Density-Dependent Phenotypic Plasticity in the South American Locust, Schistocerca cancellata (Orthoptera: Acrididae)

Abstract

Historically, the South American locust, Schistocerca cancellata (Serville, 1838), has been considered the most serious agricultural pest in Argentina. An outbreak of a magnitude not recorded since 1954 started in 2015 through 2017 in northern Argentina and neighboring Paraguay and Bolivia. Schistocerca cancellata is widely considered as a true locust, with pronounced locust phase polyphenism, although the expression of its phenotypic plasticity has never been quantitatively tested under different density conditions. In this study, we explicitly quantified density-dependent reaction norms in behavior, coloration, and morphology in last instar nymphs of S. cancellata under isolated and crowded conditions. We also quantified density-dependent plasticity in adults (size) and in some life history traits. Our results showed that crowded nymphs were significantly more active and more attracted to congeners than isolated nymphs, and developed a much higher percentage of black pattern color. We also found that density had strong effects on body size and there was a sex-dependent pattern in both nymphs and adults, revealing that differences in size between males and females were less pronounced in crowded locusts. We have recorded for the isolated nymphs the presence of about 50% more hairs in the hind femora than in crowded nymphs. Finally, the mean duration of each nymphal instar and adult stage was significantly longer in isolated individuals. We have found strong resemblance with the desert locust, S. gregaria (Forskål, 1775) in several traits, and we conclude that S. cancellata exhibits an extreme form of density-dependent phenotypic plasticity in behavior, coloration, morphology, and life history traits.

Comparative analysis of phenotypic plasticity sheds light on the evolution and molecular underpinnings of locust phase polyphenism

“…Among locust species, solitarious nymphs are often cryptically coloured and green, while gregarious nymphs exhibit black patterns on a yellow, orange, or red background colouration 15 , 16 , 29 , 38 , 40 , 51 – 53 . In addition, solitarious and gregarious locusts exhibit species-specific differences in their size 20 , 28 , 51 , and the ratio of the hind femur length over the head width (F/C ratio) is considered a reliable predictor of the phase state, with larger values observed in solitarious individuals 20 , 40 , 51 , 54 . Thus, we hypothesised that the four species would exhibit varying levels of plastic reaction norms in these traits, and that there would be a similar spectrum of reaction norms in morphology among the species.…”

“…Based on the well-resolved phylogeny of the genus 26 , 27 , we now know that the desert locust ( S. gregaria ), for which we have extensive data on the proximate mechanisms of locust phase polyphenism 19 , occupies the basal position and is sister to the rest of the genus, which implies that the ancestral condition for the genus is the presence of density-dependent phenotypic plasticity 26 . Importantly, the three locust species in the genus, the desert locust, the Central American locust ( S. piceifrons (Walker)), and the South American locust ( S. cancellata (Serville)), do not form a monophyletic group, which suggests that locust phase polyphenism has been lost and gained multiple times 26 , 28 , 29 . Furthermore, recent studies have shown that non-swarming sedentary grasshopper species in the genus have the ability to exhibit varying degrees of density-dependent phenotypic plasticity in behaviour, colour, and morphology 26 , 30 – 33 , suggesting that some aspects of density-dependent phenotypic plasticity may be phylogenetically conserved 26 , 29 , 34 , 35 .…”

A Review of the Biology, Ecology, and Management of the South American Locust, Schistocerca cancellata (Serville, 1838), and Future Prospects

“…In the laboratory, the nymphal body size is affected by rearing density, and there is a sex-dependent pattern. Crowded final instar female nymphs are smaller than those reared isolated, while crowded male nymphs are larger, resulting in a less pronounced body size difference between crowded males and females [9] (Table 1). In the field, sexual dimorphism has not been thoroughly studied, though recent observations in Argentina (Piou et al, unpublished data) suggest that female nymphs are larger than males.…”

LA LANGOSTA VOLADORA Schistocerca cancellata (SERVILLE, 1838) (ACRIDIDAE: CYRTACANTHACRIDINAE) EN LA REPÚBLICA ORIENTAL DEL URUGUAY

“…Schitoscerca cancellata tiene metamorfosis gradual, completa su ciclo a través de los estados de huevo, ninfa (juveniles) y adulto, y se caracteriza porque tiene dos fases, solitaria y gregaria. Pocco et al (2019) establecieron que las hembras y los machos desarrollados de manera solitaria tienen un largo de cuerpo de 66.0 mm (60.0-70.8) y 52.7 mm (47.5-56.1), respectivamente. Cuando se desarrollan en densidades poblacionales muy altas las hembras miden 67.2 mm (52.3-75.3) y los machos 58.1 (47.5-65.4).…”

“…Tienden a agruparse en "bandas" y trasladarse a través de la vegetación, caminos y rutas (De Wysiecki & Lange, 2005). En la fase solitaria, cuando los individuos están en bajas densidades, las ninfas son de color verde (Pocco et al, 2019).…”