Forest litter crickets prefer higher substrate moisture for oviposition: Evidence from field and lab experiments

<div><p>For insects, choosing a favorable oviposition site is a type of parental care, as far as it increases the fitness of its offspring. Niche theory predicts that crickets should show a bell-shaped oviposition response to substrate moisture. However, lab experiments with mole crickets showed a linear oviposition response to substrate moisture. Studies with the house cricket <i>Acheta domesticus</i> also showed a linear juvenile body growth response to water availability, thus adult ovipositing females should respond positively to substrate moisture. We used a field experiment to evaluate the relationship between oviposition preference and substrate moisture in forest litter-dwelling cricket species. We also evaluated oviposition responses to substrate moisture level in <i>Ubiquepuella telytokous</i>, the most abundant litter cricket species in our study area, using a laboratory study. We offered cotton substrate for oviposition which varied in substrate moisture level from zero (i.e., dry) to maximum water absorption capacity. We used two complementary metrics to evaluate oviposition preference: (i) presence or absence of eggs in each sampling unit as binary response variable, and (ii) number of eggs oviposited per sampling unit as count response variable. To test for non-linear responses, we adjusted generalized additive models (GAMM) with mixed effects. We found that both cricket oviposition probability and effort (i.e., number of eggs laid) increased linearly with substrate moisture in the field experiment, and for <i>U. telytokous</i> in the lab experiment. We discarded any non-linear responses. Our results demonstrate the importance of substrate moisture as an ecological niche dimension for litter crickets. This work bolsters knowledge of litter cricket life history association with moisture, and suggests that litter crickets may be particularly threatened by changes in climate that favor habitat drying.</p></div

Iguaçu National Park, Foz do Iguaçu, PR, Brazil.

<p>The star represents the geographic coordinates of the experiment location.</p

Cricket oviposition responses to substrate moisture level.

<p>(a) represents oviposition probability, a binary response variable with a value of either 0 (if oviposition did not occur in any replicate containers for that moisture level) or 1 (if oviposition occurred at least one of the container replicates in that level). Circles represent the field experiment, while triangles represent the lab experiment. The size of circles and triangles represents the number of observations (= the number of replicates in which the binary response occurred) for the same moisture level. Curves represent the minimal adequate model of the adjusted logistic regression (solid line for field experiment, dashed line for lab experiment; n = 30 for each experiment). (b) Numbers of eggs deposited per container by moisture level in field (circles) and lab (filled triangles) experiments. Curves represent the minimal adequate model of the adjusted regression (GLMM with negative binomial distribution) for field (solid line) and lab (dashed line) experiments (n = 30 for each experiment). The equations represent the estimated parameters for each model.</p

Numbers of eggs oviposited on cotton substrate (values summed per moisture level) after 48 hours in field and lab experiments.

<p>Numbers of eggs oviposited on cotton substrate (values summed per moisture level) after 48 hours in field and lab experiments.</p

Experimental design for field (a) and lab (b) experiments.

<p>Arrangement of moisture levels in both experiments was randomized.</p