Contributions to pest suppression through predator phenology and functional diversity

Final Report for GW12-030

Project Type: Graduate Student
Funds awarded in 2012: $13,095.00
Projected End Date: 12/31/2014
Grant Recipient: Utah State University
Region: Western
State: Utah
Graduate Student:
Major Professor:
Dr. Ricardo Ramirez
Utah State University
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Project Information

Summary:

The goal of this research was to understand how insecticides impact natural enemy phenology and life stage structure, and how changes in these factors this might influence secondary pest suppression. Specifically, we used field surveys in insecticide treated and untreated alfalfa fields, laboratory feeding trials, and an outdoor cage experiment to assess how multiple enemy life stages and species diversity contribute to pea aphid suppression, a common secondary pest in alfalfa. Results from our survey show that phenological patterns of some natural enemy groups (e.g. big eyed bugs) allow them to temporally escape the most deleterious effects of early-season insecticide use, while others with early season phenology (e.g. damsel bugs) were impacted to a greater extent when early generations were removed by insecticides. Pea aphid abundance was found to not be significantly different between treated and untreated fields. In addition, surveys reveal a dynamic natural enemy community that fluctuates in both species and life stage composition across the growing season. Immature life stages were, at times, equally or more abundant than adult life stages, yet little is known how the presence of these multiple stages affect biological control outcomes. A cage experiment with treatments controlling for species richness and number of life stages reveals that the presence of multiple life stages increases pest suppression. Feeding trials indicate that different predator life stages consume different sizes of prey items, suggesting one possible mechanism through which multiple life stages can facilitate complimentary pest suppression when together. 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 pesticides. 

Introduction

Alfalfa is an important crop to Utah agriculture, with nearly 50% of farms in Utah producing alfalfa hay in amounts exceeding 2 million tons per year (National Agricultural Statistics Service, 2014). Broad-spectrum insecticides are a major component of pest management in alfalfa, particularly for the control of alfalfa weevil. However, applications of broad spectrum insecticides have been implicated in creating conditions that favor secondary pest outbreaks, namely aphids in alfalfa (Evans 1993). Natural enemies mitigate the impacts of pest outbreaks, but the use of broad-spectrum insecticides remove or reduce densities of these predacious insects (Croft and Brown 1975). The resulting lack of predation pressure allow secondary pests to quickly multiply, potentially requiring additional pesticide use for their control (Settle et al. 1996). A focus on pest monitoring and the judicious use of insecticides using economic thresholds has aided in conservation biological control efforts that aim to protect natural enemies; however, adoption of these practices in Utah alfalfa production are not widespread. Although it is recognized that natural enemies provide valuable pest suppression services, there is relatively little information how insecticides impact phenology and the presence of multiple natural enemy life stages in alfalfa. It is also unknown how the presence of multiple enemy life stages impact pest suppression services. This project was aimed at understanding these components of the natural enemy community in an effort to aid conservation biological control efforts and integrated pest management strategies by providing greater knowledge about the natural enemy community, their impacts on aphid suppression, and how this might be influenced by insecticide use.

Natural enemy communities may be composed of many species, but they can also be composed of many life stages. Considerable evidence indicates that significant functional differences can exist between age groups of a single species (Wollrab et al. 2013; Polis 1984). This can be especially pronounced in insects since the process of complete and incomplete metamorphosis yield discreet physiological differences between adult and immature insects. Insect traits such as size, mobility, or even form can completely change over the course of ontogenetic development. Evidence suggests that diverse communities of natural enemies can improve pest suppression (Letourneau et al. 2009; Hooper et al. 2005; Cardinale et al. 2006). This is often attributed to complementary interactions between organisms where diverse communities utilize resources more efficiently than less diverse communities (Losey and Denno 1998). This can be enabled through mechanisms such as resource partitioning, where functional trait differences (e.g. foraging techniques, prey preferences, mobility, diel patterns) enable the division of resources (e.g., food, habitat, mates) across multiple organisms in space and time (Finke and Snyder 2008). The presence of multiple life stages may be a source of functional diversity that can influence biological control activities beyond the species richness level.

