Plant mediated effects on parasitoid efficacy in a banker plant system

Plant mediated effects on parasitoid efficacy in a banker plant system

Summary

The efficiency of parasitoids at suppressing pest aphid populations appears to be reduced when certain plant growth regulators are used. The changes in plant physiology caused by the use of the plant growth regulator could be to blame. Variation in parasitoid size may be a contributing factor, but the most likely cause is the decrease in female to male sex ratio. Changes in plant architecture may also have attributed to the decrease in aphid suppression as the plant growth regulators created areas where aphids were concealed from the parasitoid.

Objectives/Performance Targets

The overall goal of this project is to develop a banker plant system using A. colemani as a sustainable, effective, and economical biological control agent for aphids in greenhouses. To achieve this, the specific objectives are to:
1) Determine the effect of plant architecture on parasitoid attack rate and aphid pest suppression
Rationale: Currently plant growers utilize PGRs to alter plant aesthetics and growth rate. PGRs alter plant architecture affecting herbivore placement on the plant and consequently parasitoid foraging efficiency. For instance, plants with more compact leaves are likely to provide refuges for the pest aphids. The drop in parasitoid foraging efficiency negatively affects parasitoid population growth as A. colemani cannot reproduce without its aphid host. Because so little is known regarding these topics, more research is needed to investigate PGR impacts on herbivore hosts and parasitoid attack rate.
1.1) Determine the effect of plant architecture on parasitoid attack rate and parasitoid and aphid development;
Rationale: This first experiment will be carried out in order to determine the variation in A. colemani’s attack rate as related to the different plant architectures. This experiment is conducted to stimulate an augmentative biocontrol setting where the parasitoids are released right after purchase.
1.2) Determine how crop-plant architecture affects banker plant efficacy
Rationale: This second experiment is to test whether or not the banker plant is providing sufficient parasitoids to decrease aphid numbers. When comparing the results from 1.1 to 1.2, we will be able to determine if parasitoids reared on the banker plant are capable of suppressing pest aphid population numbers at the same rate as the ‘simulated’ augmentative biocontrol methods are.
2) Determine parasitoid preference for and attack rate of pest aphid species compared to BCOA on banker plants.
Rationale: This experiment will be conducted in the lab to determine if, when given the choice, A. colemani attacks M. persicae more than R. padi. Past studies on A. colemani have demonstrated that the parasitoid exhibits preference for the host aphid on which it was reared (van Emden et al. 2002). Because past studies have shown that A. colemani reared on banker plants do indeed suppress pest aphid populations, this choice experiment will show what percentage of parastoids preferentially forage on the pest aphid versus their non-pest host aphid. The data from this study will also demonstrate the efficacy of a banker plant system in comparison to augmentative control.
3) Determine the efficiency of aphid banker plant systems in commercial greenhouse production
Rationale: To this date, the number of banker plants to place in a greenhouse has not been properly established. This experiment will help determine the correct density of banker plants needed within a commercial greenhouse. Banker plant placement and number is important because parasitoids emerging from banker plants must be able to locate and travel to pest aphids on crop plants throughout the greenhouse.

Accomplishments/Milestones

To date, all experiments for objectives 1.1 and 2 have been completed. Results from the first objective indicate that plant growth regulators can negatively affect biological control in two ways. First, some plant growth regulators reduce parasitoid survival to adult and reduce the proportion of females produced. Second, by creating architecturally compact plants, plant growth regulators reduce parasitism of pests. Results from objective 2, the parasitoid preference experiments, indicate that second generation parasitoids of green peach aphids will prefer to parasitize the pest aphid over the non-pest bird cherry oat aphid. These results suggest that the barley banker plant system may be an extremely useful tool for green peach aphid suppression on pepper plants. The experiment for objective 3, which consists of comparing the effectiveness of augmentative biocontrol to conservation biocontrol using the banker plant system, is currently under way.

Impacts and Contributions/Outcomes

This research will help growers understand how they can improve the outcome of biological control programs using banker plant systems against green peach aphids. It will also help shed light on the potential negative effects their chemical inputs (e.g. plant growth regulators) can have on the outcome of biological control. Results have been presented to other scientists at the Entomological Society of America meeting in 2010 and 2011. Results have been presented to growers at national, regional, and state extension workshops. In addition, we are currently preparing two manuscripts to be submitted to peer-reviewed journals.

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