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. The open-rearing of Aphidoletes proved difficult, due to continual colonization by natural populations of Aphelinus parastioids. More research into host plant/aphid combinations that will promote non-competition with aphid parasitoids are needed for this technique to be successful. To meet 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 consistently and signficantly reduce population growth of green peach aphid, but not foxglove aphid, demonstrating the difficulty in controlling multiple aphid pests simultaneously. Although Aphidoletes was better able to control foxglove aphid in the absence of green peach aphid, high, single releases of this natural enemy were not enough to adequately control this pest. More research into the addition of a specialist parasitoid plus Aphidoletes are needed for foxglove aphid control. A preliminary investigation into sugar-feeding to increase survival and oviposition of commercially reared Aphidoletes adults did not show any significant benefit.
Aphids are listed among the top 10 greenhouse pests and diseases in the Northeastern U.S. Pest Management Strategic Plan (Hall et al. 2008). Aphid feeding results in wilting, reduced growth, and distortion of plant tissue, as well as transmission of plant viruses; there is also indirect damage from cast skins and honeydew production (ex. growth of sooty molds). Crop loss from cosmetic damage can occur quickly, so greenhouse flower growers apply more pesticides than any other commodity (Smith 1998). Aphids contribute greatly to this pesticide use: in a 1996 survey in Massachusetts, the number of pesticide applications for aphids was 3 per crop (Smith 1998). Such intense pesticide use is unsustainable and leads to several pesticide related issues. Chemical control of green peach aphid has become notoriously difficult in the past decade; it is strongly resistant to at least 3 classes of insecticide (Foster et al. 2000), and there are also concerns surrounding worker safety in an enclosed environment. Clearly, alternatives to pesticides are needed in this industry.
Parasitic wasps are often considered by growers to be the front line of defense in the biological control of aphids. However, the most “popular” parasitoid, Aphidius colemani, does not attack all species of aphid pests in greenhouses — they only parasitize small-sized species (e.g. green peach aphid, Myzus persicae). This presents a significant limitation in floriculture crops, which are subject to a large complex of aphid pests (see Van Driesche et al., 2008). A second parasitoid, Aphidius ervi, can parasitize larged-sized aphid species (e.g. foxglove aphid, Aulacorthum solani), but these are 4x the price of A. colemani, which is often cost-prohibitive to growers. Furthermore, the use of either of these species requires that growers correctly identify the aphid pest (or mix of pests) infesting their greenhouse at any one time. Misidentification can lead to failures of biocontrol, sending growers back to pesticide use.
Aphidoletes aphidimyza is a generalist aphid predator, the larvae of which are known to feed on over 75 different species of aphids (Harris 1973). The use of this predator could potentially greatly simplify biocontrol of aphids in floriculture crops, as proper aphid identification would not be necessary. Though different types of aphid prey have been studied extensively for this predator to optimize mass rearing, multiple aphid species have not been studied in the context of biocontrol efficacy. Thus, detailed greenhouse studies are needed before this natural enemy can be recommended as an efficacious, easy-to-use alternative to parasitoids for multi-species aphid outbreaks.
Weekly, innundative releases of any of commercial natural enemy can prove expensive for many growers. Thus, more cost-effective means of adding natural enemies would benefit the greenhouse industry. Banker plants are an open rearing system for natural enemies which provide alternate food sources if pest aphids are scarce. The Northeastern U.S. Pest Management Strategic Plan (Hall et al. 2008) lists “development of banker plant biological control systems for a broad range of insect pests” as the first Research Priority. Banker plants are commercially available for the aphid parasitoid, Aphidius colemani, but have not yet been successfully adapted for Aphidoletes. Preliminary research conducted by Bennison and Coreless (1992) showed that banker plants could produce 4x as many Aphidoletes as by introduction, which suggests this control strategy is promising, but requires further development.
This project seeks to investigate the ability of Aphidoletes to control mixed-aphid species outbreaks and potential incorporation into a banker plant system in order to give growers simpler, more effective tools against aphid infestations.
- Objective 1. Determine the relative preference of Aphidoletes for pest vs. non-pestiferous aphids and predation capacity on foxglove aphid. This objective is necessary to be sure that all aphid species sufficiently attacked (both the pest species and those to be used on the banker plant). The banker plant aphid should not be more attractive than the pest aphids, or the predator will not search for prey within the target crop. Pest aphids used were green peach aphid and foxglove aphid, the top two greenhouse aphid pests in NY state. Predation capacity on the pest aphid foxglove aphid (A. solani) has not previously been characterized.
