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Fruit Notes |
Structural Refinement of Spheres for Controlling Apple Maggot |
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Fruit Notes |
Starker Wright, Ronald Prokopy, and
Xing Ping Hu |
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Fruit Notes |
Currently, there is no clear indication of the extent to which organophosphate insecticides will continue to be labeled for use in commercial apple production. If recent state and federal decision-making trends can be used as a guide for future action, then we should prepare ourselves for the possibility that the pre-harvest interval for organophosphate insecticides (particularly azinphosmethyl) may be lengthened considerably, possibly to 60 days or more. The purpose for doing this would be to eliminate all detectable residue on fruit at harvest, a principal aim of the Food Quality Protection Act (FQPA). This same scenario could well apply to permissible use patterns of carbamate insecticides, the only other class of labeled materials known to be at least moderately effective against apple maggot. For many years (as reported in Fruit Notes), we have conducted studies aimed at development of behavioral control of apple maggot using traps as a substitute for insecticide applications. These studies have led to the development of pesticide-treated spheres as a potential alternative in commercial apple production. Such spheres are intended to be inexpensive, easy to use, safe to handle, and to offer reliable control of apple maggot. In 1997, we began an extensive comparison of odor-baited sticky coated spheres, pesticide-treated wooden spheres, and biodegradable pesticide-treated spheres for direct control of apple maggot flies (AMF) in commercial orchards. This study was conducted in eight commercial apple orchards, each containing four blocks of 49 trees each. In each orchard, one block received three insecticide sprays for AMF control, one block was surrounded by sticky-coated wooden spheres, one block was surrounded by pesticide-treated wooden spheres, and one block was surrounded by biodegradable pesticide-treated spheres. The results [Fruit Notes 62(4)] showed that pesticide-treated biodegradable spheres performed as well as sticky spheres and considerably better than pesticide-treated wooden spheres on which we were unable to preserve an effective level of feeding stimulant. None of the three trap types provided quite the level of AMF control as 3 insecticide sprays did. In the case of biodegradable pesticide-treated spheres, compromising factors were that some spheres cracked prematurely, some were eaten by birds and/or rodents, and some were overgrown by fungi, reducing the number of effective spheres comprising the barrier to fly immigration into some blocks. Despite shortcomings, our 1997 findings were encouraging, as all sphere types performed well under high pest pressure in commercial orchards. Our goal for 1998 was to conduct experiments leading to improved versions of pesticide-treated spheres and to evaluate these for direct control of AMF. Here we report on structural refinements of both pesticide-treated biodegradable spheres and pesticide-treated wooden spheres. Materials & Methods To maximize effectiveness and durability of biodegradable spheres, we addressed each of the shortcomings of such spheres experienced in 1997: 1) To combat premature breakdown of biodegradable sphere bodies, we evaluated seven different structural compositions. We assessed each body type for drying time, hardness, and resistance to humidity. 2) To ease deployment, we replaced the troublesome string hangers with wire hooks, and used a metal disc beneath each sphere to support the weight of the sphere body. 3) To prevent consumption of spheres by rodents, we evaluated five different formulations of hot pepper additive, whose active ingredient (capsaicin) is known to deter rodent feeding. 4) To reduce damage caused by birds, we evaluated black spheres versus red spheres. 