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Fruit Notes |
Evaluation of Varying Doses of Different Toxicants for Use on Spheres to Control Apple Maggot Flies |
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Fruit Notes |
Ronald Prokopy, Starker Wright, Brad
Chandler, and Xingping Hu |
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Fruit Notes |
In the 1997 Fall issue of Fruit Notes, we reported that imidacloprid was a promising alternative to dimethoate as a toxicant for application to pesticide-treated spheres for controlling apple maggot flies (AMF). We also reported that imidacloprid at 1.5% active ingredient (a.i.) in paint afforded longer residual activity than imidacloprid at 0.5% a.i. in paint, and that a Merit 75 WP formulation of imidacloprid in paint was more effective than a Provado 1.6 F formulation in paint at 0.5% a.i., though not at 1.5% a.i. The unusually large numbers of AMF in some Massachusetts commercial apple orchards in 1998 allowed early-season direct observations of the behavior of AMF after alighting on imidacloprid-treated spheres. These observations suggested to us that by increasing the dose of imidacloprid on spheres beyond the 1.5% active ingredient used heretofore, we might be able to capitalize on contact-type toxicity of imidacloprid, thus diminishing the need to maintain feeding stimulant (sucrose) on the sphere surface. Such a high-dose approach was thought previously to be impractical, owing to the handling risk associated with higher doses of our originally-adopted toxicant, dimethoate. Here, we report on 1998 studies of AMF responses to four different doses of imdacloprid in latex paint, with and without the addition of sucrose as feeding stimulant. We also report on AMF responses to three new candidate toxicants: spinosad, sugar ester, and fipronil. All three, like imidacloprid, are considered to be comparatively safe for handling by humans. Materials & Methods In our first test, we painted the inside surface of small plastic cups with a mixture of Glidden Red Latex Gloss Enamel paint and either 0.1, 0.5, 1.0, or 5.0% a.i. of one of the following potential toxicants: imidacloprid (Merit 75 WP), spinosad (SpinTor 22.8 F), sugar ester (SCO 3483, 100%), or fipronil (Regent 80 WG). No sugar was added to any mixture. After the mixture dried, 10 AMF were confined in each of two cups of the same treatment type for 10 minutes, following which AMF were transferred to clean cups supplied with food and water. Mortality was assessed after 72 hours. For our second test, we focused on the two toxicants that showed the most promise in the first test: imidacloprid and fipronil. Each of these toxicants was evaluated at 2, 4, 8, and 16% a.i. in Glidden Red Latex Gloss Enamel paint (no sugar added) applied to wooden spheres. After drying, spheres were hung in orchard trees and returned to the laboratory after 3, 6, 9, or 12 weeks for evaluation of toxicity to AMF. For evaluation, 30 AMF were placed individually on a sphere of each treatment type and allowed to remain 10 minutes, following which AMF were transferred to small, clean cups supplied with food and water. Mortality was assessed after 24 hours. Half of the spheres of each treatment type received a 20% sucrose solution just prior to testing. The other half received no sucrose. For our third test, we again focused on imidacloprid and fipronil, but in this experiment, we evaluated the behavior of sphere-exposed AMF shortly after removal from spheres. Spheres were treated with 2.0% a.i. imidacloprid or fipronil in Glidden Red Latex Gloss Enamel paint and exposed for 3 weeks in orchard trees. Just before testing, all spheres received a 20% sucrose solution. For each treatment, 32 AMF were placed individually on a sphere and allowed to feed for up to 10 minutes. One hour after feeding, each fly was transferred to a leaf near the center of the canopy of a small non-fruiting fig tree and allowed to forage freely for 15 minutes. The number of leaves visited was recorded. Immediately thereafter, each fly was placed on a ripe hawthorn fruit, and propensity to lay an egg was observed. Results In our first test, conducted with no sugar added, 20, 80, and 100% of AMF died by contact toxicity alone when confined in plastic cups treated with 0.5, 1.0, and 5.0% a.i., respectively, of imidacloprid in paint (Table 1). None died at 0.1% a.i. of imidacloprid. The only other toxicant causing AMF mortality was fipronil, where 10% of AMF died by contact toxicity alone after exposure to 5.0% a.i. of this material. Even though both spinosad and sugar ester conceivably might be somewhat toxic to AMF if ingested together with sugar or when applied alone to foliage, neither material caused any toxicity by contact alone when mixed in latex paint. In our second test, results showed that in the presence of sucrose, mortality of AMF was consistently high (70% or more) on spheres treated with imidacloprid, even after 12 weeks of sphere exposure to outdoor weather in orchard trees (Figure 1). Provided that sugar was present, the dose of imidacloprid had little effect on AMF toxicity, with 2% a.i. being about as toxic as 16% a.i. at all evaluation periods (3, 6, 9, and 12 weeks). In contrast, in the absence of sucrose, toxicity of imidacloprid-treated spheres to AMF did not exceed 15% at 12 weeks of orchard exposure, even at the highest dose tested (16% a.i.). Results for fipronil roughly paralleled those for imidacloprid at 3, 6, and 9 weeks of orchard exposure. However, at 12 weeks, even in the presence of sucrose, lower doses of fipronil (2 and 4% a.i.) yielded no more than 40% mortality of AMF. A dose of 8% fipronil was required before toxicity approached that of 2% imidacloprid at 12 weeks. In the absence of sucrose, toxicity from contact with fipronil, as with imidacloprid, was virtually nil at 12 weeks, irrespective of dose. In our third test, results showed that only a minority of AMF exposed to spheres treated with 2% a.i. imidacloprid and sucrose were capable of foraging within a plant canopy after exposure, and of those that could forage, an average of only one leaf was visited for every three flies tested (Table 2). Moreover, only 6% of assayed AMF attempted to oviposit in a hawthorn fruit. In contrast, almost all AMF exposed to spheres treated with 2% a.i. fipronil were capable of foraging within a plant canopy, with the average number of leaves visited nearly equaling the number visited by AMF that were exposed to spheres lacking pesticide. Also, 16% of AMF that were exposed to fipronil-treated spheres attempted to lay an egg. Together, the results of the behavior tests reflect the fact that fipronil is a much slower-acting compound than imidacloprid. AMF mortality following ingestion of fipronil was only about a third the amount at 24 hours after exposure as at 72 hours afterward, whereas with imidacloprid, mortality at 24 hours was nearly equal to that at 72 hours (Table 2). Conclusions Our combined findings show that imidacloprid performed better as an AMF toxicant on pesticide-treated spheres than did any of three other candidate toxicants tested here: fipronil, spinosad and sugar ester. Imidacloprid at 2% a.i. in Glidden Red Latex Gloss Enamel paint gave excellent season-long (12 weeks) AMF control provided that it was ingested with a feeding stimulant (sucrose). It gave very poor control, as did fipronil, in the absence of sucrose, irrespective of dose used (16% a.i. was the highest dose tested). A low dose of imidacloprid, such as 2% a.i., not only is less expensive than a higher dose but is safer for those handling pesticide-treated spheres. Imidacloprid rapidly immobilized AMF that ingested it, resulting in very little or no subsequent foraging and egglaying activity of exposed AMF. Because imidacloprid seems to be an ideal toxicant for use in conjunction with sucrose on pesticide-treated spheres for controlling AMF, we are hopeful that the registrant (Bayer Corporation) will see fit to officially register its use for this purpose. Acknowledgments This work was supported by a grant from the Washington Tree Fruit Research Commission and USDA SEA CSREES grant # 97-34365-5043. We thank Gary Puterka of the USDA Regional Fruit lab in Kearneysville, WV for providing the sugar ester. |
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