Generally, natural enemy diversity research has represented diversity at the species richness level with enemies in the adult life stage. However, both life-stage structure and species composition of the natural enemy community can dynamically change throughout a growing season according to species’ phenological patterns. Integrating multiple life stages into diversity research can more adequately reflect the natural community.  Pest suppression services may also be limited by natural enemy phenology and the disturbances which might affect phenological patterns, such as insecticide use. If early season insecticide applications for alfalfa weevil have negative effects on natural enemy phenology or life stage structure, it could disrupt the biological control of aphids or other secondary pest insects later in the season. This project was targeted at investigating aspects of the natural enemy community that contribute to pest suppression with an emphasis on how this relates to predator phenology and life stage dynamics. Ultimately, this can be used to advance the integration of ecologically-based pest management to minimize the impacts of pesticides on the natural enemy community. 

References

Cardinale, B. J., et al. (2006). "Effects of biodiversity on the functioning of trophic groups and ecosystems." Nature 443(7114): 989-992.             

Croft, B. and A. Brown (1975). "Responses of arthropod natural enemies to insecticides." Annual Review of Entomology 20(1): 285-335.    

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., Karren, J., Hurst, C. (1993). "Pea aphid outbreaks associated with spraying for the alfalfa weevil in Utah." Utah State University Cooperative Extension Fact Sheet No. 85.      

Finke, D. L. and W. E. Snyder (2008). "Niche partitioning increases resource exploitation by diverse communities." Science 321(5895): 1488-1490.

Hooper, D. U., et al. (2005). "Effects of biodiversity on ecosystem functioning: a consensus of current knowledge." Ecological Monographs 75(1): 3-35. 

Letourneau, D. K., et al. (2009). "Effects of natural enemy biodiversity on the suppression of arthropod herbivores in terrestrial ecosystems." Annual Review of Ecology, Evolution, and Systematics 40(1): 573-592.

Losey, J. E. and R. F. Denno (1998). "Positive predator–predator interactions: enhanced predation rates and synergistic suppression of aphid populations." Ecology 79(6): 2143-2152.              

Polis, G. A. (1984). "Age structure component of niche width and intraspecific resource partitioning: can age groups function as ecological species?" The American Naturalist 123(4): 541-564.

Settle, W. H., et al. (1996). "Managing tropical rice pests through conservation of generalist natural enemies and alternative prey." Ecology 77(7): 1975-1988.

Snyder, W. E. and A. R. Ives (2001). "Generalist predators disrupt biological control by a specialist parasitoid." Ecology 82(3): 705-716.         

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.

Wollrab, S., et al. (2013). "Ontogenetic diet shifts promote predator-mediated coexistence." Ecology 94(12): 2886-2897.

Project Objectives:

Alfalfa harbors a diverse community of generalist natural enemies, including damsel bugs, big-eyed bugs, and lady beetles that contribute to the control of aphids (Snyder and Ives 2001), yet little is known about how dynamic communities of adult and juvenile predators interact to suppress pests throughout a growing season and how this might be affected by insecticide applications. The objectives of this research were to:

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 differences between adults and juveniles of four common insect predators found in alfalfa, and
3) determine the impact of multiple predator life-stages in diverse predator communities on prey suppression and yield.  

Cooperators

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  • Dr. Ricardo Ramirez

Research

Materials and methods:

Objective 1 Methods:

Field Sites: We surveyed production alfalfa fields in Cache County, Utah to determine the effect of insecticides used for the management of alfalfa weevil on generalist predators and aphids. Fields were surveyed from May to September (specifically, May 8 – September 12 in 2012 and May 13 to September 24 in 2013; Table 1). Each alfalfa field was in three - four years of production, with the exception of one field that had six years of production. On average, each field was 15.7 acres and was separated by a minimum of 0.1 km. The fields varied in pest management (alfalfa weevil treatment or no treatment), irrigation, and harvest schedule (Table 1).  In total, 12 unique fields were sampled across the two growing seasons (2012 and 2013). Fields were sampled either both years or only one year (Table 1).  Ten fields (replicates) were treated with insecticide for alfalfa weevil management (six fields in 2012 and four fields in 2013) and 10 fields (replicates) were not treated with insecticides (four fields in 2012 and six fields in 2013) (Table 2). Alfalfa growers primarily used organophosphate (e.g., chlorpyrifos) or pyrethroid (e.g., lambda-cyhalothrin) insecticides for weevil management.

