Enhancing Biological Control With Insectary Plantings

Enhancing Biological Control With Insectary Plantings

Summary

A large-scale investigation of aphid and hoverfly distribution in commercial broccoli containing blocks of alyssum is reported, including aphid and hoverfly egg spatial distributions relative to flowers, hoverfly egg numbers on aphid-baited plants and adult numbers in pan traps. Higher numbers of eggs were laid on plants close to the flower blocks, but this trend was not apparent in bait plant data. Progress has been made with hoverfly culturing and an experimental regime to investigate predator-prey interactions is under development. Analyses of threshold aphid densities for oviposition and patterns of response of ovipositing females to aphids is continuing.

Objectives/Performance Targets

Overall Project Objectives from Original Proposal
1. To evaluate the relative attractiveness of selected insectary plants to entomophagous arthropods and key insect pests
2. To evaluate the potential of using beneficial insectary plants to enhance biological control of specific insect pests in broccoli production systems, including: (1) the cabbage aphid complex and (2) the worm complex.
3. To develop a multi-faceted educational program for growers and agricultural professionals on integrating beneficial insectary plantings into various kinds of farming operations to enhance biological pest control.
Specific Research Objectives for 2001
We have previously reported on Project Objectives 1 and 2 above. This year’s report focuses on work associated with Objective 2. The following are specific sub-objectives for the 2001 season.
1. To quantify the within-field effect that added flowering plants have on the attraction and oviposition activities of hoverflies, and on the abundance of other arthropod natural enemies and herbivores in broccoli.
2. To evaluate the prey-finding and oviposition behaviors of adult hoverflies in commercial broccoli fields.
3. To evaluate the ability of hoverflies to limit populations of aphids on single plants in controlled cage and field experiments.

Accomplishments/Milestones

Results and Discussion
Objective 1. To quantify the within-field effect that added flowering plants have on the attraction and oviposition activities of hoverflies, and on the abundance of other arthropod natural enemies and herbivores in broccoli.
Crop plants. Hoverfly eggs first appeared in low numbers, 2-3 weeks before harvest, but did not appear on broccoli crop plant leaves to any considerable extent until the last few sampling dates, within a week of harvest (Figures 1 and 2). Figure 2 also shows a trend of greater oviposition at distances closer to the flowers on the final sampling date. Through the rearing of collected eggs, the majority of eggs on all sampling dates were found to be either Eupeodes fumipennis or Sphaerophoria sulphuripes (data not shown).

Figure 1a. Mean number of aphids and hoverfly eggs per crop plant leaf for all sampling dates; block, direction and distance of sampling points pooled (numbers in graph field are specific values for # eggs at each date).

Figure 1b. Relative proportions of plants with aphids and hoverfly eggs for all sampling dates; block, direction and distance of sampling points pooled.

Figure 2. Mean number of hoverfly eggs per crop plant leaf, for all sampling dates and distances, both blocks and both directions pooled, not correcting for aphids.
Bait plants. Hoverfly eggs appeared about the same time on the bait plants as on the leaves of the broccoli crop (Figure 3). However, the trend of greater oviposition on leaves at sampling points closer to the flowers was not observed on the latest dates for this sampling method. This was true for both blocks, and all transect directions (graphs not shown).

Figure 3. Mean number of hoverfly eggs per bait plant leaf, by distance on transect and date, all transects pooled, not correcting for aphids.

Pan traps. Hoverfly adults appeared in the pan traps in the broccoli crop only after the alyssum flowers were planted (Figure 4). Large numbers were found in the traps by the 6th sampling date, two weeks later. At this time in the broccoli crop season, capture rate peaked at distances 5 m from the flowers, and gradually decreased out to 80 m. Traps at distances of 0 m and 1 m from the flowers also trapped fewer flies on this date.
More than 90% of the flies captured in the traps on this date were Toxomerus marginatus (data not shown). The data from the final three pan trapping dates has not been processed yet. Yellow sticky card traps did not capture local hoverfly species (sticky trap data not shown). Several species of ladybird beetles were captured in large numbers, but no distance relationship to flowers was observed. Green lacewings were trapped at distances close to the flower plots, but in lesser numbers than those seen for the ladybird beetles.

Figure 4. Mean number of hoverfly adults trapped per trap day, by distance on transect and date, all transects pooled.

Insect Contamination in Broccoli Heads. Out of 192 broccoli heads sampled in the no-aphicide strip, only 7% contained aphids. Of these, 14 heads, none had more than 10 aphids. When these data are broken down by distance class, no effect with distance to flowers was seen (Figure 5).

