Technician Duane Larson uses an ATV to apply an ultra-low volume spray. Photo courtesy of Dr. Alan Knight
Growers have been clear and consistent. They want easy, inexpensive, and effective approaches to manage their most egregious of pests-codling moth. Growers know how to spray; so they ask, “why not develop sex pheromone formulations that can be sprayed with standard equipment?”
Unfortunately, if you mix the sex pheromone with water, its effectiveness would be measured in hours. The secret recipe has always been how to protect the sex pheromone from oxygen and sunlight, while maintaining a consistent release rate.
Microencapsulation of pheromones was developed and initially tested on key agricultural moth pests, including codling moth, in the 1980s. But serious development of a commercial product for codling moth did not start until the mid-1990s. The focus of this research was to extend the efficacy of affordable rates of sex pheromone.
Prototypes were evaluated primarily by measuring the disruption of male moth captures in traps. Trials were assessed based on how much pheromone was applied, not on how much was deposited on the tree. Direct measurements of the emission rate of microcapsules in the field were not attempted. These studies consistently showed that rates of 10 to 20 grams of sex pheromone per acre could disrupt male codling moth catch in traps for one to two weeks.
Seasonal programs of four to six pheromone applications generally performed poorly in comparison with hand-applied dispensers, such as Isomate-C. The poor performance of these sprayables was attributed to their short residual effectiveness.
Analytical research by two manufacturers focused on how to modify the capsule to extend its emission rate. Unfortunately, in one case, this engineering created a long-lived capsule that essentially failed to release sex pheromone. Frustrated by its failure, this company dropped all future development of microencapsulated pheromones.
At about the same time, some clever person at Suterra, LLC, decided to add a fluorescent material to the capsules so that their density on leaves could be easily counted using a black light. In 1999, Dr. Tom Larsen and I evaluated the effectiveness of applying the pheromone in five gallons of water per acre using a microsprayer mounted on an all-terrain vehicle.
Interestingly, we found that this low-volume application was more effective than the traditional airblast application and provided nearly three weeks of trap shutdown. The low-volume spray also deposited twice as many capsules in the canopy, including a significant number of leaves with more than 20 capsules per leaf. These data were presented at a meeting in Portland, Oregon, in January 2000 and subsequently forgotten for several years.
Application density
Beginning in 2003, at a time when the research community had basically given up on microencapsulated codling moth pheromones, I started a new research program looking at how we could improve their use. Instead of reengineering the capsule, my objective was to figure out how to get more capsules on the trees. First, I reduced the water volume to 2.5 gallons per acre and then to 1.25 gallons using the ultra-low-volume sprayer outfitted with D2 nozzles now used to spray Western cherry fruit fly GF-120 bait. Sprays were applied at 20 to 30 pounds per square inch. This new ultra-low-volume ground application deposited six to ten times more capsules in the canopy than the airblast sprayer.
A number of studies have been conducted to look at factors that influence the deposition and retention of the microcapsules. Codling moth’s sex pheromone makes the microcapsules somewhat sticky, and adding a sticker or spreader only marginally improved deposition and retention.
Deposition and retention of capsules were both greater on apples than either pear or walnuts. A higher number of capsules were deposited on the underside versus the top of leaves due to the presence of more leaf hairs. Leaves with few hairs, such as walnut or pear, retained fewer microcapsules.
Differences in leaf pubescence between cultivars could also be an important factor influencing the efficacy of the sprayable formulation.
Rainfall and overhead irrigation significantly impacted the retention of capsules. Surprisingly, more capsules were washed off horizontal versus downward-sloping leaves. Apparently, the impact of water drops hitting the leaves bounces the capsules off. Increasing spray pressure also reduces the deposition of capsules. Leaf size and tree-row volume are important factors affecting the clustering and density of capsules deposited within the canopy.
The ULV sprayer deposits capsules throughout the tree canopy. Of course, the greatest numbers of capsules are deposited in the band where the spray contacts the canopy. A spray angle of at least 45° should be used to insure maximum deposition of capsules on the underside of leaves. A greater proportion of capsules are deposited on the upper surface of leaves in other areas of the canopy due to drift. Since codling moth is sexually active in the tops of trees, the spray should be directed to this zone of the canopy, while avoiding drift from spraying over the tops of trees.
