What started out as a search for weevil eggs and other insect items inside cereal grains morphed first into a way to identify Anopheles mosquitos infected with the malaria parasite, and then into an insect-sexing technique for use on tsetse flies. The common thread is near-infrared (NIR). It turns out that every living organism has its own NIR radiation absorption signature based on its hydrocarbon components, which a spectrometer can analyze and provide data on for expert analysis. For instance, a grain of wheat containing a weevil egg has a signature that is distinguishable from an intact kernel. To facilitate grain inspection, ARS engineer Floyd Dowell designed an automated single-grain handling system that exposed each kernel to a source of NIR light.
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So why would anyone want to sex a tsetse? As part of a program to curtail the sleeping sickness caused by a bite from a tsetse fly infected with the trypanasome parasite. The males are sterilized with radiation, after which they behave and mate normally but can't fertilize the females' eggs. Repeated releases over time cause the targeted insect population to crash. Instead of hand-sorting the adult insects, entomologists discovered they could determine a tsetse's sex in its pupal stage, between the larval and the adult form. When pupating, the insect is encased in a cuticle or cocoon that for tsetse flies is close to the size of a grain kernel. Very convenient, because the pupae could be fed automatically through Dowell's grain handler. The technique works best at five days before the pupae emerge as adults because the spectral signatures of females and males differ most prominently at that point. The reasons have yet to be discovered, but could be changes in cuticle thickness or ovarian development in the females.
Contact Floyd Dowell, Agricultural Research Service, Grain Marketing and Production Research Center, Manhattan, KS; 785-776-2753, [email protected], www.ars.usda.gov.