- Objective 2. To investigate the ability of Aphidoletes to establish and sustainably reproduce on various aphid-banker plants. This will include investigations of various cereal aphids on grains (i.e. Rhopalosiphum padi and Sitobion avenae on varieties of barley), as well as the pea aphid (Acyrthosiphon pisum) on fava bean. Generally, these aphid species are not able to transfer to floriculture crops.
- Objective 3. To determine if Aphidoletes will disperse from a banker system and attack patches of low-density pest aphids, while also continuing to oviposit on the banker plant. This step is necessary to determine the effectiveness of banker plants using this natural enemy, as well as the sustainability of the open-rearing system in the greenhouse.
- Objective 4. To compare the effectiveness of different Aphidoletes release methods in controlling pest aphid populations. This objective will compare the current release method (innundative releases) vs. banker plants in research greenhouse trials.
- Objective 5: Assess the effect of providing a sugar solution to newly emerged adults on the survival and fecundity of Aphidoletes. 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). Sugar feeding in emergence containers prior to innundative release may boost performance of this natural enemy, especial when pest levels in the crop are relatively low.
Vegetative pansies (our model crop) were infested one of 4 treatments: 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. Low density treatments were approximately 15-30 aphids/plant; high density treatments were 80-100 aphids/plant. Treatments were randomized on 4 greenhouse benches (replicated across 2 greenhouse compartments) and un-infested “background plants” were used to force Aphidoletes to search within the crop. A banker plant (barley) infested with R. padi (ca. 150-200 aphids per plant) 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.
Trials using embedded pansy leaves infested with 7 large foxglove aphids, 15 medium-sized aphids, or 30 small sized foxglove aphids were used to determine the predation capacity of a single Aphidoletes larva over 24 h. Pansy leaves were embedded in 2% Difco agar, abaxial-side up, in 60mm Petri dishes with tight fitting lids. Lids had 2cm-diameter holes covered with screen for ventilation. Larvae were 96 h old (3rd instar), the stage in which most predation takes place.
Colonies of Rhopalosiphum padi and Sitobion avenae were obtained from existing colonies at Cornell University and maintained on barley (var. Bailey). Pea aphids (Acyrthosiphon pisum) were also obtained from existing Cornell colonies and maintained on fava beans. All plants were germinated and grown in the greenhouse in cages and brought to aphid colonies in a walk-in growth chamber when the appropriate size (ca. 2 weeks after germination).
Given our problems rearing aphids on banker plants (see Results Section for Objective 2, below), no methodology is presented for this section. However, discussion of how results from other sections may pertain to the objective are presented in the Results section of this report.
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 (40-50 aphids/plant) in the same compartment. Aphids were allowed to naturally distribute and reproduce on plants. Vegetative, budding, and flowering plants were tested over time to determine if crop stage and subsequent changes in aphid feeding sites altered results. In all cases, experiments also include a control consisting of aphid-infested plants kept in cages at the end of each bench. Aphidoletes adults were released at a rate of 1 predator: 10 aphids on Day 0 of the experiment. Plants were destructively sampled for aphids and Aphidoletes life stages on Day 2, 6 and 9 of the experiment (when Aphidoletes larvae began to pupate and offered no further control). To determine control efficacy, aphid number on predator-treated plants were compared to controls on the last day.
We also assessed the ability of Aphidoletes to control foxglove aphid in the presence and absence of green peach aphid to determine if control of foxglove aphid was improved without the presence of alternative prey. Two greenhouse compartments contained a mix of green peach aphid and foxglove aphid, while 2 compartments contained only foxglove aphid at equal densities. Aphid numbers on plants were compared to their controls (predator-free plants) 9 days after release of Aphidoletes (at the same predator:prey ratio as used previously).
15 females and 10 males (from commercial sources; not provided with any food until placed into one of the treatment cages) were introduced into large cages with one of two treatments. The sugar-feeding treatment consisted of a 5% sucrose solution in a plastic vial. The solution could be accessed by the flies via a tissue wick inserted into the vial through a slit in the lid (2 vials/cage). This method prevented the possibility of flies drowning or becoming stuck in the solution. The control treatment was water only. Each treatment was replicated across 3 cages. After being allowed to feed for 24 h, 16 females/treatment were selected randomly and placed individually into a small cylindrical plastic cage. Within the cage, each female was presented with 2 Petri dishes containing embedded leaves infested with green peach aphids(15-20/leaf) of all stages as an oviposition site. Dishes were counted for eggs every 24 h and new dishes were put in their place. Eggs were counted until all female midges had died. The experiment was conducted under laboratory conditions and repeated 3 times.