Insect Collections: were collected in each field roughly every other week using sweep net sampling and suction sampling. Sample locations were 20 feet from field edges to minimize edge effects and five transects (subsamples) were setup within a U-shape in each field. Sweep net and suction samples were performed simultaneously by two different collectors and sample collections from each method did not overlap so that transects were undisturbed.

Sweep net samples were used to collect arthropods among foliage by using a 38 cm diameter sweep net (Bioquip Products, Inc.). A sweep was performed by drawing the net through the top third of alfalfa in a 1800 arc in front of the collector. Twenty consecutive sweeps comprised one sweep net subsample (approximately 25 m length). Five sweep net subsamples were taken at each field site during each sampling date. Suction samples were used to collect insects below the plant canopy and at the soil surface. Suction samples were collected with a reversible leaf blower (Echo, Shred ‘N’ Vac® ES-255). A fine mesh bag (nylon organdy bag, Rincon-Vitova Insectaries) was securely inserted into the 86 cm-long plastic tube attachment vacuum end to capture insects. Each suction sample consisted of placing the opening of the insect-suction tube perpendicular to the ground at the soil surface, making contact with the soil surface for 0.5 sec, then lifting the tube vertically above the plant canopy in one motion. The process was repeated approximately every meter in succession fifty times, generating one suction subsample. Five suction subsamples were taken from each field. Each subsample (for sweep and suction sample) was stored in separate plastic bags in a freezer and insects were later sorted, identified, and counted.

Analysis: Predator and aphid abundance was analyzed by comparing alfalfa weevil insecticide treated fields (+,-) using repeated measures within a generalized linear mixed model using PROC GLIMMIX (SAS 9.3 Institute Inc., Cary, NC). Time was included as a random effect with subjects identified as location by insecticide. Distributions appropriate for count data (poisson or negative binomial) and covariate structure for each insect group were chosen based on Akaike information criterion (AIC) fit statistics. Each season (2012 and 2013) and sampling method (sweep net and suction) were analyzed separately. To further investigate the effect of insecticides on predator phenology, predator life-stages (adult and immature) were analyzed separately. For damsel bugs and big eyed bugs, immature life-stages were further subdivided into young instars (1st and 2nd instar nymphs or larvae) and older than 3rd instar. Differences between insecticide treatments for a given sample date were examined with the least squares means (LS Means) differences.

Objective 2 Methods:

 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 1st - 2nd instar nymphs or 2nd instar larvae, 4th - 5th instar nymphs or 4th 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 (Tamaki 1972; Elvin and Sloderbeck 1984).

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.

Objective 3 Methods:

Study Site: The field experiment was conducted at Utah State University’s Greenville Research Farm in Logan, Utah, U.S. Experimental units were field cages constructed with PVC frames (40 cm x 40 cm x 80 cm) enclosed with nylon mesh fabric (0.5mm diameter).  Cages were placed over five to six fresh cut alfalfa crowns. The bottom edges of the mesh were buried into the soil while the top edges were cinched closed with a drawstring and tied with elastic cord. The top draw-string allowed access inside the cages when needed and prevented arthropod movement into or out of cages.  All arthropods from within the cages were removed by hand and by using a suction sampler (Echo ES-250 leaf blower) set in reverse for suction.  

Pea Aphid Establishment: When alfalfa growth was approximately 40 cm in height, each cage was inoculated with pea aphids to establish populations at roughly 1,100 individuals per cage. Aphids used in the experiment were reared in the lab (23? C) on fava beans (Vicia faba) under a growing light (Sun System 10 Crop Master, 12d: 12n cycle).  

