Promising new technologies for mating disruption
An attract-and-remove technique eliminates trapped males and requires only a small amount of attractant.
Scientists have used field cages to measure how male moths react to pheromone dispensers. The cages are made of 40 percent shade cloth supported by poles and high-tension trellis wire.
Photo courtesy MSU
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It has been nearly 20 years since the first mating disruption formulation was registered for codling moth control. At present an estimated 500,000 acres worldwide, including over 175,000 acres in North America, are treated with pheromone for codling moth management. Although the majority of mating disruption formulations registered over the past 20 years are devices that require hand-application at rates of up to 400 sources per acre, substantial efforts have been invested in other formulations to make the technology easier to apply. Several formulations have been developed that allow the pheromone to be sprayed on the crop either by ground or air. A quite different approach entails the formulation and release of pheromone via aerosol-emitting devices that are typically deployed at a density of two or fewer units per acre.
Our own efforts to develop more effective and economical disruption formulations for codling moth have been guided by a series of studies examining the mechanisms underlying mating disruption. There are two general ways it might be achieved:
1) by competitive attraction where males are diverted from orienting to females because of competing attraction of nearby false trails emanating from pheromone dispensers; or
2) by a non-competitive means where exposure to synthetic pheromone subsequently reduces or blocks the male's ability to sense pheromone normally, and this happens without attraction. Our findings indicate that both mechanisms are operating, but that in most cases competitive attraction is the key initial step.
Some of the most informative experiments have taken place in a suite of 20 large field cages, each constructed over 12 apple trees (Figure 1). The means of achieving disruption for a particular formulation is discerned by measuring its impact at a range of dispenser densities. Up to 100 codling moth males are released into the pheromone-treated caged trees and disruption is measured by recording moth catch in a centrally located trap baited with a female mimic lure.
In cage experiments involving various types of dispensers, codling moth behavior consistently has conformed to the predictions of competitive attraction; generating smoothly concave dosage-response profiles (Figure 2). In other words, a few dispensers provided a relatively large impact, but high levels of disruption required a substantial number of additional dispensers. Thus, the initial response of codling moth males to dispensers was attraction. However, attraction alone was insufficient for achieving a high level of disruption. The weakest impact was recorded when the “dispensers” consisted of 0.1mg lures placed inside pheromone traps lacking sticky liners. Only a maximum of 60 to 70 percent suppression was recorded in plots treated with these low releasing devices (0.02 micrograms of pheromone per hour) deployed at the highest rate of 17 per plot (equivalent to 200 per acre). The addition of sticky liners to the same “dispensers’ (0.1 milligram lure-baited traps) greatly enhanced the level of disruption achieved. The desired outcome was the best when males were ensnared, and thus removed from the plot following visitation to a disruptor. The effect of a standard commercial dispenser, Isomate ropes, on male orientation was intermediate to that of the linered and linerless 0.1 mg-baited dispensers (traps). The higher releasing Isomate dispensers (6 micrograms of pheromone per hour) were threefold more disruptive than the lure-baited linerless dispensers. However, they were only one fourth as disruptive as linered dispensers. Visitation to an Isomate dispenser apparently deactivated codling moth male response for the remainder of the evening, but additional visits to pheromone sources were made the following evening after males had recovered from the effects of interacting with the dispenser.