Data from: Geographic variation in thermal tolerance of western corn rootworm

Seven .csv files from data collected on different thermal tolerance traits of western corn rootworm, Diabrotica virgifera. Traits include critical thermal maxima and minima (CTmax and CTmin), knock-down resistance, and chill coma recovery from 13 lab colonies (genetic lines) that were collected initially between 1994 to 2013 in Illinois, Indiana, Kansas, Nebraska, Pennsylvania, South Dakota, and Wisconsin. Files also include Metadata, site coordinates estimated as the centroid of county where collection occurred, and environmental correlates for each rootworm line from Worldclim.

Tentative abstract from the paper:

Western corn rootworm, Diabrotica virgifera virgifera, are one of the most economically important crop pests in the world with estimates of damage and control approximating over $1 billion USD annually. Yet despite an abundance of research devoted to studying rootworm biology in the central Corn Belt of the United States, information on key aspects of their thermal biology is still lacking. In this study, we quantified thermal metrics of western corn rootworm populations from across their range in the United States: we measured critical thermal limits, knock-down resistance, and chill coma recovery for male and female rootworms from 13 lab colonies that were collected across 1985 km at locations that varied by up to 5.7°C in mean annual temperature. We further use these data to test a model from thermal ecology—the thermal adaptation hypothesis—which posits that (1) thermal limits track environmental temperatures and (2) more thermally variable environments support organisms with broader thermal ranges. In doing so, we found that thermal traits varied across populations. However, only heat tolerance traits (critical thermal maximum and knock-down resistance) tracked historical averages of mean annual temperature. More thermally variable environments did not support organisms with broader thermal ranges. While theory often predicts cold tolerance should track environmental temperatures, our results suggest this pattern may disappear if organisms are reared in the lab for multiple generations and instead, a legacy effect may exist for heat tolerance that is rarely reported.

Tentative methods from the paper:

Geographic populations

Thirteen geographically distinct western corn rootworm populations have been maintained at the USDA-ARS North Central Agricultural Research Laboratory in Brookings, SD for ca. 30 years. Original field collections occurred between 1994 to 2013 in Illinois, Indiana, Kansas, Nebraska, Pennsylvania, South Dakota, and Wisconsin. For all thermal trials, we used an equal number of female and male adults that were collected three days after eclosion. Additional rootworms were also kept at ambient room temperature (22°C) for each thermal tolerance trial as a control—all of which survived.

WorldClim values for mean annual temperature (i.e. bioclimatic variable BIO1) and annual temperature range (BIO7) averaged from 1970 to 2000 were extracted using the latitude and longitude for the centroid of the county in which each population was collected. Other bioclimatic variables like mean temperature of the warmest quarter (BIO10) and mean temperature of the coldest quarter (BIO11) were considered, as they represent extreme temperatures that rootworm experience, however they were multicollinear with mean annual temperature (Variance Inflation Factor > 3) and not analyzed further. The spatial resolution was ca. 18.5 km x 18.5 km (10 arcmin).

Critical thermal limits

Critical thermal maxima (CT_max) and critical thermal minima (CT_min) were measured using a dynamic heating/cooling ramping assay that has been commonly used to measure thermal tolerance. For each population, we used 30 individuals. Assays were conducted by placing individual rootworm into 1.5 ml microcentrifuge tubes that had been modified with cotton to remove a thermal refuge in the cap. Microcentrifuge tubes were then placed into a prewarmed/precooled EchoThermTM IC20 heating/chilling dry bath set at 15°C for CT_min and 35°C for CT_max. After an assay began, we checked rootworms every 10 minutes to see if they had reached their critical thermal limit by rotating the vials and looking for a righting response. Dry bath temperature was then increased/decreased 1°C (ramping rate = 0.1°C min^-1) and the process was repeated until all rootworms had lost muscle control.

Knock-down resistance and Chill coma recovery

We used a new set of 30 individuals from each geographic population to determine how long individuals could survive at temperatures near their critical thermal maxima (i.e. knock-down resistance). We did this by placing individual rootworm into 1.5ml microcentrifuge tubes in a dry bath set at 41°C—a value close to their CT_max. We then checked rootworms every 10 minutes for 120 minutes to determine if and at what time individuals could no longer right themselves.

We used 0°C—a value close to the lowest CT_min for western corn rootworm—to approximate the temperature at which individuals enter a reversible, paralyzed state know as a chill coma. Using a new set of 30 individuals from each population, we quantified the length of time it took to recover from a chill coma (i.e. chill coma recovery) by first placing individual rootworm into 1.5ml microcentrifuge tubes in a dry bath set at 0°C for 2 hours. We then removed rootworms from the dry bath after the elapsed time and checked individuals every 5 seconds to see if they could right themselves within a two-hour observation period at ambient room temperature (22°C).