Four weeks
Unfortunately, the retention of capsules over time has not been directly measured, as the fluorescent material degrades in sunlight. However, we have conducted assays measuring the response of male moths to leaf surfaces treated with a high density of capsules. Both leaf surfaces remained attractive for at least four weeks. Following periods of precipitation, the tops of leaves lost more of their attractiveness than the underside of leaves.
Field studies using artificial paper leaves have shown that leaves with more than five capsules are attractive to codling moth. Initially, in ULV-treated plots, individually treated leaves are not attractive, due to the fairly uniform distribution of pheromone in the canopy. However, over time, the background level of pheromone declines, and leaves with more than 20 capsules become attractive as individual point sources. Creating hundreds of point sources in each tree could create a very effective program. I hypothesize that it is the presence of these attractive point sources that extends the activity of the pheromone from one to three or four weeks.
During 2004, I compared the effectiveness of the microencapsulated pheromone when applied with either an airblast or a ULV sprayer in small, replicated apple plots. Applications were reapplied during the season whenever moth catches in the airblast-treated plots increased significantly, resulting in six sprays that season. Levels of injury in the ULV treatment were 70 percent lower than in the air blast-treated plots.
During 2005, I evaluated the commercial use of the ULV program (coined Speed Treatment System by Suterra) in replicated apple and pear orchards. A five-spray program was as effective as the use of Isomate-C dispensers in both crops. The influence of overhead irrigation was evaluated in three apple orchards. ULV applications were always made following an irrigation set.
While moth counts were much higher with the speed treatment program, no difference in fruit injury occurred between treatments at harvest.
The economics of the speed treatment program versus the use of hand-applied dispensers looks good due to a lower application cost. The program also gives growers greater flexibility as pheromone can be applied over the top of a reduced insecticide program at any time during the season if pest pressure warrants additional control.
Refinements
Further improvements in the application of microcapsules are possible. Methods that deposit more capsules in the canopy and that increase the clustering of capsules can be developed. Further reductions in spray pressure, the use of a spray boom to more carefully deposit the capsules in the canopy, further reductions in spray volume, and the use of alternate-row-middle or pulsed sprays should be evaluated.
Several problems were encountered with the speed treatment program during 2005. Bottles of pheromone had to be stored at temperatures greater than 60°F for at least 24 hours prior to use to avoid clumping of material. This problem was caused primarily by storage of bottles in unheated warehouses and when field representatives transported bottles in their trucks. Warning stickers were subsequently placed on all bottles.
The use of a highly concentrated pheromone caused several mechanical problems with sprayer nozzles, valves, and regulators. These problems were greatly reduced when sprayers were cleaned after each use with denatured alcohol.
Ground ULV applications can also be used to apply insecticides alone or in combination with pheromone. Studies in 2005 found that a six-spray program of Asana XL (esfenvalerate) was highly effective for codling moth, with more than 98 percent control. Residue samples at harvest found that all fruit samples were below the threshold established for Asana. Mite flare-ups occurred in the Asana-treated plots, but mite predators increased late in the season, and thresholds for mites were not exceeded. Not surprisingly, the addition of the sprayable pheromone to this heavy use of Asana did not further improve control of codling moth.
The effectiveness of the ULV-applied Asana program is assumed to be due to its activity primarily against adults. Both lethal and sublethal effects against adults may be important. For example, exposure to low rates of Asana still allows codling moth females to mate, but they lay 98 percent fewer eggs. Future studies will focus on reducing the rate and number of applications of insecticides when used in conjunction with a four- to five-spray pheromone program. Lower rates of Asana will be tested in the same plots for a second year to examine its carryover effect on mites. Ideally, an insecticide that has good adult activity but does not cause disruption of mites would be the best choice for this approach.
During 2006, other insecticides will be examined, including Imidan (phosmet) and Assail (acetamiprid), as well as several new products under development for codling moth control. The use of a four-spray Assail-plus-pheromone program in 2005 was effective, and did not cause any mite flare-ups.
The ULV spray approach is also being developed using a microencapsulated sex pheromone formulation for oriental fruit moth. Initial studies conducted in California during 2005 were successful and will be expanded this year.
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