Previous short-term experiments showed that trail following behavior of the Argentine ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae), can be disrupted by a high concentration of synthetic trail pheromone component (Z)-9-hexadecenal. In this study, a long-term field trial was conducted in 100-m2 plots of house gardens in an urban area of Japan to see whether the control effect on Argentine ants can be obtained by permeating synthetic trail pheromone from dispensers. The dispensers were placed in the experimental plots during the ant's active season (April-November) for 2 yr with monthly renewal. To estimate Argentine ant population density, foraging activity of Argentine ants in the study plots was monitored by monthly bait surveys. Throughout the study period, Argentine ant foraging activity was suppressed in the presence of the dispensers, presumably via trail forming inhibition. In contrast, the level of foraging activity was not different between treatment and no-treatment plots when the dispensers were temporarily removed, suggesting that treatment with pheromone dispensers did not suppress Argentine ant density in the treatment plots. Population decline may be expected with larger-scale treatment that covers a significant portion of the ant colony or with improvement in the potency of the disruptant.

At tea fields in Shimada in Shizuoka Prefecture, a mating disruptant composed of (Z)-11-tetradecenyl acetate (trade name: Hamaki-con) had been used for controlling Adoxophyes honmai (Yasuda) annually since 1983. In 1986, four years after use of the disruptant was initiated, the disruption percentage of pheromone trap catches was 96%, and a high control effect was observed. However, from 1996 to 1998, 14 to 16 years after treatment was started, the percentages became less than 50%. When the disruptant was applied in other tea fields where it had not been previously used, the disruption percentages were over 99%. These results strongly suggest the possibility that a selection pressure caused by continuous use of the disruptant should have caused the low disruption percentage in Shimada. We propose that the phenomenon be termed "resistance" to a mating disruptant composed of (Z)-11-tetradecenyl acetate in A. honmai. When the sex pheromone blend of A. honmai, 63 : 31 : 4 : 2 mixture of (Z)-9-tetradecenyl acetate, (Z)-11tetradecenyl acetate, (E)-11-tetradecenyl acetate and 10-methyldodecyl acetate (4-component blend), was applied as a disruptant for the "resistance" population, the disruption percentage was 99% and the larval density in the next generation decreased. These results suggest that the 4-component blend is useful as a control tool for the "resistance" population.

Combination treatment may be a more effective and environmentally friendly method for controlling invasive ants than conventional methods. Extermination of ants by insecticidal bait and inhibition of re-infestation by pheromone may be the mechanism of the combination effect. This is the first study to show a significant effect of synthetic trail pheromone on ant population.

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