The use of banker plants and the predatory midge Aphidoletes aphidimyza for aphid biocontrol in greenhouse crops.
The original objectives of this project were to 1) investigate the use of banker plants to rear the aphid predator Aphidoletes aphidimyza in the greenhouse, and 2) determine this natural enemy’s efficacy in controlling multi-species aphid outbreaks. As part of meeting the latter objective, we conducted several greenhouse experiments on ornamental plants using single releases of Aphidoletes. These studies showed that a high release of Aphidoletes was able to prevent population growth of green peach aphid, but not foxglove aphid. This was true across vegetative, budding and flowering stages of plants, and at several times of year. These results demonstrate the difficulty in controlling multiple aphid pests simultaneously, especially when one species may be preferred over the other. As of December 2012, we have initiated greenhouse experiments to determine the effectiveness of Aphidoletes for foxglove aphid alone (without the complicating presence of green peach aphid), in order to give growers more information about pest management options for this emerging pest. We hope to complete these trials by Spring 2013.
Due to the difficulty in culturing aphid banker plants for use by Aphidoletes without contamination by aphid parasitoids, progress on objective 1 has been delayed. While still pursuing this, we are also looking into other methods to increase the efficacy of Aphidoletes. Thus includes investigating the effect of sugar feeding Aphidoletes adults before release, which may increase longevity and fecundity. If this proves beneficial, a sugar solution delivery method could be easily placed in Aphidoletes emergence containers by growers. This would potentially allow growers to help compensate for any larval rearing deficits by biocontrol producers.
Part 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.
In 2011, solutions to this problem were sought. After a period of trial and error, we determined that short exposure periods to DVDP (i.e. 3 days, max.) would kill the parasitoids, but not all the banker plant aphids, thus maintaining the colonies. However, given the delicate nature of Aphidoletes adult midges, it is likely that exposure to DVDP would also harm our target natural enemy. Thus, alternative solutions, such as using hair nets over the banker plants in the greenhouse to prevent parasitoid contamination, are currently being looked at.
In 2011, we determined that the aphids Rhopalosiphum padi and Sitobion avenae would be preferable candidates as banker plant aphids than the pea aphid (Acyrthosiphon pisum). Pea aphid, though successfully used for Aphidoletes rearing in the literature, was discovered to be ill-suited to use as a banker plant in greenhouse situations, mainly due to the defensive dropping behavior of this aphid (which occurred even when just watering the plants), as well as the need to replace their host plant (fava been) much more frequently compared to the barley plants for the grain aphids. Thus, the focus of this project has shifted towards the two cereal aphid banker plants.
Although this objective has not been investigated specifically (due to the banker plant aphid problems discussed above), our trials with single releases of Aphidoletes adults (see below) have given us some clues as to the outcome of this objective. Aphidoletes adults prefer to oviposit on a) high density aphid patches, and b) aphid patches located on new growth of plants. Thus, banker plants themselves (with their high density of non-pest aphids) would likely be more attractive to Aphidoletes than plants infested with foxglove aphid (which tend to colonize lower leaves of plants, and are generally found in less-dense colonies). Additionally, both the greenhouse trials we conducted and some preliminary lab trials suggest that there is no egg deterrence in Aphidoletes. This suggests that high numbers of eggs could repeatedly be laid on banker plants, without females being forced to search out less preferred foxglove aphid colonies. However, Aphidoletes reared on banker plants will likely seek out colonies of green peach aphid in the greenhouse (as this pest species is found on higher densities on new growth of plants, a preferred location). Future research should focus on banker plants for prophylactic prevention of green peach aphid outbreaks, which is likely to be more successful than banker plants for foxglove aphid control.
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 pansies infested with low levels of M. persicae or A. solani. Vegetative, budding, and flowering plants were tested over time to determine if crop stage and subsequent changes in aphid feeding sites altered results. Aphidoletes adults were released at a rate of 1 predator: 10 aphids. These studies showed that a single, high release of Aphidoletes was able to prevent significant population growth of M. persicae infestations in all experiments (with 74-99.7% control achieved). Conversely, control of A. solani was extremely variable (36-84% control across all experiments), with an acceptable suppression of the population seen in only 1 experiment (Figures 1-3).