Predator Introduction: Within cages containing aphids and alfalfa plants, we created communities of 0, 1, or 3 natural enemy species, drawn from a pool of four common generalist predators. Generalist predators used in this experiment included two lady beetles (Hippodamia convergens, Coccinella septempunctata) and two bugs (Nabis sp., Geocoris sp.). In addition, we varied predator communities to represent immature life-stages, adult life-stages, or a combination of immature and adult predators in equal proportions.  Thus, treatments were control (four replicates containing only aphids and no predators), one species (four replicates of predators in monoculture in the immature, adult, or combination of life-stages), and three species (four replicates of each unique combination of the four species as immature, adult, or combination of immature and adult life-stages). Natural enemy diversity was manipulated within a replacement series design where predator density was kept constant at 12 individuals per cage where each species was represented with either 12 or 4 individuals in the one and three species treatments, respectively. In communities of combined immature and adult life-stages, each life-stage represented half the total predator density per species. In total, there were 32 cages (experimental units; 24 with predators + 4 aphid controls). 

After two weeks, each cage was destructively sampled. Alfalfa plants were clipped at the soil surface and shaken vigorously within the cage to dislodge arthropods back into the cage arena. After plants were free of insects, they were placed in paper bags to be dried (Thermo Scientific, Precision Drying Oven) for seven days at 70 °C and weighed.  Once all alfalfa was removed, remaining aphids and predators were extracted from the cage arena using a suction sampler.  Contents of the suction sampler were transferred to sample bags for predator and aphid enumeration in the lab at a later date. The number of pea aphids retrieved from cags at the end of the experiment were analyzed with two-way ANOVA (SAS 9.3 Institute Inc., Cary, NC).

Research results and discussion:

Objective 1: Survey

Damsel bugs: In both years, the effect of insecticides on adult damsel bugs were dependent on time (2012 Sweep net: INSECTICIDE×TIME: F4,32=3.23, p=0.023; 2013 Sweep net: INSECTICIDE×TIME: F5,37=3.08, p=0.02). Generally, untreated fields had two peaks in adult abundance, the first occurring around late-June or July and the second in mid-August. Patterns of adult abundance in treated fields were characterized by a gradual increase in numbers that climaxed in August. The effect of insecticides on immature life stages of damsel bugs were also dependent on time in suction samples (2012 INSECTICIDE×TIME: F4,38=4.12, p=0.007; 2013  INSECTICIDE×TIME: F5,37=6.21, p<0.001) and in 2012 sweep nets (INSECTICIDE×TIME: F4,32=3.88, p=0.011), but not in 2013 sweep nets where there was a significant effect of insecticides across the growing season (INSECTICIDE: F1,22=23.79, p<0.001) (Figures 1 and 2).

The effects of insecticides on adult life stages were likely indirect. In 2013, starting adult populations were similar in treated and untreated fields. However, in the sampling event just prior to applications (early-June) a large percentage (untreated: 71%; treated: 74%) of the immature life stages captured in suction samples were young instars (1st and 2nd).  In the sampling event immediately following applications (late-June) in untreated fields there were few young instars (13%) and many old (4th and 5th) instars (78%). During this time, only one immature damsel bug was captured across all treated fields. By tracking the phenological development of these initial colonizing populations of damsel bugs, we identified that this first generation of damsel bug nymphs were effectively removed by insecticide sprays. This likely had cascading consequences to adult abundance later in the growing season in July when pea aphid populations reached peak abundance.  

Big eyed bugs: Populations of big eyed bugs were more prominent later in the growing season starting in July. Numbers gradually increased over the season, typically peaking around August and tapering off in September. In sampling events prior to insecticide applications in both years, populations were low, with less than one adult or nymph captured per subsample. Across both years, sampling events, and life stages, significant differences were only detected for adults in 2012 sweep nets (INSECTICIDE×TIME: F4,32=3.99, p=0.010)  and nymphs in 2012 suction samples (INSECTICIDE×TIME: F5,38=2.92, p=0.025).  Suction samples in 2012 captured significantly more nymphs in untreated fields on two occasions during July and mid-August, while sweep nets captured adults in significantly greater numbers in late-June and mid-August (Figures 1 and 2).