In our greenhouse trials examining oviposition on the two pest aphids within the crop vs. banker plants, Aphidoletes demonstrated a positive response to aphid density among all 3 aphid species at both the plant and the patch (leaf) level (P < 0.05 in all cases). Overall, 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 eggs on 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 3rd 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 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. Additionally, the host plant for pea aphid, fava bean, needed to be replaced frequently (ie. every 2 weeks) due to poor plant quality resulting from aphid feeding etc., which would likely be considered too effort-intensive for growers. Conversely, barley plants only needed to be changed ca. every 4 weeks. Thus, any future research programs investigating novel banker plants for Aphidoletes should focus on an aphid-plant combination that last at least as long.
We were not able to conduct predation/ oviposition preference trials between the two potential grain aphids due to recurring infestations of parasitic wasps (identified as Aphelinus spp.) which wiped out our aphid colonies several times, even when plants were kept in cages. Solutions to this problem were sought. After a period of trial and error, we determined that short exposure periods to 2,2-dichlorovinil dimethyl phosphate (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, need to be investigated.
Although this objective has not been investigated specifically, our trials with single releases of Aphidoletes adults have given us some clues as to potential outcomes 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 (with their high density of non-pest aphids) may generally be more attractive to Aphidoletes than plants infested with low levels of foxglove aphid (which tends to colonize lower leaves of plants). Though our results from Objective 1 demonstrated little preference of Aphidoletes for R. padi vs. foxglove aphid, the number of pest aphids per plant was relatively high, and these were commercially-obtained Aphidoletes (not reared from the banker plants). Previous studies show that Aphidoletes can have a preference for its natal host plant. Further complicating matters is that our greenhouse studies (and a preliminary lab trial) strongly 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, we hypothesize that Aphidoletes reared on banker plants will likely seek out colonies of green peach aphid in the greenhouse, since this pest species is found at higher densities on new growth of plants, its preferred oviposition location. Future research is needed to confirm these hypotheses.
Greenhouse studies showed that a single, high release of Aphidoletes was able to prevent significant population growth of green peach aphid infestations in all experiments (with 74-99% control achieved). Conversely, control of foxglove aphid was extremely variable (36-80% control across all experiments), with an acceptable suppression of the population (≥80% control) seen at only 1 plant stage (budding). The number of green peach aphids on plants left after treatment with Aphidoletes were significantly different than their controls (no predator) at all stages of plant growth (Figs. 1-3A); for foxglove aphid, no significant effect of Aphidoletes was seen when plants were in flower (Fig. 3A). Additionally, at 2/3 of the plant stages tested, numbers of foxglove aphids remaining on Aphidoletes-treated plants were significantly higher than those infested with green peach aphid. In many cases, Aphidoletes was able to drive green peach aphid-infested plants to near extinction (e.g. 7 out of 12 plants in the budding stage had ≤ 2 aphids/plant on Day 9).
Detailed study of the distribution of aphids and Aphidoletes eggs/larvae over the course of the experiments strongly suggests that differences in control were due to the interaction between preferred feeding sites of the two aphid species and a location preference with Aphidoletes oviposition. Specifically, Aphidoletes showed a strong oviposition preference for the center growing point of the plant, generally ignoring other locations. Green peach aphid was shown to prefer the growing point of pansy as a feeding site (i.e. 66% of aphids were found here on vegetative pansy; 46% when plants were in flower), and therefore received a statistically greater number of Aphidoletes eggs than foxglove aphid at all stages of plant growth (Figs. 1-3 B). Foxglove aphid was shown to feed primarily on bottom leaves of vegetative plants (56% of aphids), though they moved to flowers of flowering plants (52%). Eggs that were received by foxglove aphid infested plants were generally deposited on the growing points, despite the low numbers of aphids found there. Patterns of Aphidoletes larval distribution were generally consistent with egg distribution, demonstrating little within-plant movement from their natal location.
When foxglove aphid was the only prey in the greenhouse, control was significantly higher using Aphidoletes than when it was present was part of a mixed population (i.e. 40% vs. only 12% control, respecitvley). But overall control was still lower than expected. However, in all experiments, foxglove aphid populations generally increased at slower rate than green peach aphid, suggesting that there may have been time for a second release of Aphidoletes before foxglove aphid populations got out of control. These results indirectly highlight the potential benefit of having a constant source of Aphidoletes in the greenhouse via banker plants. Control of foxglove aphid may only be achieved through prophylactic control of initial populations entering the greenhouse, or through constant attack 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 green peach aphid is present in the same compartment.
We observed that ca. 50% of a midge population held in an emergence cage will visit a tissue paper wick soaked in a 5% sugar solution. Upon comparing the survival and daily egg production of individual females from sugar fed and non-sugar fed colonies of Aphidoletes, we observed some increase in female survival (average increase = 24 h), but no significant difference in oviposition over adult female lifetime with sugar feeding. Given that the larvae of this species are the only predaceous stage, no further testing was initiated.
