Plant volatiles for detection of migrating apple fruit moth

Apple fruit moth Argyresthia conjugella (Zeller) (Lepidoptera: Argyresthiidae) is the most important insect pest in apples in Fennoscandia. Its principle host is the masting forest tree rowan Sorbus aucuparia. Damage to apple occurs only in intermasting years when rowan produces low crop. In these years, populations of apple fruit moth move from the forest to the nearby orchards to lay their eggs on apple fruits. Volatile plant compounds may be used to trap migrating females searching for the alternative host during intermasting years (Bengtsson et al., 2006). Plant volatiles released by rowan as well as by apple were found to attract both sexes of A. conjugella in field experiments (Bengtsson et al., 2006). Additional research effort is needed to improve the attraction and thus to develop a monitoring tool to detect the timing of female migration to apple in order to precisely apply control measures. In this study we present data from field monitoring experiments done in 2007 and 2008 in both rowan and apple crops. Tetra traps were lured with a rubber septum loaded with a blend of compounds diluted in hexane. Traps were deployed during the migration of the moths (June and July). In 2007 we carried out a field test in apple with the aim to screen for behaviorally active compounds beside those already described by Bengtsson et al. (2006). In apple, higher attraction was observed for a blend of 2-phenyl ethanol, methyl salicylate and decanal followed by a blend of 2-phenyl ethanol and anethole. Although to a lower level, also blends containing Z-jasmone and (Z)-3- hexenyl-2-methyl-butanoate were significantly attractive. In a rowan forest, only the 2-phenyl ethanol and anethole blend was attractive. In 2008, in rowan the best attractant was the two component blend of 2-phenyl ethanol and anethole in a 1:1 ratio, confirming the results of 2007. This blend caught moths in a sex ratio similar to that detected by visual inspection. In apple, instead, this blend caught only few males. However, a 6 component blend including methyl salicylate, decanal, Z-jasmone and (Z)-3-hexenyl-2-methylbutanoate beside 2-phenyl ethanol and anethole, was in apple the best female attractant. We detected insects by both visual inspections and trapping in a rowan stand, whilst only traps baited with the new 6 component blend could detect migrating insects in apple orchards. Although the number in apple was low due to the fact that 2008 was a low-attack season and the majority of the moths remained in the rowan stands, our preliminary results open the possibility of developing a specific lure to be used in apple orchards as a warning tool for population control of apple fruit moth. In our field experiments, we observed an effect of the crop on the attraction to the different blends. It is possible that the odors from the crop have a behavioral effect on the attraction of the blends, by providing a background level of volatiles which may interact with the plume released by our traps. This effect has been recently reported in a wind tunnel study (Knudsen et al., 2008), where apple fruit moth females were attracted to 2-phenyl ethanol only in the presence of rowan vegetation as background. It thus appears that improvements of blend composition for field monitoring in apple orchards may be achieved by additional field tests in damage years in combination with wind tunnel studies.