The use of banker plants and the predatory midge Aphidoletes aphidimyza for aphid biocontrol in greenhouse crops.
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 this natural enemy’s efficacy in controlling multiple aphid pest species during simultaneous outbreaks. As part of meeting the latter objective, we conducted several greenhouse experiments on vegetative and flowering plants using single releases of Aphidoletes. These studies showed that high releases of Aphidoletes were able to prevent the population growth of green peach aphid (Myzus persicae), but foxglove aphid (Aulacorthum solani) was not controlled with a single release. These results highlight the potential benefit of having a constant source of Aphidoletes in the greenhouse via banker plants. Banker plant aphid colonies (i.e. Rhopalosiphum padi and Sitobion avenae on barley; pea aphid on fava bean) were initiated in 2011. Pea aphid was eliminated as a possible banker plant aphid due to its defensive dropping behavior, which would make it difficult to maintain in the greenhouse. Further tests with R.padi and S. avenae were delayed by repeated contamination with aphid parasitoids (Aphelinus spp.). However, short periods of exposure to low amounts of an organophosphate insecticide were able to control parasitoids without decimating the aphid colonies. We plan to resume trials testing banker plants in conjunction with Aphidoletes in Spring 2012.
Much of this objective was accomplished in 2010. However, we have yet to conduct predation/ oviposition preference trials between the 2 potential banker plant aphids due to recurring infestations of parasitic wasps (identified as Aphelinus spp.) which wiped out our aphid colonies several times. Parasitoids are thought to be coming into the rearing facility via the barley plants. Though our greenhouse compartments for barley production are aphid-free, other compartments in the facility have had periodic aphid outbreaks, and thus a low level of parasitoids is likely constantly present. Furthermore, Aphelinus parasitoids are more difficult to detect on banker plant aphid colonies (vs. pest aphids), as both aphid species are dark green in color, and are indistinguishable from the black mummies formed by Aphelinus upon casual inspection.
In 2011, solutions to this problem were sought. In the M. persicae and A. solani colonies, small chunks of insecticidal strips containing a volatile organophosphate insecticide (DVDP) are successfully used to eradicate/prevent parasitoid infestation. However, when we attempted this technique with the banker plant aphids, the insecticide killed both the parasitic wasps and the aphids. After a period of trial and error, we determined that short exposure periods to DVDP (i.e. 3 day, max.) would kill the parasitoids, but not all the aphids, thus maintaining the colonies. We hope to resume trials with Aphidoletes and banker plant aphids in early 2012, but expect contamination with Aphelinus spp. to be a recurring issue.
In 2011, colonies of Rhopalosiphum padi and Sitobion avenae (both on barley), and pea aphid (Acyrthosiphon pisum) on fava bean were successfully established for several months (before parasitoid infestations began). Pea aphid, though successfully used for Aphidoletes rearing in the literature, was discovered to be ill-suited to use as a banker plant, for several reasons. First, pea aphids have a defensive behavior that involves dropping off the plant when disturbed. This occurred even when watering the plants, and would make maintaining colonies of this aphid difficult in a commercial setting. Secondly, fava bean itself is not well suited as a banker plant; it grows extremely quickly, and is appropriate for aphid rearing for only 1-2 weeks before the plant needs to be replaced or becomes too tall to be manageable. Thus, this aphid/plant combination was omitted as a potential banker plant. Trials will focus on cereal aphids on barley plants (which can last >3 weeks) in Spring 2012.
No further progress was made on this objective in 2011 due to aphid rearing problems.
Although banker plants could not be tested due to aphid rearing issues, in 2011 we tested innundative releases of Aphidoletes (i.e. single, high-rate releases, which are currently employed in greenhouse operations) for control of multiple pest aphid species. This involved tests on both vegetative and flowering pansy, infested with low levels of M. persicae or A. solani (i.e. 40- 50 aphids per plant). Aphidoletes adults were released at a rate of 1 predator: 4 aphids. These studies showed that a single release of Aphidoletes was able to prevent significant population growth of M. persicae infestations. Conversely, A. solani was not controlled at either stage of plant growth, and reached levels of ca. 250 aphids/plant on flowering pansies after 10 days.
