Ambient temperature affects the utility of plasticine caterpillar models as a tool to measure activity of predators across latitudinal and elevational gradients

Abstract

Predation is a key ecosystem process and central to the practice of biological control. Artificial caterpillars moulded from plasticine have been widely used to compare attack rates in a variety of field settings. A recent study extended their use to latitudinal and elevational gradients and concluded that herbivores at lower latitudes and elevations suffer higher rates of attack by invertebrate predators. The present laboratory study examined effect of ambient temperature on the capacity of plasticine caterpillars to be marked by either a simulated attack of standardised force or by minute-long exposure to live ants. The effect of temperature on ease of marking of plasticine caterpillars was tested by preparing the plasticine at 2, 4, 8, 16, and 32 °C. The first three experiments tested the prepared plasticine at room temperature, immediately after removal of model prey from the preparation temperature and before it reached room temperature. A fourth experiment tested the plasticine caterpillars at the same temperatures at which they were prepared (i.e., 2, 4, 8, 16, and 32 °C), exposing the plasticine caterpillars to live ants. In all studies, the plasticine caterpillars were subsequently inspected for visible impressions by nine independent assessors who were unaware of the experimental aim or treatments. The assessors designated individual plasticine caterpillars as marked or unmarked. Here we report that plasticine caterpillars in the first three experiments were significantly more likely to be assessed as marked if the attack took place at higher temperatures. There was no significant difference in the incidence of visibility of attack marks on plasticine caterpillars at 2 °C and 4 °C but each successive temperature increment above 4 °C had a significant (p < 0.05) effect. Similarly, in the fourth experiment, there was no significant difference in the incidence of visible markings between 2 °C and 4 °C, significant (p < 0.05) increases for successive temperature increments above 4 °C, but a plateau such that the highest two temperature treatments did not differ. We conclude that the physical properties of plasticine make it an unsuitable substrate for prey models in studies where treatment sites have different temperatures such as those that have elevation or latitude gradients.