GROWTH OF COYOTE WILLOW AND THE ATTACK AND SURVIVAL OF A MID-RIB GALLING SAWFLY, EUURA SP

Citation
Jo. Woods et al., GROWTH OF COYOTE WILLOW AND THE ATTACK AND SURVIVAL OF A MID-RIB GALLING SAWFLY, EUURA SP, Oecologia, 108(4), 1996, pp. 714-722
Citations number
32
Categorie Soggetti
Ecology
Journal title
ISSN journal
00298549
Volume
108
Issue
4
Year of publication
1996
Pages
714 - 722
Database
ISI
SICI code
0029-8549(1996)108:4<714:GOCWAT>2.0.ZU;2-3
Abstract
We studied the relationship between variation in age and shoot charact eristics of the host plant Salix exigua Nuttall (coyote or sandbar wil low) and the attack and survival of Euura sp. (an unnamed leaf-midrib galling sawfly). Variation in shoot characteristics resulted from redu ced growth as willow ramets aged. Mean shoot length per ramet and mean longest leaf length per shoot decreased by 95% and 50% respectively b etween 1- and 9-year-old willow ramets. All measured shoot characteris tics-shoot length, longest leaf length, number of leaves per shoot, an d mean internode length-were significantly negatively correlated with ramet age (r(2) ranged from -0.23 to -0.41). Correlations between shoo t characteristics were highly positive, indicating that plants also gr ew in a strongly integrated fashion (r(2) ranged from 0.59 to 0.85). F our hypotheses were examined to explain sawfly attack patterns. The ho st-plant hypothesis was supported in explaining enhanced larval sawfly survival through reduced plant resistance. As willow ramets aged, the probability of Euura sp. attack decreased over 10-fold, from 0.315 on 1-year-old ramets to 0.024 on 2- to 9-year-old ramets. As shoot lengt h increased, the probability of sawfly attack increased over 100-fold, from 0.007 on shoots <100 mm, to 0.800 on shoots in the 1001-1100 mm shoot length class. These attack patterns occurred even though 1-year- old ramets and shoots >500 mm each represented less than 2% of the tot al shoots available for oviposition. Host plant induced mortality of t he egg/early instar stage decreased by 50% on longer leaves and was th e most important factor determining survival differences between vigor ous and non-vigorous hosts. Sawfly attack was not determined by the re source distribution hypothesis. Although shoots <200 mm contained 82%, Of the total leaves available, they contained only 43% of the galls i nitiated. The attack pattern also was not explained by the gall volume hypothesis. Although gall volume increased on longer shoots, there wa s no significant variation in mid or late instar mortality over shoot length, as would be expected if food resources within smaller galls we re limited. The natural enemy attack hypothesis could not explain the pattern of oviposition since predation was greater on longer shoots an d leaves. In addition, larval survival was related to oviposition beha vior. Due to a 69% reduction in late instar death and an 83% reduction in parasitism, survival of progeny in galls initiated close to the pe tiole base was 2.8 times greater than in galls initiated near the leaf tip. A 75% reduction in gall volume over this range of gall positions may account for the observed increases in late instar mortality and p arasitism.