Assessing temporal dynamics of predation and effectiveness of caterpillar visual defense using sawfly larval color and resting posture as a model

Abstract

Caterpillars (Lepidoptera and Symphyta larvae) employ diverse visual defensive tactics, and effectiveness of such tactics may be highly dynamic across time due to seasonal changes in the predator assemblages and their preferences. However, this has rarely been studied especially in tropical regions. Here we assessed temporal changes in the defensive value of caterpillar color and shape, using six types of plasticine dummy caterpillars: three colors (green, black, and white) × two shapes (curled and straight). These dummy caterpillars were deployed five times over different seasons in tropical forests of Xishuangbanna (China) and, as a comparison, twice in a temperate forest of Hirosaki (Japan). The colors and shapes of dummy caterpillars simulate visual traits of black sawfly larvae which take the curled resting posture in tropical rainforests of Xishuangbanna, apparently masquerading excrements commonly found on plants, while in Hirosaki there is no black‐curled sawfly larvae and few excrements on plants. We found no significant effects of caterpillar colors or shapes on predation in Hirosaki. In contrast, black and curled caterpillars received significantly lower predation by birds in Xishuangbanna constantly across time. However, we were unable to provide evidence that the black‐curled sawfly larvae are masquerading as excrements. Shapes of the dummy caterpillars also affected the predation by ants and parasitoid wasps at certain times. This is the first report on ecological function of the curled posture of sawfly larvae, and we demonstrated the importance to assess the temporal dynamics of predation and effectiveness of defensive tactics in tropical forests.

Estimating predation pressure in ecological studies: controlling bias imposed by using sentinel plasticine prey

“…(3) reflectance spectra of live prey, plasticine models, and natural leaves should be checked (Aslam et al, 2020;Pan et al, 2021), so that potential bias between real and plasticine models can be estimated.…”

“…Unlike invertebrates, attack rates by vertebrates (mainly birds) were not significantly different between live mealworms and plasticine caterpillars, suggesting the suitability of plasticine caterpillars to estimate predation rates by birds and other vertebrates. However, our study was conducted at the end of the rainy season when predation by vertebrates, especially birds, is relatively low (Pan et al, 2021). Further scrutiny is required to confirm the usefulness of plasticine models in different seasons, especially during the breeding season of birds (spring) in this locality.…”

“…Caterpillar‐shaped plasticine models were molded by a metal syringe (a sugar press) to make a smooth round shape (0.3 cm diameter, 3 cm long), resembling the common size and shape of lepidopteran larvae (Sam et al, 2015; Roslin et al, 2017). Following the standard method used by previous studies, these caterpillars were bent in the middle to mimic the natural ‘inchworm’ posture of caterpillars (Roslin et al, 2017; Pan et al, 2021). Plasticine cubes (0.5 × 0.5 × 0.5 cm) were made using the same, green‐colored plasticine that was pressed and cut using a metal syringe.…”

Seasonal variations in bird selection pressure on prey colouration

“…Only a handful of studies have explored seasonal changes in the intensity of bird predation on herbivorous insects, and the outcomes of these studies have been somewhat contradictory (Remmel et al 2009 ; Mappes et al 2014 ; Kozlov et al 2017 ; Pan et al 2020 ). In our study, predation peaked in mid-summer, and remained at a high level until late summer.…”

“…These results indicate that avoidance learning of neutral stimuli (neither unpalatable nor rewarding), even if it occurs, is relatively slow (Alcock 1970a) and does not compromise the outcomes of long-term observations. Only a handful of studies have explored seasonal changes in the intensity of bird predation on herbivorous insects, and the outcomes of these studies have been somewhat contradictory (Remmel et al 2009;Mappes et al 2014;Kozlov et al 2017;Pan et al 2020). In our study, predation peaked in mid-summer, and remained at a high level until late summer.…”