The use of banker plants and the predatory midge Aphidoletes aphidimyza for aphid biocontrol in greenhouse crops.

The use of banker plants and the predatory midge Aphidoletes aphidimyza for aphid biocontrol in greenhouse crops.

Summary

The objective of this project is to investigate the use of a banker plant system to rear the generalist aphid predator Aphidoletes aphidimyza in the greenhouse, as well as determine its efficacy in controlling multiple aphid pest species during simultaneous outbreaks. As part of meeting these objectives, we showed that Aphidoletes larvae prey on foxglove aphid at similar rate as green peach aphid under lab conditions. Banker plant aphid colonies (R. padi on barley) were initiated, and a greenhouse experiment testing Aphidoletes preference for pest vs. banker plant aphids was conducted. Although the results showed that Aphidoletes adults lay statistically more eggs on green peach aphid infested plants vs. foxglove aphid infested plants, both pest aphids were found by the natural enemy, even at low densities. The results also showed that adult Aphidoletes oviposit on banker plant aphids with equal or lesser preference than pest aphids, illustrating that adult Aphidoletes will still find low density patches of pest aphids when an open rearing system is used. Over the winter, Aphidoletes colonies will be established on the candidate banker plant systems, and dispersal from banker plants will be tested in the Spring. Upon presenting our results to date at a large floriculture grower meeting in Ontario, grower interest in Aphidoletes for controlling foxglove aphid was high.

Objectives/Performance Targets

Initial oviposition cage experiments using aphids on embedded leaves in Petri-dishes under lab conditions were conducted as outlined in the proposal. Preliminary results showed zero oviposition in the dishes containing the banker plant aphid R. padi. As we found this result somewhat suspect, we instead conducted greenhouse trials using whole plants, to eliminate any influence the embedded leaves may be having on choice. On our greenhouse “crop” plants, we sought to evaluate whether Aphidoletes females will lay eggs among patches of the two aphid pest species at each of two aphid densities. Thus, pansies were infested with 1 of the 4 aphid treatments , randomized on 4 greenhouse benches (replicated across 2 greenhouse compartments). Treatments included a low density of green peach aphid, a high density of green peach aphid, a low density of foxglove aphid, or a high density of foxglove aphid. A banker plant (barley) infested with R. padi was then placed at the end of each greenhouse bench and adult Aphidoletes (obtained commercially) were released. Allowing 2 nights for Aphidoletes oviposition, the number of eggs and aphids on each leaf of aphid infested plants were then counted. Using this methodology, there was an average of 3.0 Aphidoletes eggs/leaf on plants infested with foxglove aphid, 3.5/leaf on those infested with R. padi, and 4.7 for green peach aphid (with this species being statistically higher than the other two). This shows that commercially reared Aphidoletes will indeed oviposit on R. padi, but choose it with equal or lesser preference than the pest aphids. We consider this a positive trait for a candidate banker plant system, in order to ensure Aphidoletes will disperse from the banker plant to search of pest aphids in the crop (Objective 3).
We also evaluated the capacity of 4th instar Aphidoletes larvae to kill foxglove aphid, and found that they are able to consume an average of 11.8 small, 6 medium, or 3.5 large-sized aphids in 24 h (with a maximum predation capacity of 21 small-sized aphids). These results are similar to previously published predation capacity results on the green peach aphid (ie. maximum capacity =26 small-sized aphids in 24h). We have yet to conduct predation trials on potential banker plant aphids.

We have successfully begun a colony of R. padi on barley (one of our potential banker plant systems) and are in the process of starting a pea aphid colony on fava bean.

In Objective 1, we determined that innundatively released, commercially reared Aphidoletes will oviposit on both heavily infested banker plants and low-density pest aphid populations. We still need to conduct experiments using Aphidoletes distributed using the banker plant system.

No progress has been made on this objective, as its implementation depends on the results of the other objectives.

Accomplishments/Milestones

• August 2010: Aphidoletes predation trials on foxglove aphid were consistent with previous results with green peach aphid. September-October 2010: Oviposition cage trials were put on hold because preliminary results seemed implausible; trials were changed to a greenhouse setting to gain more realistic results. Greenhouse trials for Objective 3 (originally planned for August/September 2010) will have to be conducted next spring (assuming production of Aphidoletes on our banker plant aphid systems is up and running). October-December 2010: Successfully started an R. padi colony on barley; initiating a pea aphid colony.

• Winter 2011: Establish Aphidoletes populations on banker plant aphid systems (min. 4 generations) to determine feasibility and sustainability; Finish Aphidoletes predation trials on banker plant aphid species. Spring /Early Summer 2011: Finish oviposition preference experiments in greenhouse (i.e. with pea aphid banker plants); Conduct dispersal experiments from 2 candidate banker plant systems (we’ve learned July-September is too hot to conduct greenhouse experiments using Aphidoletes or pansies as the experimental plant). Fall 2011: Refine Aphidoletes rearing methods on banker plant systems. 2012: Conduct longer-term greenhouse trials investigating control of aphids using Aphidoletes banker plants; Final analysis and report writing.

Impacts and Contributions/Outcomes

The work to date has shown that a) Aphidoletes larvae attack foxglove aphid at a rate similar to green peach aphid (data previously unrecorded in the literature), b) that Aphidoletes will oviposit on both pest aphid species when present simultaneously in the greenhouse, even at low densities and c) that inclusion of R. padi banker plants in the greenhouse does not appear to affect the ability of Aphidoletes to search for low-density infestations of either pest aphid. These are promising results for the implementation of an open-rearing strategy for Aphidoletes and ultimately the reduction of pesticide use in the greenhouse floriculture industry.
Interest in biological control of aphids has increased lately in the greenhouse community; in late August I was invited to speak at the 2010 Flowers Canada Pest Management Conference in Ontario, Canada, where data on Aphidoletes use for biological control of foxglove aphid was well received by growers (where this pest is as much of a problem as it is in the North Eastern U.S.). This bodes well for the adoption of Aphidoletes and banker plant techniques for aphid control in the North Eastern U.S. upon completion of this project.

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