Lady beetles: In 2012 the effect of pesticide on average adult abundance was dependent on time (Sweep nets: INSECTICIDE: F4,32=3.23, p=0.025). Average adult abundance in untreated fields exceeded treated fields in late-June (Untreated: 5.8 ± 1.87 s.e.; Treated: 2.24 ± 0.74 s.e.), while the opposite was true in early August (Untreated: 0.65 ± 0.13 s.e.; Treated: 7.43 ± 5.32 s.e.). However, a particularly high average number of adults was recorded in one treated field (33.6 ±4.79 s.e.) in early August in 2012 that was more than 300% higher than the next highest recorded field average for 2012.  Other than these two time periods, average adult numbers were relatively low, but stable across time in treated and untreated fields (Sweep net: TIME: F4,32=3.23, p=0.116). In 2013, patterns of adult lady beetle abundance were very similar across time in treated and untreated fields (Sweep nets: INSECTICIDE: F5,37=0.81, p=0.548), Peak adult abundance occurred in August, one month past peak aphid populations (Figures 1 and 2).

Larval populations were highly variable and ephemeral across the growing season, characterized by relatively low levels across the growing season interspersed with short-lived population spikes. A model appropriate for comparing abundance in treated and untreated fields has not been found given the extreme variability and dearth of non-zero capture rates. Across the 2012 and 2013 growing season, two treated fields in particular experienced explosive increases in larval abundance following periods of pea aphid outbreaks. In 2012, one field recorded an impressive value of 206 (± 23.68 s.e.) larvae per sweep net subsample; 858% larger than the next highest recorded field average for sweep nets in that year. This high abundance occurred one month after peak aphid abundance (11,560 ± 1,868 s.e.) was recorded for this field.

In 2013, another treated field experienced a large secondary spike in pea aphid populations in mid-August (1,648 ± 225 s.e. pea aphids per sweep net subsample). This occurred during a time when the average capture rate of pea aphids among all other fields was 181 ± 44 s.e. individuals per sweep net subsample. Lady beetle larvae in this field increased from one individual captured in early August to 21.2 ± 6.4 s.e. individuals captured per suction subsample by mid-August when pea aphids were again abundant. Interestingly, larval numbers continued to increase in this field to an average of 55.7 ± 7.7 s.e. individuals per suction subsample after pea aphid populations had plummeted to low levels (17 ± 5.8) by early September. These high numbers of lady beetle larvae were not seen in any other field, and were 470% greater than the highest recorded subsample average for suction samples for this year.

Spiders: In general, the effect of insecticides on spider populations in suction samples was dependent on time (2012 INSECTICIDE×TIME: F5,38=5.44, p<0.001; 2013 INSECTICIDE×TIME: F5,29=10.51, p<0.001). Insecticides may impact some spider groups more than others. For example, when spiders of the family Lycosidae (wolf spiders) were analyzed separately, insecticides significantly decreased wolf spider abundance below that seen in untreated fields (2013 Suction INSECTICIDE: F1,8=11.14, p=0.01) for the duration of the growing season.

Pea aphids: In 2012, peak pea aphid abundance occurred in mid-July. However, three of the four untreated fields had been harvested by this time period. The remaining unharvested field recorded a sweep net average of 6,203 (± 1,524 s.e.) pea aphids. During this same time period, the field averages for treated fields were 5,510 aphids (± 2,102 s.e.). Pea aphid populations were highly variable across fields during this year. For example, sweep net averages per field during mid-July ranged from 570 (±135 s.e) to 11,561 (± 836 s.e.) pea aphids.  

In 2013, peak pea aphid abundance also occurred during July. In this year, three of the nine fields sampled exceeded economic threshold for pea aphids (1,000 per 20 sweeps). Of these three fields, one belonged to the untreated field group which recorded the highest sweep net average for pea aphids for the July sampling date (3,504 ± 398 s.e.). Pea aphid abundance was not found to be statistically significant between treated and untreated fields in both 2012 and 2013.

