Stored product beetles

For farmer storing harvested food grains these names strike fear into them: Rusty grain beetle, flour mill beetle, flat grain beetle. These beetles can be found feeding on grain and cereal products, but also other dried material of plant origin, as well as packaged and processed goods. They have been recorded in wheat, corn, rice, barley, flour, oilseeds, cassava root, dried fruits and even chilies. Their larvae feed preferentially on the germ of the whole kernels, but sometimes they hollow out the entire kernel. Growth of mold in the kernel renders it more suitable as larval food. 

All these species belong to the genus Cryptolestes and apparently they unable to feed on sound grain, but instead need kernels with very slight imperfections or injuries. There are at least 15 species of the genus which are regarded as storage pests of economic importance and cause losses in temperate and tropical regions around the world.  Five of these species, namely Cryptolestes ferrugineus (rusty grain beetle), Cryptolestes pusillus (flat grain beetle), Cryptolestes turcicus (flour mill beetle), Cryptolestes pusilloides and Cryptolestes capensis, represent the most common Cryptolestes beetles found in stored products.

Identification of Cryptolestes species is traditionally based on morphological characters of adults. The species are small (c. 2 mm), similar in appearance, and difficult to identify on the basis of external morphological characters alone. The characters are variable and differences among species are not obvious . It is generally agreed that identification requires examination of male or female genitalia . Only experts can identify species of the genus Cryptolestes accurately. 

Unfortunately, more frequently found in stored products are body fragments and larvae, both of which lack specific features for species identification. Reason enough to test if DNA Barcoding can help with this issue. An international team of researchers now tested the suitability of DNA Barcoding for the identification of the five most common species (listed above).

In the present study, the five pre-identified Cryptolestes species were successfully diagnosed to species level by use of DNA barcoding. Also the larvae were successfully identified. The abdomens removed from adults in order to avoid their negative influence on sequencing (gut content) could also be used as voucher material, because the genitalia inside abdomens are decisive diagnostic characteristics for Cryptolestes sp. (Bank, 1979).

For their proof of concept study they used cultures of the five species that were obtained in labs on three different continents (Asia, Europe, North America). The researchers obtained 93 specimens of adults and larvae which were expert identified and subsequently subjected to a standard DNA Barcoding routine. Not surprisingly the colleagues were very pleased with the results:

For researchers or practitioners, with access to DNA sequencing facilities, our data make it possible to rapidly identify the five species of Cryptolestes based on simple DNA sequence comparisons and will facilitate the identification in quarantine inspection and other pest control efforts. The present study also provides a large set of sequences to design species-specific PCR primers annealing to regions displaying variation among species but not among populations or individuals of the same species, which makes it possible to design rapid identification kits.