No abstract

Resistance to several classes of insecticides was correlated with azinphosmethyl resistance in codling moth, Cydia pomonella (L.), in California. In tests of laboratory and field populations, cross-resistance was positively correlated with azinphosmethyl and two organophosphates (diazinon, phosmet), a carbamate (carbaryl), a chlorinated hydrocarbon (DDT), and two pyrethroids (esfenvalerate and fenpropathrin). Additionally, negatively correlated cross-resistance was identified between azinphosmethyl and two other organophosphates, chlorpyrifos and methyl parathion. Patterns of resistance observed in laboratory colonies were confirmed with field bioassays. In bioassays of field populations, azinphosmethyl resistance was observed to increase from 1991 to 1993, although levels of resistance remained < 13-fold. Because orchards with azinphosmethyl resistance have had difficulties with suppression of codling moth, and cross-resistance was found for all tested classes of insecticides, strategies for managing resistance will need to be developed so as to protect current and future control tactics. The two insecticides with negatively correlated cross-resistance are discussed as potential tools for resistance management.

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Cacopsylla pyricola (Förster) is one of the most important pests of pear in North America, where several native predators have been considered for integrated pest management (IPM) programmes. Two molecular markers of 271 and 188 bp were developed from C. pyricola cytochrome oxidase I (COI) fragments, in order to study the detection of this species in the gut of arthropod predators. Primer sensitivity and the detection period for pear psylla remains in the guts of Anthocoris tomentosus Pericart were determined. The sensitivity threshold was defined at 10-5 dilution of a C. pyricola fifth-instar nymph in all samples. Predator adults were evaluated immediately after ingestion of one to five C. pyricola nymphs (t = 0) and after 2, 4, 6, 8, 16, 24 and 32 h. Detection of the presence of C. pyricola DNA always lasted longer using the shorter fragment and was observed after 32 h of digestion using both markers. The primers amplifying the 188 bp fragment amplified all four psyllid species tested, whereas the primers designed to amplify the 271 bp fragment did so exclusively for C. pyricola and its close relative, Cacopsylla pyri (Linnaeus). Both primers failed to amplify DNA from representative species of the Coccinellidae, Chrysopidae, Hemerobiidae, Anthocoridae, Miridae, Salticidae, Aphididae, Tetranychidae and the Tortricidae, suggesting their suitability for general trophic studies.

Natural enemies have been proposed as important agents of natural selection on herbivorous insects that may facilitate host plant shifts and increases in diet breadth. However, there is little experimental field work to support claims of host-shifting via escape from natural enemies, i.e., to enemy-free space. In this study, we took the unique approach of experimentally creating a host shift for a specialized leafmining fly, Liriomyza helianthi (Diptera: Agromyzidae). We manually transferred leafminer larvae from their normal host plant, Helianthus annuus (Asteraceae), to a variety of novel plants: Helianthus maximilianii, Ambrosia artemisiifolia, Taraxacum officinale, and Centaurea solstitialis. By exposing transferred larvae on normal and novel plants to natural enemy attack under field conditions, we were able to examine whether host-shifting can provide an herbivore with enemy-free space.Our data show that enemy-free space does exist for L. helianthi immediately following a host shift, as mortality in novel plants averaged 17% less than in the normal host. Nevertheless, there was significant within-and between-year heterogeneity in results over the 3-yr period of the study. We found that escape from natural enemies was related to annual variation in the diversity and abundance of parasitoid species. In years when parasitoid assemblages were dominated by endoparasitoids, mortality of larvae averaged 22% lower in novel hosts. However, when generalist ectoparasitoids, Diglyphus spp. (Hymenoptera: Eulophidae), were present, there was no advantage of developing in novel plants, a result that could be explained by the less discriminating nature of the ectoparasitoids. When overall levels of mortality from natural enemies were high, the benefit of novel plants was also reduced. This pattern suggests that, as available larval hosts become scarce, parasitoids may be more likely to forage on novel host plants in search for prey, thus diminishing the opportunity for enemy-free space.Nevertheless, our study showed that enemy-free space can exist for an herbivorous insect utilizing a novel host plant, and that natural enemies may, in some cases, offset physiological fitness costs often associated with developing in novel plants. If all else is equal, the balance of these factors may facilitate the inclusion of novel host plants into the feeding repertoire of an herbivore.