Differences in control were shown to be due to the interaction between preferred feeding sites of the two aphid species and a location preference with Aphidoletes oviposition. Specifically, in both trials, Aphidoletes showed a strong oviposition preference for aphids present on the growing points of the plant, generally ignoring other locations. Myzus persicae was shown to prefer growing points of pansy as feeding sites (i.e. 66% of aphids were found there on vegetative pansy; 46% when plants were in flower), and therefore received a statistically greater number of Aphidoletes eggs than A. solani. Specifically, M. persicae-infested pansies received an average of 49 Aphidoletes eggs per vegetative plant, and an average of 17 eggs per flowering plant. Conversely, A. solani was found to feed primarily on bottom leaves of vegetative plants (56% of aphids), and on open flowers of flowering plants (52%). Subsequently, this aphid pest received an average of only 3.5 eggs/vegetative plant and 3 eggs/flowering plant. Eggs that were deposited on A. solani infested plants were generally found on the growing points, despite the low numbers of aphids found there.
In both experiments, A. solani populations generally increased at slower rate than M. persicae, suggesting that there may have been time for a second release of Aphidoletes to control A. solani before numbers of this pest got out of control. With Aphidoletes larvae decreasing the number of potential oviposition sites on M. persicae infested plants through predation, a second Aphidoletes release would likely result in higher numbers of eggs being laid on A. solani colonies. These results indirectly highlight the potential benefit of having a constant source of Aphidoletes in the greenhouse via banker plants. Control of A. solani may only be achieved through prophylactic control of aphid populations entering the greenhouse, or through constant attack on aphids from established populations of natural enemies. Our results show that current methods (single releases of high, “curative” rates of Aphidoletes) are ineffective for this challenging pest, especially when M. persicae is present in the same greenhouse.
- • January-April 2011: a pea aphid colony was successfully established and maintained, but was eliminated as a potential banker plant candidate due to its defensive dropping behavior. Sitobion avenae was successfully reared and remains a promising banker plant candidate (along with R. padi). • April-May 2011: Greenhouse tests of innundative releases of Aphidoletes for control of pest aphids were conducted on vegetative plants; control of A. solani (foxglove aphid) was insufficient. • May-Sept. 2011: Plans to conduct greenhouse trials comparing the efficacy of S. avenae and R. padi as banker plants for Aphidoletes were thrown off when Aphelinus parasitoids repeatedly contaminated the aphid colonies. • October 2011: Problems encountered rearing cereal aphids during the summer months (i.e. parasitoid contamination) were resolved using short exposure to pesticide strips. Banker plant aphid colonies are currently being built back up. • September-October 2011: Greenhouse tests of innundative releases of Aphidoletes for control of pest aphids were conducted in a flowering crop; results confirmed those of the trial on vegetative plants.
- • Winter 2012: Establish Aphidoletes populations on the 2 candidate banker plant systems to determine suitability for greenhouse use. • Spring /Early Summer 2012: Finish oviposition preference experiments in greenhouse (i.e. comparisons between R. padi and S. avenae banker plants); conduct dispersal experiments from 2 candidate banker plant systems. • Fall 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, as Aphidoletes is used currently in greenhouse floriculture production, this natural enemy will not sufficiently control the emerging pest of the foxglove aphid (A. solani), especially when green peach aphid (M. persicae) is present. This work illustrates that failures of biocontrol are not necessarily due to an ineffective natural enemy, but rather due to our failure to fully understand its biology. Understanding the oviposition behavior of Aphidoletes allows us to potentially develop more effective strategies, such as banker plants, which would provide constant, prophylactic levels of this natural enemy (vs. attempting to use it in a “curative” capacity).
Results of these experiments have been presented to growers, biocontrol specialists and researchers when I was invited to the International Organization for Biological Control (IOBC) annual Meeting in the UK in September, 2011. Results were also published in the IOBC Bulletin (IOBC/wprs Bulletin 68: 85-88), which is widely read by biocontrol researchers and practitioners. Further results were presented in the Greenhouse Biocontrol seminar of the Entomological Society of America Annual Meeting in November, 2011. Additionally, I was invited to write an article on the general use of banker plants for the IOBC/nrs newsletter (Volume 33(3): 2).
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