For grain and flour mill managers, heat treatments can be a powerful tool for controlling stored-grain insect pests in grain facilities and food-processing plants. Insects are sensitive to extreme temperatures, and properly applied heat treatments can achieve very high mortality rates. However, grain managers are often frustrated to discover that a few insects survive even after a heat treatment appears successful. Understanding why can help managers improve pest control programs and avoid costly reinfestations.

Insects Are Resilient

Stored-product insects thrive within a relatively narrow temperature range. Most species develop and reproduce best from about 77 to 91 degrees Fahrenheit. As temperatures move outside that range, insect development slows, and sufficient extremes will eventually cause the insects to die. However, death does not occur immediately. The temperature, exposure time, insect species and life stage all influence survival. Stored-product insects can survive considerable temperature extremes if exposure times are too short or if they find protected locations.

Why Heat Treatments Work

Commercial heat treatments typically raise temperatures in grain storage structures, flour mills and food-processing facilities to between 122 and 140 degrees Fahrenheit. At these temperatures, most insects are unable to survive when exposure is maintained long enough. Heat treatments eliminate pests without chemical residues and can reach locations where conventional insecticides are difficult to apply.

In a study conducted in empty concrete grain silos, maintaining temperatures near 122 degrees Fahrenheit for at least six hours produced 98% to 100% mortality of lesser grain borers, red flour beetles and psocids. The results demonstrated that heat treatments can be an effective and economical alternative to chemical control when sufficient temperatures are achieved throughout the structure. However, the heat treatment was not equally effective everywhere. Where small residual pockets of grain remained, insects survived.

In general, heat treatments should only be applied to empty structures. Applying heat to facilities containing raw grain or flour may risk damage to the grain or flour quality or cause a fire. High heat can scorch grain, crack kernels, lower its test weight or alter moisture profiles, rendering the grain unmarketable. If the grain is to be used for planting, high heat can severely damage or destroy seed germination.

Grain and Flour Are Excellent Insulators

Grain and flour act as insulators. Insects protected within grain kernels, grain residues or flour are more difficult to kill because heat penetration is reduced. Some insect survivors can be found in locations where temperatures do not remain high enough for a sufficient period.

Heat movement can be slowed by grain, dust accumulations and hidden refuges within a storage structure. Insects in protected areas may experience significantly lower temperatures than those elsewhere in the structure.

Insect Survivors

Not all insect species respond to heat equally. Some pests are more heat-tolerant than others. In addition, insect life stages differ in their susceptibility. Eggs, larvae, pupae and adults may require different exposure times and temperatures for complete control. Mortality can occur within minutes at temperatures near 131 degrees Fahrenheit, although susceptibility varies with species, life stage, previous acclimation and humidity. A heat treatment that kills most adults may still leave behind enough individuals to allow infestation to occur.

Hidden Refuges Create Problems

Successful heat treatment depends on achieving lethal temperatures everywhere insects can hide. Unfortunately, grain facilities offer many protected locations:

  • Residual grain accumulations
  • Dust deposits
  • Cracks and crevices
  • Equipment voids
  • Aeration ducts
  • Grain remaining in difficult-to-clean areas

Heat: Part of an Integrated Program

Because stored-product insects often hide in these protected locations, complete control can be difficult if heat does not penetrate uniformly throughout the structure. Therefore, heat treatments work best when combined with thorough sanitation and integrated pest management practices before treatment begins, temperature monitoring throughout the facility and continued insect monitoring after heat treatment.

When applied correctly, heat can be an effective nonchemical management tool for controlling insect pests.

Sources: Research published in the Journal of Economic Entomology and the Journal of Stored Products Research.

Edmond Bonjour is a senior extension specialist — stored-products entomologist in the Department of Entomology and Plant Pathology at Oklahoma State University in Stillwater. He can be reached at 405-744-8134 or edmond.bonjour@okstate.edu.