Bad Bugs: Insects That Can Cause Problems

By Alain VanRyckeghem, BCE

Warehouse Beetle
Trogoderma variabile

This persistent little pest is a member of the genus Trogoderma which also includes the dreaded quarantine pest, the Khapra Beetle (T. granarium). The Warehouse beetle is a common pest of stored foods in the United States and is found in many other countries. It is about 1/8 inch long, brownish black in appearance with elytra that have a mottled color pattern and lightly covered with hairs. There are about six Trogoderma species that can be commonly encountered in stored food environments. Identification cannot be easily done by visual observation alone. Careful microscopic examination is required to separate the common food pest species from the less serious ones, and from the most undesirable Khapra beetle.

Adults may live only 10 days, but with food such as pollen, they may survive up to 60 days. During that time the female may lay as many as 100 eggs. Under optimal conditions of 90°F, eggs will hatch within 5-7 days, larvae will develop over 35-40 days, then pupate for another 5-7 days before emerging as adults again. Two or three generations per year are possible. The quality of food, temperature, moisture, and humidity has a significant impact on development time of the larvae. It is not uncommon for larvae to take 1-2 years before pupating. Larvae will molt or shed their exoskeleton 5-6 times, leaving the characteristic cast skins as evidence of their infestation. Larvae have been observed to shed their skins monthly for 9 consecutive months before pupation.

These adult beetles are excellent fliers and are attracted to UV lights, pheromone traps, and food odors. Trogoderma beetles are common insects outdoors and can fly considerable distances (e.g.1/4 mile) on the trail of food odors. The larvae are not attracted to light and favor dark cracks and crevices in walls and floors. Larvae feed on high protein foods, including processed wheat, corn, and other cereal grains, fish meal, milk powder, dried spices, nuts, and processed pet foods. It can also sustain itself on dead animal
carcasses and insects.

Trogoderma spp are difficult to control with cold temperature. They are cold tolerant. Eggs will not survive -2°F (-19°C) for 10 days or more. Control can be easily achieved with heat at 122°F (50°C) after 12 hours. Fumigation is one of the most effective ways of killing all stages of this insect in commodities and buildings. Fogging with a 3% pyrethrin is recommended, but this method will only kill exposed stages. Larvae, eggs and adults which are hidden in product or cracks and crevices of walls and floors will not be controlled with a pyrethrin fogging. Use of esfenvalerate (Conquer) and insect growth regulators in a fog will give longer lasting effects.

Warehouse beetles are excellent fliers, and thus hanging traps with pheromones are useful and effective at attracting male beetles. Note: Not every Trogoderma beetle is attracted to the same pheromone lure. Insect light traps can however, attract both sexes and most Trogoderma species to the glue boards. Interpretation of population activity within an area can be more difficult due to the long range of flight of the beetles. These beetles could fl y several hundred feet from the source to fly into a light trap.

Monitoring of larvae can only be achieved by the use of floor level pitfall traps, or a uniquely designed wall trap. Larvae will be attracted to the food baits in these traps and not the pheromone. The most important fact to know about monitoring for this group of beetles is that the Khapra beetle (Trogoderma granarium) adult does not fl y and thus monitoring for this pest is done only with pitfall and wall traps with both food and pheromone attractants.

In summary, the Warehouse beetle is a small beetle that resembles many other species, of which some are very important, and others less so. Do not base your monitoring and control programs on a “guess.” Get a positive identification.


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