PHENOTYPIC PLASTICITY IN THE SOUTH AMERICAN LOCUST, SCHISTOCERCA CANCELLATA (ACRIDIDAE)

FOQUET, BERT; LANGE C.E.; CIGLIANO M.M.; SONG, H.; POCCO M.E.; NIEVES E.L.

Agadir

Congreso; 13th International Congress of Orthopterology; 2019

The Orthopterists' Society

Historically, Schistocerca cancellata has been considered the most serious agricultural pest in Argentina. An outbreak of a magnitude not recorded since 1954 started in 2014-15 in northern Argentina and areas of neighboring Paraguay and Bolivia. The objective of this study is to contribute to the knowledge of the expression of density-dependent phenotypic plasticity by explicitly quantifying density-dependent reaction norms in behavior, coloration, and morphology. Gregarious nymphs and adults of S. cancellata were collected in Catamarca province, Argentina in March 2016. Locusts were reared under isolated and crowded conditions for at least three generations before this study. Each treatment was placed in separate rooms at 30°C, 14 L: 10D photoperiod and 40% RH. Characterization of each isolated and crowded nymphal instar and adult stage was performed, and mean duration of stages (nymphal instars and adult stage) was recorded for both density conditions. To quantify behavioral reaction norms, the behavioral assay arena designed by Roessingh et al. (1993) was used in this study. We generated data for 52 isolated and 56 crowded final-instar nymphs (two days after molting). The behavior of each nymph was recorded for 10 minutes using a video camera. Behavioral data (activity and position-related variables) were acquired for each insect using the software EthoVision (Noldus). To quantify color reaction norms, high-resolution digital images of each specimen (lateral and dorsal views of head and pronotum, lateral view of wing pad and hind femur) were taken. From the captured images, two attributes of color were measured in ImageJ64: background color and black patterns. To quantify morphology, the linear length of the pronotum and hind femur was measured from the images using ImageJ64. In addition, the number of hairs located on the outer surface of the hind femur of each specimen was counted under a stereomicroscope. The effect of density in each of the variables (behavior, color and morphology) was tested (ANOVA/Kruskal Wallis) in R (3.4.2.). The nymphs reared in crowded conditions were significantly more active than the isolated nymphs. In terms of background color and amount of black patterns, we found clear differences between the two density treatments. We also found that rearing density had clear effects on body size depending on sex: crowded female nymphs were significantly smaller than isolated ones, while crowded male nymphs were significantly larger than isolated ones. Finally, isolated nymphs had about 50% more hairs on the outer face of the hind femora than crowded nymphs, in both sexes. This study provides conclusive evidence on presence of density-dependent phenotypic plasticity in behavior, color and morphology in the South American locust, Schistocerca cancellata. We discuss and compare our findings with studies conducted on other locust species.