Parasitism rate differs between herbivore generations in the univoltine, but not bivoltine, range

With climate change, plant-feeding insects increase their number of annual generations (voltinism). However, to what degree the emergence of a new herbivore generation affects the parasitism rate has not been explored. We performed a field experiment to test whether the parasitism rate differs between the first and the second generations of a specialist leaf miner (Tischeria ekebladella), both in the naturally univoltine and bivoltine parts of the leaf miner's distribution. We found an interactive effect between herbivore generation and geographical range on the parasitism rate. The parasitism rate was higher in the first compared to the second host generation in the part of the range that is naturally univoltine, whereas it did not differ between generations in the bivoltine range. Our experiment highlights that shifts in herbivore voltinism might release top-down control, with potential consequences for natural and applied systems

The effect of host generation, host range and their interaction on the probability of parasitism of the specialist leaf miner <i>Tischeria ekebladella</i>.

Shown are χ2 values, degrees of freedom (df) and P-values of fixed effects, as well as estimates from the variance components, from a generalized linear mixed effects model. Significant P-values (P < 0.05) are in bold.</p

Overview of the study location, experimental timeline and key findings.

Panel a) shows differences in the parasitism rates between the first and second generations of the leaf miner Tischeria ekebladella in the univoltine and bivoltine ranges, respectively. Black bars represent standard errors, and an asterisk represents a statistically significant difference (P b) shows the location of the study site in the univoltine (Stockholm, Sweden) and bivoltine range (Alnarp, Sweden) of the specialist leaf miner Tischeria ekebladella. Panel c) depicts the experimental timeline, illustrating when we placed seedlings with leaf mines of the first and second generations in the field and when we collected those seedlings for assessment of parasitism rate.</p

<i>A posteriori</i> comparisons of the parasitism rates between the univoltine and bivoltine ranges, separately for each herbivore generation.

Shown are the groups compared, estimates, standard errors (SE), z-ratios and P-values. Significant P-values (P < 0.05) are shown in bold.</p

<i>A priori</i> comparisons of the parasitism rate between the first and the second herbivore generations, separately for the univoltine and bivoltine ranges.

Shown are the groups compared, estimates, standard errors (SE), z-ratios and P-values. Significant P-values (P < 0.05) are shown in bold.</p