cage experiments were lab-reared from field-captured adults. Rearing practices involved careful timing of juvenile emergence of all four predator species to emerge from eggs at the same time while also maintaining adult insect colonies in habitats suitable for reproduction. This was achieved by growing approximately 150 host plants (Vicia faba) to manage pea aphid colonies and predator rearing habitats, while also regulating rearing temperatures to speed up or slow down the process of insect hatching. In the end, we were able to add 288 predatory insects of four different species in different life stages to field cages at the appropriate time.

Results from this experiment suggest that predatory insect communities containing multiple life stages (both adults and juveniles) are more efficient at suppressing pests than those communities containing adults only or juveniles only (Figure 2). The presence of multiple life stages may even have a larger impact on pea aphid suppression than species diversity per se (Figure 3).

• Figure 1. Frequency of common adult and juvenile predators, May through July, 2012
• Figure 3. Pea aphid retrieval rates for predator species richness and life stage treatments
• Figure 2. Pea aphid retrieval rates for life stage treatments

Impacts and Contributions/Outcomes

Data obtained from 2011 and 2012 cage experiments and field surveys were presented at two national meetings for the Entomological Society of America conferences in Reno, Nevada in 2011 and Knoxville, Tennessee in 2012. It was also presented at the Intermountain Graduate Research Symposium hosted by Utah State University in spring 2012. Furthermore, these data were incorporated into the winter 2012 Utah Pests Newsletter in an article titled “Monitoring Alfalfa Weevil Can Save Money.” Outreach goals for 2013 include the production of a beneficial insect pocket guide book to be utilized by alfalfa growers and extension agents to help increase awareness of beneficial insect populations in their fields. Additionally, I will be presenting my work to producers during the local Cache County Crop School hosted by Utah State University Cooperative Extension in February 2013.

Ultimately, researching the dynamics of predatory insect communities can lead to pest management strategies that enhance the roles of natural pest suppression and minimize the impacts of pesticides. This research will contribute to our growing knowledge of biodiversity and ecosystem functioning for the benefit of managed agricultural settings, especially as it relates to Integrated Pest Management (IPM) strategies and the reduction of pesticide impacts on humans, our food resources and the environment.

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