5) To inhibit growth of fungi on the surface and in the body of the spheres, we incorporated 1.0% sorbic acid into the ingredients of each sphere body as a preservative. In an attempt to extend the residual activity of sucrose as the necessary feeding stimulant on pesticide-treated wooden spheres, each sphere was fitted with a 1.25-inch plastic dish containing 15 grams of molten sucrose, which cooled and hardened after pouring. In concept, as these spheres are exposed to rainfall, the sphere surface is retreated with a dilute sucrose solution from the reservoir atop the sphere. If there was a constant release of sugar to the sphere surface under conditions of rainfall, then ideally each sphere would have enough sucrose to replenish the active supply through several inches of rainfall. In addition to structural modifications of pesticide-treated spheres, we increased the level of imidacloprid (Merit formulation, 75 WP) from 1.5% to 2.0% active ingredient to extend late-season effectiveness of the spheres (see following article). For all pesticide-treated spheres, the toxicant (imidacloprid) was applied to the spheres at 2.0% a.i. in Glidden Latex Gloss Enamel paint. In 1998, we evaluated odor-baited sticky spheres, modified wooden pesticide-treated spheres, and improved biodegradable pesticide-treated spheres in 32 commercial orchard blocks. Blocks used in 1998 were the same blocks used in 1997; spheres were emplaced in eight commercial orchards, each containing four blocks of 49 trees. Each block receiving spheres was surrounded by 26 spheres of the same type at a 5-meter interval, and each sphere was baited with a vial of butyl hexanoate. Results In laboratory tests under conditions of artificial rainfall, the following body composition proved the most durable for biodegradable spheres: 16% wheat flour, 16% pre-gelatinized corn flour, 5% corn starch, 20% powdered sugar, 13% granulated sugar, 7% corn syrup, 8% glycerol, and 9% water. Spheres of this composition were found to harden very quickly and were much more resistant to premature breakdown than the body style used in 1997. Of materials tested to deter rodent feeding, the best product was African cayenne pepper powder, mixed at 5% with sphere body ingredients. Subsequent lab and field tests indicated that AMF were completely unaffected by the pepper additive, whereas damage to spheres by rodents was greatly reduced or eliminated. Field assays revealed that bird damage to biodegradable spheres was substantially reduced by painting spheres black, thus making them less visually attractive to foraging birds. Fortunately, AMF are equally attracted to black and red spheres. Given this early-season progress, we were confident in the potential of biodegradable pesticide-treated spheres to offer AMF control comparable to sticky-coated wooden spheres. Data trends established in the 1997 comparison of sphere types held true in 1998. Results for 1998 (Table 1) showed that biodegradable pesticide-treated spheres performed about as well as sticky spheres (0.77% and 0.70% injured fruit, respectively) and nearly as well as three insecticide applications (0.59% injured fruit). As in 1997, wooden pesticide-treated spheres released the entire sucrose supply too quickly, and did not perform as well (2.93% injured fruit). Unfortunately, many biodegradable spheres required replacement after about a month of field exposure due to softening and bursting of the sphere bodies. Conclusions Pest pressure in commercial orchards in 1998 was even higher than in 1997 (an average of 1702 AMF per block captured by the 26 sticky spheres per block). Despite shortcomings of biodegradable and wooden pesticide-treated spheres, we are nonetheless encouraged by the 1998 results, particularly by the performance of biodegradable spheres under very high pest pressure. With further structural modifications and pending commercial production of biodegradable sphere bodies, we are becoming increasingly confident in recommending use of sphere traps in place of insecticide sprays for control of AMF in commercial orchards. As discussed in the Introduction, development of alternatives for control of AMF is gaining emphasis given the implications of FQPA implementation. For the 1999 field season, we will again compare the efficacy of sticky-coated wooden spheres, pesticide-treated wooden spheres, and pesticide-treated biodegradable spheres for direct control of AMF. In 1999 field trials, pesticide-treated wooden spheres will be augmented with a new sucrose-release mechanism, and biodegradable spheres used in 1999 will be commercially-made prototypes, produced by a private corporation under USDA supervision. Acknowledgments This work was supported by state/federal IPM funds, a grant from the Washington Tree Fruit Research Commission, a USDA SEA CSREES grant (# 97-34365-5043), and a SARE grant (USDA 96-COOP-1-2700). As always, we are grateful to the seven growers who participated in this study: Bill Broderick, Dana Clark, Dave Chandler, Tony Lincoln, Wayne Rice, Dave Shearer, and Tim Smith; and to our field technicians: Jon Black, Joel Benton, Anthony Minalga, Stephen Lavallee, Eric Gemborys, and Max Prokopy. |
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