Monoterpene Variation Mediated Attack Preference Evolution of the Bark Beetle Dendroctonus valens

Several studies suggest that some bark beetle like to attack large trees. The invasive red turpentine beetle (RTB), Dendroctonus valens LeConte, one of the most destructive forest pests in China, is known to exhibit this behavior. Our previous study demonstrated that RTBs preferred to attack large-diameter trees (diameter at breast height, DBH ≥30 cm) over small-diameter trees (DBH ≤10 cm) in the field. In the current study, we studied the attacking behavior and the underlying mechanisms in the laboratory. Behavioral assays showed that RTBs preferred the bark of large-DBH trees and had a higher attack rate on the bolts of these trees. Y-tube assays showed that RTBs preferred the volatiles released by large-DBH trees to those released by small-DBH trees. Subsequent analysis revealed that both large- and small-DBH trees had the same composition of monoterpenes, but the concentration of each component differed; thus it appeared that the concentrations acted as cues for RTBs to locate the right-sized host which was confirmed by further behavioral assays. Moreover, large-DBH pine trees provided more spacious habitat and contained more nutrients, such as nitrogen, than did small-DBH pine trees, which benefited RTBs' fecundity and larval development. RTBs seem to have evolved mechanisms to locate those large hosts that will allow them to maximize their fitness. Monoterpene variation mediated attack preference implies the potential for the management of RTB

Trade-offs of host use between generalist and specialist **Helicoverpa** sibling species : adult oviposition and larval performance

Much attention has been paid to the question of the relative importance of female behaviour versus larval feeding capacities in determining the host range of herbivorous insects. Host-use trade-offs displayed by generalist and specialist sister species of the genus Helicoverpa were evaluated to examine the relationship between maternal choice and offspring performance. The prediction of optimal oviposition theory, that females will choose to lay eggs on plants on which their offspring perform best as larvae, was tested by measuring oviposition preference and larval performance of Helicoverpa armigera and H. assulta on tobacco, sunflower, and hot pepper. These two measures were more highly correlated in the specialist H. assulta. Both species exhibited the same oviposition preference ranking: tobacco > sunflower > hot pepper. H. armigera larvae preferred sunflower, followed by tobacco and hot pepper; while H. assulta larvae preferred tobacco to sunflower and hot pepper, consistent with their mothers' oviposition preference. Duration of the total period from egg to adult emergence for each species was significantly shorter on the host plant preferred by the larvae. H. assulta had shorter larval duration and higher relative growth rate than H. armigera on tobacco and hot pepper, and vice versa for sunflower, indicating species differences in host utilization. Thus, while only the specialist H. assulta displayed the predicted optimal oviposition pattern, females of both species show the least preference for the plant on which their offspring perform worst. Selection for optimal oviposition may be stronger on the specialist, which has fewer choices and lower lifetime fecundity than the generalist

Body size difference of <i>Monochamus alternatus</i> larvae for dual-choice and no-choice tests of <i>Sclerodermus harmandi</i>.

<p>(A) beetle larvae for dual-choice test; (B) beetle larvae for no-choice test. Bars indicate standard errors and different letter on the bar means significantly different at p≤0.05 (ANOVA) (A) and p≤0.05 with Bonferroni Multiple Comparison (ANOVA) (B).</p

Quantitative variation in monoterpenes of large and small trees of Chinese pine <i>P. tabulaeformis</i>.

<p>Quantitative variation in monoterpenes of large and small trees of Chinese pine <i>P. tabulaeformis</i>.</p

The comparison of RTB body thickness and two size host-phloem thickness.

<p>Bars indicate standard errors and different letters on bar indicate significant differences at p≤0.05 with Bonferroni Multiple Comparison (ANOVA).</p

Analysis of RTB attacking behavior on large- and small-diameter Chinese pine <i>P. tabulaeformis.</i>

<p>Values in parentheses are numbers of sample size.</p><p>(*) means p<0.05; (**) means p<0.01; (***) means p<0.0001.</p

Correlation between <i>Monochamus alternatus</i> larvae size and <i>Sclerodermus harmandi</i> offspring fitness indices.

<p>Effect of host size is assessed separately for (A) offspring body size (y = 0.0069×+4.8103, r² = 0.0.113, F_1,84 = 10.706 <i>p</i> = 0.002), (B) offspring survival (y = 0.0761×+31.913, r² = 0.086, F_1,88 = 8.299, <i>p</i> = 0.005), and (C) Offspring sex ratio (female percentage) (y = −0.0246×+98.916, r² = 0.0764, F_1,85 = 7.035, <i>p</i> = 0.01).</p

Duration of pre-oviposition and fecundity of <i>Sclerodermus harmandi</i> females on <i>Monochamus alternatus</i> larvae.

<p>(a) Duration of pre-oviposition of adult females; (b) female fecundity. Bars indicate standard errors and different letter on the bar means significantly different at p≤0.05 with Bonferroni Multiple Comparison (ANOVA).</p