In all experiments, A. solani populations increased at slower rate than M. persicae (Figures 1-3), suggesting that there may have been time for a second release of Aphidoletes before A. solani populations got out of control. With Aphidoletes larvae decreasing the number of potential oviposition sites on M. persicae infested plants (due to larval deterrence), a second release of Aphidoletes may have resulted in higher numbers of eggs being laid on A. solani infested plants. These results strongly suggest a potential beneficial role for banker plants in the control of multiple aphid species: the prophylactic presence of banker plants may provide earlier oviposition in, and control of, green peach aphid colonies, allowing subsequent innundative releases of Aphidoletes to target foxglove aphid colonies as new oviposition sites. Thus, a combination of banker plants, plus innundative releases, may prove to be the optimal release strategy for this biocontrol agent against mixed aphid populations.
Given our observations that Aphidoletes prefers to oviposit on colonies of green peach aphid, further greenhouse studies were needed to determine the efficacy of this biocontrol agent for outbreaks of foxglove aphid alone. Without the distraction of high-density aphid colonies on the growing points of plants provided by green peach aphid (the preferred oviposition site of Aphidoletes), our hypothesis is that adult midges may be forced to search for foxglove aphid colonies present on lower leaves of plants. These follow up experiments are extremely important in our goal of giving growers as much information about Aphidoletes as a control agent for different aphid outbreak situations. Trials are currently under way, and should be completed in March 2013.
Previous research has shown that while 40-50% of the Aphidoletes egg compliment is based on larval nutrition, the other 50-60% is based on feeding of adults, who consume honeydew from aphid colonies in nature (Sell and Kuo-Sell, 1987). In previous studies of Aphidoletes in the lab, 5% sucrose solutions have been provided (using tissue paper soaked in the solution) (ex. Havelka and Zemek 1999) to promote oviposition. Thus, it seems that sugar feeding of adults could provide an important boost in efficacy of this biological control agent. This is especially true given that insufficient feeding in the larval stage during mass rearing can easily occur (due to decreased quality of host plants and aphids during winter months in greenhouse facilities, competition between larvae, etc). This can thus lead to a significant reduction in egg production in adults from reduced larval reserves (Sell and Kuo-Sell 1987). Thus, we propose that a simple feeding method could be supplemented onto current commercial shipments of Aphidoletes by growers upon receipt, improving the quality of adults and helping to mediate any larval insufficiencies that may have been suffered during rearing.
Lab trials for this objective have already been initiated. Preliminary results show that 50% of a midge population held in an emergence cage will visit a tissue paper wick soaked in a 5% sugar solution. We are currently conducting trials comparing the survival and daily egg production of individual females from sugar fed and non-sugar fed colonies of Aphidoletes. Preliminary results from these experiments are expected in January 2013. If a beneficial trend in sugar feeding is seen, we will scale up to greenhouse trials, comparing the efficacy of adult Aphidoletes which have been held and sugar-fed compared with standard release methods.
• May 2012: greenhouse experiments on innundative releases of Aphidoletes for control of multiple aphid species were finished. Experiments looked at 3 different crop stages in total (vegetative, budding, flowering); trends were the same regardless of plant stage.
• June 2012: Preliminary lab trials were conducted to investigate egg deterrence in Aphidoletes. Preliminary results indicate that females are not deterred from ovipositing in an aphid colonies where eggs from a conspecific female already exist, suggesting that banker plants will repeatedly receive high numbers of eggs. This could have repercussions on foxglove aphid control.
• December 2012: The first of two greenhouse experiments assessing the effectiveness of Aphidoletes for controlling foxglove aphid in the presence, as well as absence, of green peach aphid was completed. A preliminary analysis of the data suggests that Aphidoletes was slightly more effective for controlling foxglove aphid when it is the only aphid pest in the greenhouse. However, similar to previous results, only 29-55% control was achieved (while 84-98% of green peach aphid was achieved in compartments where both aphids were present). This confirms our previous studies suggesting that Aphidoletes is a better biological control agent for green peach aphid.
• July 2012: Lab experiments investigating the effects of sugar feeding on Aphidoletes survival and fecundity were initiated. Preliminary results suggest that ca. 50% of an adult Aphidoletes population will visit sugar solution provided.
Impacts and Contributions/Outcomes
The work to date has shown that Aphidoletes may 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 impacts of prey choices and natural enemy behavior.
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, and a full research article has been submitted to the journal Biological Control and has been accepted with minor revisions. Further results were presented in the Greenhouse Biocontrol seminar of the Entomological Society of America Annual Meeting in November, 2011, and at Floriculture Field Day at Cornell University in May 2012 (attended by growers from NY and surrounding states). 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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