Yield: 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 1st and 2nd crops of the 2012 field season, and during the 1st, 2nd and 3rd 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 > 0.05) between treated and untreated fields for each cutting in both 2012 and 2013 (Figure 3).

Objective 2: Feeding Trials

Predator encounters with beet armyworm prey

Predators were paired with multiple sizes of beet armyworm larvae prey in small petri dish environments to detect feeding differences between age groups within a species of predator. Of these trials, the most noticeable differences occurred when predators were paired with small beet armyworm larvae. Here, small instars of ladybeetle larvae and small damsel bug nymphs successfully inflicted mortality on more prey items than their larger adult counterparts. Large ladybeetle larvae, however, were 100% successful at consuming all small beet armyworms within the first hour of the feeding trial. Large damsel bug nymphs consumed the least amount of small beet armyworms compared to adults and small nymphs. The opposite was true for big eyed bugs, where both adults and large instars were 90% successful at inflicting mortality on small beet armyworms during the 2nd hour, whereas small nymphs were only 60% successful (Figure 4).       

Predator encounters with pea aphid prey

In these feeding trials, lady beetles were clearly the most voracious consumers of aphids, as would be expected of aphidophagous predators. Seven-spotted lady beetles and convergent lady beetles were very similar in feeding ability across each of the age groups. Adults were the most successful consumer of large aphids and large larvae were the second most successful. Small larvae, however, were only 40%- 50% successful with large aphids after the second hour. Young larvae would frequently latch onto legs or antenna, only to be dislodged by aphid movement while the aphid kicked or walked away from their small pursuing predator. Young larvae had much greater success (90% mortality) with small aphids, as well as adults and large larvae (100% mortality at the end of the 2nd hour) (Figure 5).

Damsel bug adults were the most effective at inflicting mortality on large aphids, whereas all large aphids survived in the presence of small damsel bug nymphs. When paired with small aphids, however, all life stages performed similarly well across age groups (adults: 50%; large nymphs: 40%; small nymphs: 60% pea aphid mortality at the end of the 2nd hour). All life stages of big eyed bugs had similar success with small aphids (adults: 40%; large nymphs: 50%; small nymphs: 40% pea aphid mortality at the end of the 2nd hour), but no mortality was recorded for large aphids for all predator treatments  (Figure 5). 

Objective 3: Cage Experiment  

Pea aphid retrieval rates were significantly lower in treatments that contained multiple life stages. There was no significant interaction between species richness and the number of life stages present, and no main effect of species richness. However, the combined presence of both adult and immature life stages, regardless of species richness, significantly depressed populations better than treatments containing one life stage (adults only or immatures only) (Figure 6a). A Tukey’s post hoc comparison shows significant differences between immature only treatments and those treatments containing a combination of both adults and immatures. (Figure 6b).

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 only one life stage.

Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:

One of our most anticipated outreach opportunities from this project was creating and printing an introductory guide to the “Beneficial Insects and Pests of Utah Alfalfa.” This guide is pocket-sized (3.5 x 5.25 inches) with the intent that it be portable for producers, extension agents, or field scouts who are becoming acquainted with some of the most common insect fauna found in alfalfa. This full-color, 41 page guide includes pictures of multiple species and life stages of insects, physical descriptions, life cycles, brief management considerations for pests, and graphs of relative insect abundance (incorporated from this project’s phenological surveys) throughout the growing season. This guide is currently undergoing the Utah State University Extension (FastTrack) review process.  After review, 735 guides will be printed, spiral bound, and provided to several Extension Agents for dissemination across multiple counties throughout Utah. 

In addition, results from this project will be included in the publication of the graduate student's thesis, with two manuscripts currently in progress with goals for submission to Biological Control and Ecology by spring of 2015. 

Extension Presentations

Ramirez, R.A. 2014. Insect pests common to Cache Valley crops. Oral presentation (35 min). Cache County Crop School. FEB 28. Audience: 120. Logan, UT.

Ramirez, R.A. 2014. Sampling alfalfa fields with a sweep net. Oral presentation (30 min). Box Elder County Crop School. JAN 28. Audience: 120. Brigham City, UT.