This work has shown that, as Aphidoletes is used currently in greenhouse floriculture production (innundative releases), this natural enemy will not sufficiently control the emerging pest of foxglove aphid (A. solani) if only a curative application is used. This is especially true when other pest aphid species are present (e.g. green peach aphid, Myzus persicae). Our project further demonstrates that failures of biocontrol are not necessarily due to an ineffective natural enemy, but rather due to our failure to fully understand the biology of the organisms involved. Here, we have come to understand the feeding site preferences of the difficult-to-control foxglove aphid, which tends to colonize lower leaves of vegetative plants, but moves up to open flowers when plants are reproductive. We also demonstrated a mismatch between the feeding locations of foxglove aphid and the preferred oviposition site of Aphidoletes (the meristem tissue of plants). Understanding more about the oviposition behavior of Aphidoletes provides information which may be useful to developing more effective strategies, such as Aphidoletes-banker plants, in the future.
Information from these results have been passed onto growers, biocontrol specialists and researchers when I was invited to the International Organization for Biological Control (IOBC) annual Meeting in the UK in 2011. Results were also published in the IOBC Bulletin (IOBC/wprs Bulletin 68: 85-88), which is widely read by biocontrol researchers and practitioners. Presentations regarding pest aphids and use of Aphidoletes were made to growers in both Ontario, Canada, and in NY state so that more informed pest-management decisions could be made.
Education & Outreach Activities and Participation Summary
Jandricic SE, Wraight SP, Gillespie DR, Sanderson J. In Prep. Control of simultaneous outbreaks of the aphid pests Aulacorthum solani and Myzus persicae (Hemiptera: Aphididae) using the aphidophagous midge Aphidoletes aphidimyza (Diptera: Cecidoymyiidae) in greenhouse floriculture crops.
Jandricic SE, Wraight SP, Gillespie DR, Sanderson JP. 2013.Oviposition behavior of the biological control agent Aphidoletes aphidimyza (Diptera: Cecidomyiidae) in environments with multiple pest aphid species (Hemiptera: Aphididae). Biological Control 65: 234-245.
Jandricic S, Sanderson JP, Wraight S. 2011. Aphidoletes aphidimyza oviposition behavior when multiple aphid pests are present in the greenhouse. IOBC/wprs Bulletin 68: 85-88.
Jandricic S. 2011. Banker plants in the greenhouse: giving beneficial insects a helping hand. IOBC/nrs Newsletter: Volume 33(3).
Presentations to Growers / IPM Practitioners:
Jandricic S, Sanderson J, Wraight S. Aphidoletes for the control of multiple aphid species. Oral presentation, Flowers Canada Pest Management Conference. At: Niagara Falls, ON. 2012.
Jandricic S, Sanderson J, Wraight S. Location, location, location: how the within-plant distribution of different aphid species affects their attack by Aphidoletes aphidimyza. Oral presentation, Floriculture Field Day. At: Ithaca, NY. 2012.
Jandricic S, Sanderson J, Wraight S. Aphidoletes aphidimyza oviposition behavior when multiple aphid pests are present in the greenhouse. Oral presentation, IOBC/wprs Working Group “Integrated Control in Protected Crops, Temperate Climate” Meeting, At: Hampshire, UK. September, 2011.
Jandricic S, Sanderson J, Wraight S. How “picky” midges affect pesky aphids: investigations of Aphidoletes aphidimyza for foxglove aphid biocontrol in floriculture. Oral presentation, Flowers Canada Pest Management Conference. At: Niagara Fall, ON. 2010.
No economic analysis was done for this project.
Results concerning the use of Aphidoletes for multiple aphid species have been communicated to growers in NY and surrounding states, Biocontrol Specialists in the US and Canada, and to commercial suppliers of this natural enemy. Many growers are therefore now aware that Aphidoletes may not control foxglove aphid, especially in the presence of green peach aphid.
Areas needing additional study
In our greenhouses, open-rearing of Aphidoletes proved difficult, due to continual colonization by natural populations of Aphelinus parastioids. Given that Aphidoletes is able to feed on >70 different aphid species, we recommend that a host plant/aphid combination be sought for that would promote non-competition with aphid parasitoids (which have a narrower host range, being specialists). Barring the success of this, investigation into a density of traditional banker plants which allows the population growth of both parasitoids and Aphidoletes needs to be conducted.
Once an open-rearing system for Aphidoletes is established, densities of banker plants for control of foxglove aphid specifically are needed. This aphid has proved difficult to control with both parasitoids and curative releases of Aphidoletes. Effective mixes of natural enemies for foxglove aphid control should also be pursued.