Ns. Cobb et al., INCREASED MOTH HERBIVORY ASSOCIATED WITH ENVIRONMENTAL-STRESS OF PINYON PINE AT LOCAL AND REGIONAL LEVELS, Oecologia, 109(3), 1997, pp. 389-397
Using 6 years of observational and experimental data, we examined the
hypothesis that water and nutrient stress increase the susceptibility
of pinyon pine (Pinus edulis) to the stem- and cone-boring moth (Diory
ctria albovittella). At two geographic levels, a local scale of 550 km
(2) and a regional scale of 10,000 km(2), moth herbivory was strongly
correlated with an edaphic stress gradient. At a local scale, from the
cinder soils of Sunset Crater to nearby sandy-loam soils, nine of ten
soil macro- and micronutrients, and soil water content were lowest in
cinder-dominated soils. Herbivore damage was six times greater on tre
es growing in the most water and nutrient deficient site at Sunset Cra
ter compared to sites with well-developed soils. Percentage silt-clay
content of soil, which was highly positively correlated with soil nutr
ient and soil moisture at a local scale, accounted for 56% of the vari
ation in herbivory at a regional scale among 22 sites. Within and acro
ss sites, increased stem resin flow was positively associated with red
uced moth attack. On the basis of moth distribution across a stress gr
adient, we predicted that pinyons growing in highly stressful environm
ents would show increased resistance to herbivores if supplemented wit
h water and/or nutrients. We conducted a 6-year experiment at a high-s
tress site where individual trees received water only, fertilizer only
, and water + fertilizer. Relative to control trees, stem growth and r
esin flow increased in all three treatments, but only significantly in
the water + fertilizer treatment. Although there was no significant d
ifference in herbivore damage among these three treatments, there was
an overall reduction in herbivore damage on all treatment trees combin
ed, compared to control trees. This experiment suggests that release f
rom stress leads to increased resistance to insect attack and is consi
stent with our observational data. While other studies have predicted
that short-term stress will result in herbivore outbreaks, our studies
extend this prediction to chronically stressed host populations. Fina
lly, while flush-feeders are not predicted to respond positively to st
ressed host plants, we found a positive association between herbivore
attack and stressed pinyon populations.