Ramirez, R.A. 2014. Insecticide resistance, emerging pests, and insect management in agronomic crops. Oral presentation (45 min). Sevier County Crop School. JAN 23. Audience: 60. Richfield, UT.

Ramirez, R.A. and E. Stephens**. 2013. Pest management in forage and grain. Oral presentation (35 min). Cache County Crop School. FEB 13. Audience: 63. Logan, UT.

Ramirez, R.A. 2013. Insecticide issues in alfalfa. Oral presentation (25 min). Utah Hay Conference. JAN 31. Audience: St. George, UT.

Ramirez, R.A. 2013. Insect thresholds and management. Oral presentation (45 min). Beaver County Crop School. JAN 24. Audience: 40. Minersville, UT.

Ramirez, R.A. 2013. Pest management in forage and grain. Oral presentation (45 min). Sevier County Crop School. JAN 23. Audience: 52. Richfield, UT. 

Invited Presentations 

Ramirez, R.A. 2013. The contribution of predator ontogeny toward pest suppression. Symposium, Biological control, application, innovation, and exploration. 97th Annual Meeting of the Pacific Branch Entomological Society of America. APR 7-10. Lake Tahoe, NV

Ramirez, R.A. 2013. Times they are a’changin: The role of phenology in predator and plant defense interactions for pest suppression. University of Idaho and Washington State University, Entomology Bi-University Guest Seminar Series (BUGSS). MAR 20-22. Moscow, ID

Ramirez, R.A. 2012. Aphid suppression: more to it than predator diversity and plant defense. University of California-Riverside, Entomology Seminar Series. NOV 26. Riverside, CA

Submitted Presentations

Ramirez, R.A. 2013. Predator life-stage complementarity enhances pest suppression in alfalfa. 61st Annual Meeting of the Entomological Society of America, NOV 9-12. Austin, TX

Stephens, E.P. 2012. The contribution of predator life-stage and diversity to secondary pest suppression in alfalfa. 60th Annual Meeting of the Entomological Society of America, NOV 11-14. Knoxville, TN.

Stephens, E.P. 2011. The contribution of predator life-stage and diversity to secondary pest suppression in alfalfa. 59th Annual Meeting of the Entomological Society of America, NOV 13-16. Reno, NV.

Project Outcomes

Project outcomes:

This project provides benefits to Utah alfalfa producers through an increased awareness of the substantial population of natural enemies that reside within alfalfa fields that provide natural pest control. It also raises awareness about the season-long implications that insecticides can have on future pest and natural enemy populations. Phenological surveys show when common natural enemies and pest populations are at peak abundance, indicating which species and life stages are likely to participate in biological control activities. Further, surveys in both insecticide treated and untreated fields show which natural enemy groups are at the greatest risk of being impacted by early-season alfalfa weevil insecticides. Out of the major predator groups analyzed, damsel bugs were affected the most due to an early-occurring phenology, while the late-occurring big eyed bugs were relatively unaffected. Lady beetle populations responded to outbreaks of pea aphids after alfalfa weevil larvae were the most problematic and were not seriously affected by alfalfa weevil treatments.  In addition, results from yield assessments showed no significant differences in average yield estimates from treated and untreated alfalfa fields. This shows that practicing natural enemy conservation through the judicious use of insecticides can still maintain yield and while also saving on pesticide costs (Figure 3).

Results from our feeding trials suggest that different natural enemy life stages are contributing to pest suppression in different ways. Body size (and consequently age) has long been identified as an important ecological trait. However, functional differentiation through ontogenetic development is not well integrated into biological control concepts. With these results, we show that the diet breadth of natural enemies is limited by their size (and consequently, age). Generally, large instars and adult predators eat both large and small prey items while younger instars were generally limited to smaller prey items. Through partitioning food resources based on size, this study shows one potential mechanism by which adult and immature life stages may fill different ecological niches that contribute to functional diversity.

