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.

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.

#2) Determine the feeding ability of adults and juveniles of four common insect predators in alfalfa.

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

Accomplishments/Milestones

Objective 1

2013 was our final year of pest and beneficial insect collections among commercial alfalfa fields in Cache County, Utah.  Collections for this year began in mid-May and ended late September.  With the exception of one producer who had plowed under their alfalfa fields for a new crop, all producers participating in the 2012 field survey permitted us to continue surveying their fields in 2013. To maintain an appropriate number of replicates, replacement fields were found for the ones that had been plowed. Two new additional collecting sites were established in a location less than 1 mile from the original 2012 collection sites. Of the three growers who treated their fields for alfalfa weevil during the 2012 growing season, two had chosen not to apply pesticides during the 2013 growing season. In total, 10 alfalfa fields were sampled for pests and beneficial insects approximately twice a month for four to five months.  Four of these fields were treated for alfalfa weevils while the other six were left untreated.

Thus far, a majority of the 2013 insect samples have been processed and entered into a database now representing 199,549 arthropods from the 2013 field season. With only 65 samples left out of the 715 that have been processed thus far, we are very close to completing this major hurdle. The processing completion date is anticipated for January 31. This is an accomplishment given that this is four months faster than the previous year’s processing time. While a formal statistical analysis will be performed once all insects have been entered into the database, a preliminary look at a few groups of beneficial insects are included in the discussion below.

Damsel bugs (Nabis sp.) were the most numerically dominant beneficial insect found in these alfalfa fields. Figure 1 suggests these insects may be affected by weevil sprays, seen as shifts in their phenology and abundance in fields that have been treated for alfalfa weevils when compared to theuntreated counterparts. Peak abundances of both adults (Fig. 1A) and nymphs (Fig. 1B) in treated fields occur roughly one month later than in untreated fields. 

Big eyed bugs (Geocoris sp.) were also captured in our samples and were found in their greatest abundance during August (Fig. 2). This peak abundance is a full two months after fields have been treated with alfalfa weevil pesticides, which typically occur in early June for Cache Valley alfalfa growers. It is also interesting to note that there are relatively few big eyed bugs in May and early June prior to insecticide applications. Due to their natural phenology, these beneficial insects may escape a majority of deleterious effects of alfalfa weevil sprays and may be available for pest suppression services later in the growing season for both treated and untreated fields.

Coccinellids, a group encompassing multiple common species of lady beetles, including Hippodamia convergens and Coccinella septumpunctata, are shown in Figures 3A (adults) and 3B (larvae) for fields treated and untreated for alfalfa weevil. These populations appear to be relatively unaffected by weevil sprays, with the exception of a large spike in larval numbers occurring in treated fields toward the end of the growing season in September. Since coccinellids are generally aphidophagous and are known to respond numerically to increases in their aphid prey (Wright and Laing 1980; Evans and Youssef 1992), it is helpful to consider coccinellid populations in conjunction with the most abundant aphid found in these fields, pea aphids (Fig. 3C). The spike in larval numbers appear to correlate with a second resurgence of pea aphid populations in treated fields, which occurred approximately two weeks prior to peak larval abundance. Additionally, coccinellids take roughly four weeks to complete development in the field (Obrycki and Tauber 1981), which may help explain the timing at which the maximum abundance of adult ladybeetles was observed during the growing season four weeks following peak aphid numbers.

Yields from alfalfa fields treated and untreated for alfalfa weevil were monitored during the 2012 and 2013 field seasons.
A standardized set of stem clippings were taken from each field during the first and second crops of the 2012 field season and during the first, second and third crops in the 2013 field season. In a comparison of yields collected from fields treated and untreated for alfalfa weevil, yields were on average not statistically different (p >.05) between treated and untreated fields for each cutting in both 2012 and 2013 (Fig. 4). General economic threshold levels for alfalfa weevil larvae are considered at 19 larvae per 180 degree sweep with a 15 inch sweep net. Of the six fields that were treated for alfalfa weevil in 2012, only one had exceeded this threshold during the first crop. In 2013, none of the fields treated for alfalfa weevil had exceeded this threshold; however, three of the untreated fields had.

