Sizing up spotted lanternfly nymphs for instar determination and growth allometry

Abstract

A major ongoing research effort seeks to understand the behavior, ecology and control of the spotted lanternfly (SLF) (Lycorma delicatula), a highly invasive pest in the U.S. and South Korea. These insects undergo four nymphal stages (instars) before reaching adulthood, and appear to shift host plant preferences, feeding, dispersal and survival patterns, anti-predator behaviors, and response to traps and chemical controls with each stage. However, categorizing SLF life stage is challenging for the first three instars, which have the same coloration and shape. Here we present a dataset of body mass and length for SLF nymphs throughout two growing seasons and compare our results with previously-published ranges of instar body lengths. An analysis using two clustering methods revealed that 1st-3rd instar body mass and length fell into distinct clusters consistently between years, supporting using these metrics to stage nymphs during a single growing season. The length ranges for 2nd-4th instars agreed between years in our study, but differed from those reported by earlier studies for diverse locations, indicating that it is important to obtain these metrics relevant to a study's region for most accurate staging. We also used these data to explore the scaling of SLF instar bodies during growth. SLF nymph body mass scaled with body length varied between isometry (constant shape) and growing somewhat faster than predicted by isometry in the two years studied. Using previously published data, we also found that SLF nymph adhesive footpad area varies in direct proportion to weight, suggesting that footpad adhesion is independent of nymphal stage, while their tarsal claws display positive allometry and hence disproportionately increasing grasp (mechanical adhesion). By contrast, mouthpart dimensions are weakly correlated with body length, consistent with predictions that these features should reflect preferred host plant characteristics rather than body size. We recommend future studies use the body mass vs length growth curve as a fitness benchmark to study how SLF instar development depends on factors such as hatch date, host plant, temperature, and geographic location, to further understanding of life history patterns that help prevent further spread of this invasive insect.

Fig 1. Photograph of 1st, 2nd, 3rd, and 4th instar spotted lanternfly nymphs.

Double-headed arrow shows the definition of body length, L. (scale bar = 10 mm).

Fig 2. Clustering of spotted lanternfly nymph mass vs length data.

Body mass vs length (filled circles) for (A) 2021 and (B) 2022 data; cluster centroids are shown as black x markers and red lines indicate scaling law fits to all data. Shaded ellipses show the 95% CI for each cluster based on the Mahalanobis distance; note that the shaded ellipses for some clusters are covered by datapoints. (C) Symbols and horizontal lines show the means and ranges of lengths, respectively, for each instar reported in previous studies (Table 1).

Fig 3. Comparison of body length and mass for each spotted lanternfly nymph stage from this study and previous research.

(A) Spotted lanternfly nymph body length vs instar from this study and previous work (Table 1). (B) Spotted lanternfly nymph body mass vs instar from this study. Error bars are 95% CI for this study and the measures of variance given in Table 1. (Lines between datapoints show overall trends).

Fig 4. Results from Dyar’s Rule analysis of data from this study and previous research.

(A) Plots of spotted lanternfly nymph log(body length) plotted vs estimated instar number as a test of Dyar’s rule. Markers show data from this study and three previous papers, while lines indicate linear fits. (B) Growth ratios, G, from the fits in (A); dashed line indicates the mean value for 2022 from this study. All error bars are 95% CI.

Fig 5. Plots and fits showing how spotted lanternfly foot and mouthpart data relate to body size measures.

A) Plots of data and fit results for the dependence on log body length for (A) the log tarsal claw tip-to-tip width, TCT, (B) arolium area, A_adh, (C) labium length, L_L, and (D) stylet length, L_S. Footpart and mouthpart morphometric data are from [14], while nymph body length and mass are from this study and adult mass and length from [27]. (Fit lines and 95% confidence intervals from ordinary linear regression fits to power laws, as described in the main text).