Through examining species richness and multiple life stages on the suppression of pea aphids, we showed that the presence of multiple life stages can enhance pest suppression. These findings highlight the importance of evaluating the role of immature life stages in biological control activities. These findings fill a gap in knowledge about a significant portion of the natural enemy community given that immature life stages can, at times, be equally or more abundant than adults. 

Economic Analysis

While a formal economic analysis was beyond the scope of this study, it has implications for producer savings through increased awareness of integrated pest management strategies for the control of alfalfa weevil. Often, alfalfa weevil insecticides are applied preventatively as an insurance against the potential for economic damage. However, with this approach the costs of applying an insecticide may actually outweigh the costs associated with yield loss incurred through insect damage. During the 2012 growing season, five of the six fields that received an insecticide treatment for the control of alfalfa weevil had larval populations below economic threshold (19 weevil larvae per sweep). In 2013, all fields treated for alfalfa weevil had populations below economic threshold. In both years, there were no significant differences in average yield between fields treated and untreated for alfalfa weevil. These cases represent areas where potential savings can be gained through an increased understanding of the economic injury level concept and the judicious use of insecticides. Routine field monitoring can help determine when a spray will not be profitable to free up funds for other farming enterprises. In addition, this has the added benefit of conserving beneficial insect populations to help combat future pest outbreaks if insecticides for alfalfa weevils are deemed economically unjustified. 

Farmer Adoption

In February 2013, this work was presented at the Cache County Crop School sponsored by Utah State University Extension. Information about beneficial insects, secondary pest outbreaks of pea aphids in alfalfa, and the benefits of monitoring was presented to an audience of 67 people. After this presentation, forty surveys were handed out to Crop School participants as an assessment of the 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 beneficial insects 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.

In addition, we hope to further advance the awareness of beneficial insect populations in alfalfa by reaching a larger audience through the publication and dissemination of 735 pocket guides discussed in the Publications/Outreach portion of this report.

Recommendations:

Areas needing additional study

Insecticides are suggested to be used once alfalfa weevil larvae reach economic threshold, yet in northern Utah alfalfa production systems this frequently coincides with the timing of the first harvest. Some producers opt to spray early to allow for appropriate pre-harvest intervals, while others will harvest and apply a stubble spray. While this study did not discern between these two strategies due to a low number of replication, it may have implications for natural enemy conservation since it is possible that effects of insecticides on natural enemy populations may vary between the two strategies. In addition, different insecticides and concentrations can also affect the outcome of natural enemy survival or recolonization of treated fields, but these questions would need to be evaluated in studies with greater experimenter control over agricultural practices.

Contrary to previous results found by Evans et.al. (1993), our survey did not show significant differences in pea aphid abundance between treated and untreated fields. It was hypothesized that insecticide-mediated loss of natural enemies would allow for an increase in secondary pests due to reduced predation. However, pea aphid abundance could not be explained with the absence of natural enemies. In some cases, treated fields had very low natural enemy abundance, but also very low numbers of pea aphids. Conversely, some untreated fields recorded high levels of pea aphid abundance, but also high levels of natural enemies. This leaves us with unanswered questions about the nature of these particular pea aphid outbreaks.

Other mechanisms have been implicated in promoting secondary pest outbreaks. In a phenomenon known as hormoligosis, sublethal exposure to insecticides or residues can have confounding effects of either directly or indirectly physically stimulating higher rates of reproduction in pest insects. Hormoligosis has been well documented to cause pest resurgence or secondary pest outbreaks in other systems, but this factor was not controlled for in this study. Soil characteristics and fertilization regimes have also been suggested to increase the likelihood of pest outbreaks through indirectly effecting plant physiology. Fertilization inputs can affect host plant quality and plant defensive chemistry to the extent that crops are amenable for pest survival and reproduction. Future research should include soil nutrient analysis and plant chemistry to compliment studies of aphid outbreaks.

Results from evaluating multi-stage natural enemy communities suggest that the presence of multiple life stages can increase pest suppression. Broadening the scope of these results to other systems and pest insects can help determine if this is a generality among natural enemy communities or a context dependent outcome for pea aphids in alfalfa. 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.