Objective 2

In 2013, we performed feeding trials to evaluate predator foraging based on size. Prey items used were pea aphids (Acyrthosiphon pisum) and beet armyworm larvae (Spodoptera exigua) in these predator feeding trials. Pea aphids were reared on fava beans (Vicia faba) in the lab under growing lights (23? C, 12 D:12 N), while beet armyworms were reared in incubators (27? C, 12 D: 12 N) on beet armyworm diet (Southland Products Inc.).  Small batches of beet armyworm eggs were hatched every two to three days to generate a colony of staggered life stages.

Two predatory bugs (Nabis sp., Geocoris sp.) and two coccinellids (Hippodamia convergens, Coccinella septumpunctata) were used in these experiments.  Predator age classes were characterized as early instars, late instars or adults. These age classes correspond to first to second instar nymphs or second instar larvae, fourth to fifth instar nymphs or fourth instar larvae, and sexually mature adults, respectively. Age classes of predatory bugs were identified using approximate size and wing bud development (Tamaki 1972; Elvin and Sloderbeck 1984), while larval stages of coccinellids were determined according to head capsule width as well as relative size.

This study was performed in a laboratory at Utah State University where we crossed three predator size classes (adult, large instars, small instars) with two prey size classes of pea aphids (large and small) or three size classes of beet armyworms (large larvae, medium larvae, small larvae) in experimental arenas (35 mm diameter X 10 mm height Petri dish) lined with Insect-a-Slip Barrier (BioQuip Products) to discourage insects from escaping the arena. Arenas were set up with a single predator individual and a single prey individual. Pea aphid arenas were arranged with a small, moistened cotton wick placed over the hypocotyl of an alfalfa seedling. Pea aphids were transferred singly onto one of the exposed cotyledon leaves and allowed 15-20 minutes to establish a feeding site. In beet armyworm treatments, larvae were placed directly into the arena without additional sources of food or water. After prey were added, a single predator of the respective treatment age class was introduced. We recorded prey mortality over a two hour time period and recorded predator behavior based on three simplistic behaviors: walking, sitting or eating. These actions were recorded two to three times every 15 minutes. Graphs of prey mortality after the two hour time block for damsel bugs and big eyed bugs can be found in found in Figures 5 and 6.

Objective 3

During the 2013 field season, field cage experiments were performed during June and September. However, due to heavy infestations of alfalfa weevil larvae, armyworms and cutworms in June and a leak in a nearby irrigation pipe (which greatly altered plant growth between cages) in September, results from these experiments are inconclusive.  

Citations

Elvin, M. K. and P. E. Sloderbeck (1984). “A Key to Nymphs of Selected Species of Nabidae (Hemiptera) in the Southeastern USA.” The Florida Entomologist 67(2): 269-273.

Evans, E. W. and N. N. Youssef (1992). “Numerical Responses of Aphid Predators to Varying Prey Density among Utah Alfalfa Fields.” Journal of the Kansas Entomological Society 65(1): 30-38.

Obrycki, J. J. and M. J. Tauber (1981). “Phenology of Three Coccinellid Species: Thermal Requirements for Development.” Annals of the Entomological Society of America 74(1): 31-36.

Tamaki, G. a. R. E. W. (1972). “Biology And Ecology of Two Predators, Geocoris pallens and Geocoris ballatus.” Agriculture Research Service Technical Bulletin No. 1446: 1-46.

Wright, E. J. and J. E. Laing (1980). “Numerical Response of Coccinellids to Aphids in Corn in Southern Ontario.” The Canadian Entomologist 112(10): 977-988.

• Damsel bugs
• Big eyed bugs
• Coccinellids
• Yield comparison
• Damsel bug feeding trial
• Big eyed bug feeding trial

Impacts and Contributions/Outcomes

Data obtained from the 2012 insect phenology collection was incorporated into a talk which was presented at the Cache County Crop School hosted by Utah State University Cooperative Extension on February 13, 2013.  I gave a 15 minute presentation to 67 growers about beneficial insects and secondary pest outbreaks of pea aphids in alfalfa that are common after weevil pesticide applications. Forty surveys were handed out to Crops School participants to provide Western SARE with an assessment of the effectiveness and impact of this outreach on local growers and extension agents. Of those 40 surveys, 34 were returned with feedback. Ninety-seven percent of survey participants indicated it improved their awareness of the topics covered, and 100% indicated that it provided new knowledge on the subject of predators and secondary pest outbreaks due to pesticides. From this survey, it was also estimated that some aspect of this project would be shared with approximately 167 other producers or professionals within the next 12 months. 

Collaborators: