Oviposition preference and larval performance of Nematus oligospilus (Hymenoptera: Tenthredinidae) in relation to host plant vigor

We investigated the relationships among oviposition preference and larval performance of Nematus oligospilus Foerster, a solitary, free-feeding sawfly, and the vigor (shoot length) of its host plant, arroyo willow, Salix lasiolepis Bentham. In the 3 populations studied, the number 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 >800 mm accounted for only 0.20-3.17% of the available shoots. The distributions of leaves and leaf area did not account for greater attack on longer shoots, nor did the size (target area) of leaves. N, oligospilus had strong and relatively inflexible oviposition preferences for rapidly growing shoots of S. lasiolepis. In multiple choice oviposition experiments, females laid 4.0-6.2 times more eggs on vigorous shoots than on medium and short shoots. In no choice oviposition tests, females 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 larvae corresponded to the oviposition preference of females because short shoots were poor quality resources for developing larvae. In the field and laboratory, larval mortality on vigorously growing shoots averaged 26% less at establishment and 33.5% less after establishment compared 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 developing on vigorous shoots averaged 4.8 eggs more than that of females reared from larvae developing on short shoots, although this difference was only nearly statistically significant (P = 0.06). Natural enemies (egg parasites) did not play a role in the correlation between larval survival and oviposition preference. A life table analysis suggests that the influence of variation in plant quality (vigor) of S, lasiolepis on natality and larval performance significantly influences the demography of N, oligospilus.