Figure 5. Relative percentage of sampled heads with aphids at each distance from the flower blocks.
Objective 2. To evaluate the prey-finding and oviposition behaviors of adult hoverflies.
Processing of data collected for this objective in the 2 field trials in 2000 is continuing and certain trends are already evident. The dataset is large, consisting of aphid and hoverfly egg counts on each individual leaf on each sampled plant. The analysis should be complete in the spring of 2002. The information in Figure 6 is included here to show one trend that has been observed in many of the sampling dates looked at so far, that is, plants with less than 50 aphids did not have hoverfly eggs. No such threshold has yet been seen for individual leaves, many leaves with no aphids had hoverfly eggs. No peaks in oviposition response have been observed per leaf or per plant, rather, what has been seen is a sustained increase in number of eggs with an increasing numbers of aphids (Figure 6). Remaining data manipulations and analyses to address this objective are outlined in the ‘Future Work’ section below.

Figure 6. Number of hoverfly eggs per plant on plants with different densities of aphids, Stahlbush Island North Trial, July 1, 2000.

Objective 3. To evaluate the ability of local hoverflies to limit populations of aphids on single plants in controlled cage and field experiments.
Reliable data were not obtained from this run of the experiment because of problems with larval mortality and migration from the plants (data not shown). For those plants where larvae did remain on the plants, the maximal aphid mortality observed was 48 aphids in the first 12 hours of the experiment.
Discussion
The first objective of quantifying the effect that added flowering plants have on the activity of predatory hoverflies was addressed by measuring the amount of: oviposition on crop plants, oviposition on bait plants, aphids on crop plants, aphids in broccoli heads, and trapped adult hoverflies at different distances from the flowering plants over most of the crop’s development.
Oviposition did not occur to any significant extent until just before (less than one week) broccoli harvest. Much lower levels of oviposition were recorded on both crop and bait plant leaves for the two weeks prior to this period. This same result was seen in both field trials reported on in the 2000 annual report to SARE. It was hypothesized in the discussion of the 2000 report that providing floral resources sooner, and in greater amounts might induce earlier, and greater amounts of, oviposition. The 2001 trial used over 6 times as many flowers provided approximately one month earlier in the broccoli field. Although the onset of hoverfly oviposition occurred at the same time as that seen in the 2000 trials (i.e. 3 weeks prior to harvest), the relative proportion of plants with hoverflies was higher at the end of the season (Figure 1b). Also, unlike the 2000 trials, oviposition on crop plants was generally greater at sampling points closer to the flowers.
This same trend, of reduced oviposition at distances greater from the flowers, was not seen with the bait plant sampling method however. These differences may not be surprising when the differences in plant quality, size and number between these two sampling methods are considered. Additionally, since the oviposition activities of predatory hoverflies throughout a crop field are also influenced by the relative amounts of aphid hosts at each sampling point, the most appropriate analysis of oviposition response as a function of distance from floral resources will also take into account the number of aphids present at each sampling point. Once these aphid data are included in the analysis, the spatial patterns of oviposition response on both crop and bait plant sampling points at different distances from the flowers may be different from that reported here.
These aphid data, once processed, will also be used to assess the indirect effect that the floral blocks may have on aphid infestations at varying distances from the blocks on the last few sampling dates. Indirect effects on aphid infestation in the broccoli heads were not seen. The very low incidence of aphid-infested broccoli heads at all distances from the flower blocks sampled in the field (in terms of both relative percentage of infested heads and number of aphids in heads) probably indicates that the overall infestation of the heads in this no-aphicide strip was insufficient to detect any distance relationship which may have occurred. Levels of other key arthropod herbivore pests and natural enemies on both crop and bait plants were also too low in the sampling area to identify any spatial or temporal relationship with the added flower blocks.
More than 90% of the flies trapped in the first 6 pan trap sampling dates were the species Toxomerus marginatus, a species which has not been observed to oviposit on broccoli plants with aphids in local broccoli fields. The data from the last three trapping dates remain to be processed, but certain trends are already evident at this point.
The pan trap data from the 6th sampling date, midway through the broccoli season, showed the total number of adult hoverflies trapped (mostly represented by T. marginatus) to taper off from a peak at 5 meters, showing a greater number of hoverflies flying in zones closer to the flower blocks. Very low trap numbers at the 0 meter distance, and moderately low numbers at the 1 meter distance, indicate that flower-mimicking yellow pan traps are less attractive when a dense block of real flowers is in the immediate vicinity.
Although eggs of E. fumipennis, S. sulphuripes and Platycheirus spp. did not appear until 3-4 weeks after the alyssum flowers were transplanted, adults of at least some individuals of these species were trapped within days of flower transplant. The reason for this lag could be either insufficient egg maturation in hoverflies present on these early sampling dates, insufficient numbers and arrangements of aphids to elicit oviposition responses, or a combination of both of these. This will be investigated further by dissecting the ovaries of the preserved specimens trapped on each date to assess egg development over time. These data will then be compared to the information derived from the analysis of oviposition response to varying levels of aphids on crop and bait plant leaves over time, to assess the relative importance of each.
We are continuing to develop appropriate experimental methodology to examine the ability of locally-occuring hoverfly species to limit aphid populations on broccoli. Our success with hoverfly culturing, suggests that this experimental approach can be developed further. The early results demonstrated that a combination of glasshouse conditions and host plant quality (particularly the removal of most leaves) affect hoverfly larval survival. Attempts will be made to enhance microclimate, and to improve the colonization and survival rate of larvae, by using clip cages for an initial period of acclimation.
This technique is could provide valuable, quantitative insights into the rate and patterns of hoverfly predation, interactions with other predators, and the ability of hoverflies to limit aphid population growth and the colonization of broccoli heads. Studies of this type are expected to provide more accurate information than simple voracity studies in Petri dishes, which can only provide minimum and maximum feeding rates.

