Final report for ONE20-380
Banker plants provide resources for arthropod natural enemies when target pests are scarce, enabling in-greenhouse reproduction of the natural enemies for continuous production and reducing or eliminating the costs of otherwise-required weekly natural enemy shipments. There are banker plant systems for small aphid pest species and for large aphid pest species. But they are not compatible and cannot be used in the same greenhouse simultaneously because they both use aphid species that only infest barley as the host plant. This common host plant allows the aphid species from one system to eventually displace the aphid species of the other system. This project evaluated the use of a novel banker plant system, using pea aphids (a large aphid) on potted fava bean plants as hosts for the parasitoid Aphidius ervi. Because this novel system uses fava beans rather than barley, it would be compatible with the other barley-based banker plant system for small aphids. Experiments and observations were done in research greenhouses and at the partner grower's Spring crop production to determine optimal timing and production of the fava beans, the pea aphids, and the parasitoids. The resulting banker plant system was evaluated in a commercial greenhouse during Spring ornamental crop production. This new banker plant system makes it possible for both A. colemani and A. ervi to co-exist, reproduce, and provide successful cost-effective biocontrol of all common greenhouse aphid pests. A fact sheet for growers has been prepared and will be used in grower presentations that will outline the details of the system.
This project sought to develop and test a cost-effective, relatively simple system for in-greenhouse production of a parasitoid species, Aphidius ervi, that attacks larger pest aphids such as foxglove or potato aphid, using non-pest pea aphids on fava bean plants. The project combined research to develop the system with a commercial observations and trials to test the system. Three objectives were accomplished:
1. The first determined the best growth stage of fava bean plant to infest with pea aphids to balance maximum aphid production with minimal plant decline. We also determined the best time to release the wasps after the plants have been infested with the pea aphids.
2. The second checked that Aphidius colemani parasitoids, commonly used for small aphid biocontrol, do not outcompete A. ervi on the fava bean banker plants with pea aphids.
3. The third evaluated how to cope with the frequent behavior of pea aphids to drop off of the fava bean plants at the slightest disturbance.
Using research and observational results, the system was successfully employed by the grower partner in their commercial greenhouse on Spring crops. A fact sheet about the system was prepared to provide guidance for greenhouse growers. The successful system should give growers a way to simultaneously produce two parasitoid species, one for large aphids and another for small aphids, providing complete biocontrol of aphids.
Crop loss from cosmetic damage by greenhouse pests can occur quickly, so greenhouse flower growers apply more pesticides per cubic meter than any other commodity to protect their crops (Smith 1998). Such intense pesticide use is an unsustainable approach and leads to several pesticide-related problems. Biological control of greenhouse pest such as aphids has been shown to be an effective alternative and the popularity of biocontrol has been surging among NE greenhouse growers. However, the required weekly, inundative releases of commercial natural enemies can be expensive for many growers. Banker plants provide a cost-effective alternative. Banker plants are an open rearing system for natural enemies which provide alternate non-pest food sources for the natural enemies when target pest levels are low, so that natural enemies can reproduce in the greenhouse. This avoids the need for repeated parasitoid purchases and shipping costs. Banker plants of potted barley infested with non-pest Rhopalosiphum padi aphids are commonly used to sustain populations of an aphid parasitoid, Aphidius colemani. But A. colemani only attacks small-sized aphids, and greenhouse crops are subject to a complex of aphid pests, including important large species such as foxglove and potato aphid. A second parasitoid, Aphidius ervi, is an effective parasitoid of large-sized aphids, and a potted barley banker plant system for these is also available, using the non-pest aphid Sitobion avenae. However, the two banker systems are incompatible because the host aphid for one overtakes the host aphid for the other if they co-occur in the same greenhouse, because both banker plant systems use the same plant species. We used research and observations to develop a novel banker plant system for A. ervi, using potted fava bean plants infested with pea aphids. This system uses a different plant species than barley (i.e., fava bean) to avoid the non-pest aphids from interfering with each other. Pea aphids do not survive on barley, and R. padi aphids do not survive on fava beans. Thus, this new banker plant system should make it possible for both A. colemani and A. ervi to co-exist, reproduce, and provide successful cost-effective biocontrol of nearly all greenhouse aphid pests.
