OVIPOSITION PREFERENCE AND LARVAL PERFORMANCE OF NEMATUS-OLIGOSPILUS (HYMENOPTERA, TENTHREDINIDAE) IN RELATION TO HOST-PLANT VIGOR

We investigated the relationships among oviposition preference and lar val performance of Nematus oligospilus Foerster, a solitary, free-feed ing sawfly, and the vigor (shoot length) of its host plant, arroyo wil low, Salix lasiolepis Bentham. In the 3 populations studied, the numbe r of eggs laid by N, oligospilus increased 20- to 170-fold from shoots less than or equal to 100 mm to shoots >800 mm, even though shoots >8 00 mm accounted for only 0.20-3.17% of the available shoots. The distr ibutions of leaves and leaf area did not account for greater attack on longer shoots, nor did the size (target area) of leaves. N, oligospil us had strong and relatively inflexible oviposition preferences for ra pidly growing shoots of S. lasiolepis. In multiple choice oviposition experiments, females laid 4.0-6.2 times more eggs on vigorous shoots t han on medium and short shoots. In no choice oviposition tests, female s laid 83.5% of their available eggs on vigorous shoots, but only 54.4 % on medium shoots and 31.0%, on short shoots. The performance of larv ae corresponded to the oviposition preference of females because short shoots were poor quality resources for developing larvae. In the fiel d and laboratory, larval mortality on vigorously growing shoots averag ed 26% less at establishment and 33.5% less after establishment compar ed with mortality on short shoots. Development time was 2.3 d less on average for larvae reared on vigorous shoots compared to those reared on short shoots. The fecundity of females reared from larvae developin g on vigorous shoots averaged 4.8 eggs more than that of females reare d from larvae developing on short shoots, although this difference was only nearly statistically significant (P = 0.06). Natural enemies (eg g parasites) did not play a role in the correlation between larval sur vival and oviposition preference. A life table analysis suggests that the influence of variation in plant quality (vigor) of S, lasiolepis o n natality and larval performance significantly influences the demogra phy of N, oligospilus.