Movement of Rusty Grain Beetle in Response to Temperature Gradients in Stored Wheat
Paul W. Flinn and David W. Hagstrum
The rusty grain beetle, Cryptolestes ferrugineus, is the most common insect pest of stored wheat in the United States and Canada. Adults and larvae feed mostly on the wheat germ and cause considerable damage. Beetle population growth rate is primarily affected by grain temperature. In the fall, the periphery of the grain mass cools more rapidly than the center. Beetles often reach high densities in the center of the grain mass, because warmer temperatures there allow the population to increase during the winter. Beetle populations may also be higher in the center if they are able to move from the cool periphery towards the warm center of a grain mass. However, temperature gradients in a grain bin are often small (5°C/m), so it may be difficult for insects to locate warmer regions of a grain mass. To predict rusty grain beetle population growth in bins, we need to know if they move towards and remain in warmer regions of a grain mass.
The test arena consisted of a 56 cm diameter cylinder with 9 cm high metal sides, and insulated top and bottom. The cylinder was filled with hard red winter wheat (12% moisture). A temperature gradient was established by heating the center of the cylinder with an aquarium heater immersed in a water bath. The perimeter of the cylinder was cooled to 20°C by keeping the arena in an environmental chamber maintained at 20°C. The chamber was kept at 30°C for the no-gradient experiment.
After establishing the temperature gradient for 24 hours, 98 adult rusty grain beetles were evenly distributed over the grain surface and the insulated top was secured to the cylinder.
Twenty-four hours later, the top was removed and a metal divider was inserted into the grain that partitioned the cylinder into 13 compartments. The grain was removed from each compartment using a vacuum device and sieved for insects.
After 24 hours, rusty grain beetles moved into and remained in the warm center of the grain mass in the high, moderate and low temperature gradients. Insects remained evenly distributed throughout the grain mass when there was no temperature gradient.
When a temperature gradient was established, insect densities were 10 times higher in the center compartment than in the middle or outer compartments. With no temperature gradient, insect density was not significantly different between the center, middle, and outer compartments.
Insects moved into and remained in warmer areas of the grain mass after 24 hours. Beetle preference for the warmest area of the grain mass occurred at 42-20°C, 24-20°C, and 21-20°C temperature gradients. The beetles were able to locate the warmest area even at the smallest gradient of 3.7°C/m (1°C/0.27m). Grain stored in bins cools fastest on the outside and remains warmer longer in the center. In the fall, gradients often reach 7-10°C/m (Hagstrum 1987). This study suggests that rusty grain beetles should move toward the warmer, inner regions of a grain mass as the periphery of the grain cools in the fall. Beetles should also move toward the periphery of the grain mass as it warms in the spring. This movement will be incorporated into a spatial model of rusty grain beetle population dynamics (Flinn et al. 1992).