Suppression of citrus leafminer, Phyllocnistis citrella, with an attract-and-kill formulation

The citrus leafminer, Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), is a worldwide pest of citrus crops and is responsible for proliferation of citrus bacterial canker, Xanthomonas axonopodis (Hasse) pv. citri (Gamma Proteobacteria: Xanthomonadaceae). We developed and evaluated an attracticide formulation, termed MalEx, for control of P. citrella. MalEx is a viscous paste with UV-protective properties that is dispensed as 50-ll droplets using custom-made calibrated pumps. A formulation containing 0.016% P. citrella pheromone [3:1 blend of (Z,Z,E)-7,11,13-hexadecatrienal and (Z,Z)-7,11-hexadecadienal] and 6% permethrin was found to suppress male response to pheromone in the field better than formulations containing 10· less pheromone. Although formulations without permethrin showed some suppression of male activity because of mating disruption, addition of 6% permethrin to the formulation was required for optimal efficacy. When MalEx, containing 0.016% pheromone and 6% permethrin, was applied at 3 000 point sources ha, the application height did not influence efficacy of male P. citrella suppression within mature 4-m tall citrus trees. Decreasing the rate of MalEx from 3 000 to 1 500 droplets ha reduced efficacy as measured by both male P. citrella activity and larval infestation. Although 4 500 droplets ha did not result in statistically better efficacy than 3 000 droplets ha, there was a noticeable trend for higher efficacy as droplet density increased. Continuous treatment of 0.5-ha blocks of citrus with MalEx over the course of 112 days reduced larval infestation of new flush, as compared with those in untreated control plots, by 3.6–7.2· depending on droplet application density. In laboratory behavioral bioassays, the attractiveness of MalEx droplets to male P. citrella was drastically reduced after 21 days of field aging. However, our laboratory investigation confirmed that 100% of males contacting MalEx droplets, aged up to 35 days in the field, were killed within 24 h. Direct observation of male P. citrella behavior in the field confirmed that attracted males made contact with droplets. Control of P. citrella with MalEx should reduce the number of required broad spectrum sprays for leafminer management in both field and citrus nursery settings.