- - Producer
The first experiment determined the best growth stage of fava bean (Vicia faba) plant to infest with pea aphids (Acyrthosiphon pisum) that would balance maximum aphid production with minimal plant decline. We used fava bean plants within individual bugdorm cages in a research greenhouse. A completely randomized 2x2 factorial design was used, with all combinations of each treatment replicated 4 times. For the two treatments to measure the main effect of plant age, we used potted fava bean plants that were either young at no more than a week from sprouting, or mature at at least 2 weeks. For the two treatments to measure the main effect of initial pea aphid density, plants were infested with a low treatment of only 5 aphids, or a high treatment of at least 30 aphids. Thus there were four treatments – young plants with few aphids, young plants with abundant aphids, mature plants with few aphids, and mature plants with abundant aphids, each treatment replicated 4 times. Plant health was qualitatively assessed every 5 days for at least a month or until plants collapsed. Aphids on 5 leaves per plant were counted non-destructively every 5 days. For each sample date, a 2-factor ANOVA was used to test for main effects of plant age and initial aphid density as well as possible interactions.
To determine the best interval for wasp release after aphids are added to the banker plants, pea-aphid-infested fava bean plants were housed singly within bugdorm cages with 3 cages per time period treatment. Twenty A. ervi wasps were released into the cages after three time periods: immediately after aphid infestation, or 1 or 2 weeks after aphid infestation. Numbers of mummies in each cage were counted 3 weeks after parasitoid release.
The second experiment evaluated whether A. colemani will compete with A. ervi for pea aphids on the banker plants. Pea-aphid-infested banker plants were exposed to an abundance of A. colemani wasps for 2 weeks and subsequently checked for signs of parasitism. Additionally, our partner grower who is trialing this banker plant system sent us mummified pea aphids from his banker plants so we could identify the parasitoids that emerged.
The third objective evaluated ways to cope with frequent pea aphid disappearance from the banker plants. Observations in both research and commercial greenhouses were used to document this phenomenon in terms of how sensitive the aphids are to being jostled on their plants, how many banker plants can be affected, and the escape response of the pea aphids when they detect the parasitoids.
Beginning in February, 2021, fava bean/pea aphid banker plants will be grown and placed throughout the greenhouse at Mischler’s Florist, as well as the usual barley banker plants for A. colemani. Thus, banker plants for parasitoids of both large and small aphid pests will be simultaneously active. Partner grower Mark Yadon scouted his crops for aphid and parasitoid activity, collected mummies to send to Sanderson for identification, and grow at least one additional round of banker plants to replace old ones. Yadon assessed of the success of the trial, including degree of simplicity, criticisms, and possible improvements. Yadon used the system again in 2022, with some improvements to increase numbers of parasitoids at the start of his crop and avoid aphid disappearance on his banker plants.
Due to delays caused by COVID-19 issues, our timeline for the research steps was altered.
Here are results from the first experiment: A. to determine the best growth stage of fava bean plant to infest with pea aphids; B. best interval for wasp release after aphids are added to the banker plants:
A. Plants that were infested with a high number of pea aphids resulted in more aphids than plants that were infested with a low level of aphids, and the plants were able to sustain this level for at least 3 weeks. Among plants that were infested with the high number of aphids, more aphids were produced on plants that were infested 14 days after emergence, compared with plants infested at emergence or a week after emergence. The 14-day-old plants generated more aphids and were still healthy after 3 weeks. Thus, for maximum aphid production for eventual abundant wasp production, growers should grow the fava bean plants for 2 weeks before infesting with aphids.
B. Using plants that maintained their aphids in the presence of wasps, we determined the best time to release the wasps after the plants have been infested with aphids. A. ervi mummy production was greatest when wasps were released one week after initial aphid infestation, using the higher initial aphid infestation level noted from the first experiment in 2020. This resulted in at least 100 mummies per plant after two weeks, with likely additional aphids that would eventually become mummified but were not mummified at the end of two weeks.
For the second objective, pea aphids on more than 20 fava bean plants were exposed to an abundance of A. colemani, but no parasitized aphids resulted. Moreover, samples of pea aphid mummies collected during the 2021 Spring crop cycle at the partner grower’s greenhouse revealed no A. colemani. Thus, it appears that the pea aphid/fava bean banker plants for A. ervi production are compatible with the barley banker plants for A. colemani production.
In our experiments, as well as from the experiences of some growers who have contacted us, for unknown reasons the pea aphids on a fair number of the banker plants disappear after the wasps are released, and these plants obviously produce no wasps. Yet an abundance of wasps may be produced on other fava bean banker plants concurrently in the same greenhouse. We sought to investigate this phenomenon.
