Interplay between foraging behaviour, adult density and fruit ripeness determines the effectiveness of gibberellic acid and host-marking pheromone in reducing susceptibility of grapefruit to infestation by the Mexican fruit-fly, Anastrepha ludens

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The potential of gibberellic acid (GA3) and host-marking pheromone as management tools for control of Mexican fruit-fly (Anastrepha ludens) was evaluated in the field. Varying numbers of GA3-treated and untreated ''Ruby Red'' grapefruit were hung in branches of field-caged citrus trees and exposed to four different densities of gravid A. ludens females (1, 5, 15 and 30 individuals per cage) over the entire harvest season (October to February). Infestation was not lowered by GA3-treatment when fly density was set at 0.06, 0.19 and 0.38 females per fruit. Results depended mainly on fruit ripeness and female density. A reduction in infestation (almost eight-fold) was recorded at a fly density of one female per 200 fruit (0.0005 female/fruit rate), when fruit was treated with 15 ppm of GA3 (applied once) and exposed to flies early in the harvest season (November). At a higher GA3 concentration (20 ppm applied twice) and a fly density of 30 females per 100 fruit (0.3 female/fruit rate), no statistically significant reduction in damage was recorded. Late in the season, when untreated fruit were overripe (i.e. were of poor quality), there was a tendency for females to preferentially oviposit into GA3-treated (and therefore superior quality) fruit. We also tested the combined effect of GA3 and host-marking pheromone and pheromone-treated fruit on the oviposition behaviour of A. ludens and found that the presence of pheromone, with or without GA3, reduced infestation to nearly zero. We discuss our findings in the context of the interplay between oviposition behaviour, fly density and fruit ripeness (i.e. the season) and the complexities involved in the design of biorational fruit-fly management schemes. eng

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