Mona Cromb, of Green Island, asks :-

I found an infestation of small insects in a drawer in my writing desk. (sample enclosed) I had recently thrown out a stained wheat bag stored in the drawer. What are they and why did they accumulate there?

Diane Barton and Bruce Philip, entomologists with Agresearch, responded.

Although the specimens were badly damaged in transit they are identified as a weevil belonging to the genus Sitophilus. Given the location (Dunedin), likely host food (wheat) and the size of the specimens it is likely they were the granary weevil Sitophilus granarius. Sitophilus is a cosmopolitan genus of weevils (which are a family of beetles), a number of which are serious pests of grains and stored products.

There are three common species of Sitophilus in New Zealand – the granary or wheat weevil (Sitophilus granarius), rice weevil (Sitophilus oryzae) and maize weevil (Sitophilus zeamais). None are native to New Zealand. The granary weevil is common throughout the world, while rice and maize weevils are widespread in tropical and subtropical countries.

In New Zealand rice and maize weevils are more abundant in the north than further south. Generally the rice weevil prefers rice or wheat, while the maize weevil prefers maize grain or its products. Granary weevils are a pest of many types of grain, including wheat, oats, rye, barley, rice and maize. All three species will attack pasta or caked flour.

Adult size is variable and depends on the size of the grain kernels they developed in. However, the granary weevil is generally larger (up to 5mm) than rice and maize weevils (up to 3.5mm). Rice and maize weevils are capable of flying, while the granary weevil lacks wings. Adult weevils can live up to 8 months, and females can lay several hundred eggs, with a single egg being deposited inside each grain kernel. The immature stages (larvae) of the weevil look very different from the adults, being plump, legless and white with a tan head. The larvae feed on the contents of the grain kernels and metamorphose into adults via a non-feeding, immobile pupa stage. All larval and pupal development takes place inside a single grain kernel; only adults are generally seen outside the grains.

The weevils can complete a generation in about 5 weeks in summer. Granary weevils, which can survive in colder climates, may take up to 20 weeks per generation at cooler temperatures. It is estimated that one pair of granary weevils is capable of producing up to 6,000 offspring per year.

Storing grains in containers with tight lids is the best way to prevent infestations of Sitophilus weevils (as well as other stored product pests). If an infestation is suspected, carefully examine the grains for adult weevils or holes in the kernels. Even if an infestation is identified early, disposal is likely to be the only effective solution. Infested grains should be frozen for several days prior to disposal to kill the weevils.