First, we, and our grower partners, have consistently observed that pea aphids will readily fall of fava bean plants if the plant is even slightly jostled or disturbed. Thus to successfully infest the plants with pea aphids, the fava bean plants must first be situated in their final location in the greenhouse and remain unmoved. Then aphids can be added to them. But any slight jostling or movement of the plants will cause most or all of the pea aphids to drop off the plant. This dropping behavior due to jostling could be responsible for some of the disappearance of pea aphids from the banker plants.
Second, we also carefully observed pea aphid behavior in reaction to the presence of A. ervi wasps. Individual wasps were released onto aphid infested plants to observe the interaction between the foraging wasp and the pea aphids. In all cases, as soon as the wasp was detected, the aphids either fell off the leaves or began quickly walking down the plant stems to exit the plant. However, twice we have set up fava bean banker plants on an open greenhouse, successfully infested these with pea aphids, allowed the aphid population to establish and begin to grow for one week, and then released A. ervi wasps. Many of the plants subsequently lost aphids and had few or no mummies after two weeks, though always some plants maintained an abundance of aphids and produced well over 100 mummies. We still do not understand this phenomenon of aphid disappearance on some banker plants but not others.
Our partner grower has apparently avoided this problem by carrying some pea aphids from his aphid stock culture to add to his banker plants weekly. Apparently his wasp production has remained relatively consistent by this weekly addition of aphids.
Partner grower experience in 2021:
Our grower partner used fava bean banker plants during the Spring 2021 growing season and monitored the general numbers of mummies on his banker plants and on his cash crop. He collected and sent us samples of mummies from his banker plants and from his crop for wasp identification. He was satisfied with the degree of foxglove aphid control that the banker plants provided. In 2022 he started his banker plant releases sooner to better time the wasp activity with the stage of his seedling production of perennial crop species.
Objective 1: We determined that for maximum aphid production for eventual abundant wasp production, growers should grow the fava bean plants for 2 weeks before infesting with aphids. Also, for optimal wasp production on the banker plants, releases of A. ervi should begin 1 week after pea aphids are added to the fava bean plants.
Objective 2: Our research results, and samples collected from our grower partner's greenhouse in which both A. colemani and A. ervi were active, indicate that the pea aphid/fava bean banker plants for A. ervi production are compatible with the barley banker plants for A. colemani production. No A. colemani emerged from the pea aphids; only A. ervi emerged.
Objective 3: Despite our research observations, we still do not understand this phenomenon of aphid disappearance on some banker plants but not others. However, our grower partner is apparently successful in overcoming this problem by adding additional pea aphids to his banker plants weekly. Continuous wasp production is achieved on all his banker plants. Even when aphids disappear from some of the banker plants, sufficient wasp production is usually produced on other banker plants.
A fact sheet has been prepared that outlines the use of fava bean banker plants for in-greenhouse reproduction of Aphidius ervi.
Education & Outreach Activities and Participation Summary
A fact sheet has been produced to provide growers with details of the use of pea aphid/fava bean banker plants. The fact sheet will be available online at the Cornell Greenhouse Horticulture website. We will also alert Cornell Cooperative Extension educators with greenhouse responsibilities about the fact sheet. Also, though beyond the timeline of this project, during the winter greenhouse conferences in New York State in 2023, usually held on Long Island, Albany, Buffalo, and the Hudson Valley, the results of this project will be presented. A demonstration of this banker plant system will be done at the 2023 IPM In-Depth conference at Cornell. Cooperative Extension educators and New York State IPM personnel may wish to conduct local demonstration events on the use of these banker plants.
The partner grower is very satisfied with this banker plants system in that it provides adequate biological control of almost all aphid pests in his greenhouse. Growers in at least two other states are also using this system.
The growers who are using this new banker plants system are already experienced with using biological control on their crops. They have the motivation and creativity to make the system work for them under their conditions. This is important for the success of this system, because extra care is needed to cope with aphid disappearance from banker plants. This system may not work for growers who lack sufficient motivation.
The biggest challenge with this banker plant system is the unwanted disappearance of the pea aphids from the banker plants. The aphids readily fall off or leave the plants if the plant is even gently jostled. Also, though we observed aphid defensive dispersal when they detect wasps, we don't understand why some banker plants produce hundreds of parasitoids, while other identical plants in the same greenhouse at the same time lose all aphids and thus produce no mummies. The partner grower overcomes this problem by re-infesting each banker plant weekly with more aphids. But this requires commitment that may be beyond other growers.