Data from: Effects of short-term heat stress on the thermal tolerance of western corn rootworm (Coleoptera: Chrysomelidae)

Data files for manuscript under review titled "Effects of short-term heat stress on the thermal tolerance of western corn rootworm (Coleoptera: Chrysomelidae)".

Two .csv files: Metadata and Master Data. The Master Data tab contains all of the information used in the analyses.

Abstract from paper under review:

The effect that prior high temperature exposure has on insect thermal tolerance is complex and depends on the degree of heat stress experienced; high heat exposure may allow for individuals to tolerate higher temperatures through hardening or may reduce an individual’s capacity to withstand higher temperatures through accumulated heat stress. In this study, we assessed how short exposures to high temperatures and a laboratory colony’s geographical origin affected the critical thermal maximum (CTmax) of western corn rootworm (WCR, Diabrotica virgifera virgifera LeConte), an economically important crop pest. Despite a wide latitudinal range of source populations, WCR colonies did not differ in their CTmax. Regardless of colony origin, we found that exposing WCR to higher temperatures resulted in lower CTmax, which suggests that heat stress accumulated. This study highlights how WCR experiences heat stress at temperatures near the high temperatures they experience in the field, which may have important and currently unknown implications for its behavior.

Methods from paper under review:

Geographic populations and rearing

WCR colonies were collected from geographically distinct populations from across their U.S. range and were maintained in the lab under constant conditions. Field collections of the individuals used to start the colonies occurred from 1995 to 2013 in Kansas, Nebraska, Pennsylvania, South Dakota, and Wisconsin. Since collection, the colonies have been maintained under optimal conditions for WCR development. We incubated eggs for 10 d in moist soil. Then, we added 40 mL pre-soaked corn kernels, 175 mL soil, 60 mL of water, and ca. 500 (487 ± 3, mean ± SE) viable eggs into small plastic containers (13 x 11 x 7 cm). Across all life stages, WCR were exposed to constant temperature (25°C) and humidity (60%) within environmental chambers and soil moisture was inspected visually every 2-3 d to ensure rootworm did not desiccate.

At 350 degree days (DD) (roughly half of WCR development time), we transferred WCR to new containers with ample amounts of 1 week old corn for successful development to adulthood. At 550 DD (typical WCR emergence is at 700 DD), we placed the containers into emergence cages (34 × 25 × 10 cm), which consisted of a dark box with a clear collection tube. We turned on lights in environmental chambers to draw adult WCR into collection tubes. Emergence tubes included an agar water source to limit the effects of dehydration on adult WCR.

Critical thermal limits

During emergence, we removed all emerged adult WCR every 1-2 d and conducted CTmax assays on a subset of post-teneral adults. We measured CTmax using a dynamic ramping assay. We placed individual adult WCR in 1.5 mL microcentrifuge tubes into a prewarmed EchoThermTM IC20 heating/chilling dry bath. Cotton balls were added to the top of the microcentrifuge tubes to remove access to thermal refugia in the caps during assays. We evaluated whether individuals had lost muscle control (i.e., reached their CTmax) and increased the temperature by 1°C every 10 minutes (ramping rate = 0.1°C/min) until all individuals lost muscle control, as indicated by an inability to show a righting response.

We first conducted these thermal tolerance trials for 10 individuals from each colony (total n = 60) with assays starting at 35 °C to evaluate differences in heat tolerance across colonies. Then, to assess the interactive effects of colony geographic origin and cumulative heat stress, we exposed new sets of WCR to a range of temperatures from 30 to 40 °C (10 levels, 31 °C omitted; n = 6 WCR per colony and temperature; total n = 360) for 1 hr and then ran thermal tolerance assays starting at 35 °C as above.