Impacts and Contributions/Outcomes

Impact of the Results
Laste season predation of cabbage aphids by predactious hoverfly larvae may have an important impact on the suppression of this pest and preventing excessive contamination of broccoli heads by these pests. We have documented high numbers of predacious hoverfly eggs in broccoli prior to harvest and we have developed methods to identify the key predacious hoverfly species. In our field experiments to date, however, we have been unable to demonstrate an enhancement of biological control of aphids by planting blocks of insectary flowers either on the margin of the field or in within the fields. We have made significant steps in understanding the key aspects of oviposition behavior of Eupeoides fumipennis, the major predacious hoverfly species occurring in broccoli in Oregon. There seems to be an “ovipositional threshold” of approximately 50 aphids per plant that is required before the adult female hoverlies will begin to lay eggs on broccoli plants.
Economic Analysis
Because we have not been able to demonstrate actual pest reduction resulting from our efforts to enhance biological control through the provisioning of floral resources, we have not pursued an economic analysis of this approach compared to other pest control practices. We do not feel that an economic analysis would have any relevancy here.
Publications and Outreach
We are planning to produce an OSU Experiment Station Bulletin on the identification of predacious hoverflies in Oregon farm land. We are also preparing three to five manuscripts to be submitted to refereed scientific journals within the next year.
Farmer Adoption
Stahlbush Island Farms, near Corvallis, has planted beneficial insectary flowers in more than 200 acres of broccoli in 2001. However, because of our inability to document beneficial impacts of this practice, we have not pursued an educational program that would encourage grower adoption.
Areas Needing More Study
Objective 1. To quantify the within-field effect that added flowering plants have on the attraction and oviposition activities of hoverflies, and on the abundance of other arthropod natural enemies and herbivores in broccoli.
Three field experiments have now been completed, adding to a number of other investigations over the last five years. A meta-analysis of data from all of these investigations will be undertaken to determine patterns of hoverfly oviposition relative to plant development and aphid abundance. This analysis will be used to build hypotheses for the development of further field experiments. It will also be used to provide a summary of findings to be communicated to growers and to be incorporated into extension publications. Aphid outbreaks have consistently occurred at a late stage in the season, and hoverfly population build up has consequently, also been at a late stage in crop development. Glasshouse investigations will provide information on the contribution that this late phase of predation may make to aphid colonization of broccoli heads. Further field investigations, or field monitoring exercises, which may be the subject of further grants, will determine the probability that hoverflies could limit aphid population growth in the early season outbreaks that are the most damaging to the crop. An outline of these hypotheses and proposed approaches will be provided in the final report.
Objective 2. To evaluate the prey-finding and oviposition behaviors of adult hoverflies in commercial broccoli fields.
Analysis of this large data set will be completed in the next four months. The analysis should provide a basis for predicting the threshold densities of aphids that elicit egg laying. These predictions will be tested in the meta-analysis of independent data sets referred to above. It will also be possible to test these predictions in small, replicated plot experiments where artificial aphid outbreaks will be created in screen houses, and in the open field, at different times in the season. This will also assist with resolution of the question of seasonal timing of hoverfly oviposition, relative to aphid arrival and population build up.
Objective 3. To evaluate the ability of local hoverflies to limit populations of aphids on single plants in controlled cage and field experiments.
This experimental approach will be the main approach undertaken in 2002. Initial work will involve the resolution of experimental design problems mentioned above. Usable experimental designs resulting from this work will then be used to first investigate and quantify the ability of E. fumipennis larvae to limit aphid populations on individual broccoli plants. Potential follow-up studies after this could include investigations of the aphid-limiting abilities of other local hoverfly species, host range studies of the most common local species as well as the role of inter- and intra-specific interactions between larvae on plants with aphids.

Collaborators: