Friday Cat-erpillar Blogging

The velvetbean caterpillar is an occasional pest of soybean in the Southern US.  Soybean plants  are well protected against many insect pests and in many years can be grown without insecticide treatments.  The soybean is protected by substances called proteinase inhibitors that interfere with digestion.  Animals (including insects) that try to eat soybeans are unable to digest the soybean proteins because their protein digesting enzymes are inhibited by soybean substances. The soybean proteinase inhibitors protect the soybeans in storage from insect damage until it is time to make them into meal. 

The inhibitors in soybeans can be deactivated by heat treatment or by fermentation.  Soybeans fed to livestock, or processed into soybean meal for use as human food are heated before consumption. Lafayette, IN and occasionally the Purdue campus is treated to the odor of roasting soybeans emanating from a local soybean processor. The proteinase inhibitors are being deactivated. Fermentation of also deactivates the proteinase inhibitors. Microorganisms metabolize the proteinase inhibitors during the fermentation process, leaving an edible product. Much of the soybeans consumed by humans is in the form of fermented soybeans or Tofu which lacks the proteinase inhibitors.

Velvetbean Caterpillar
Photo: USDA ARS

Insects that can feed on soybeans, such as the velveltbean caterpillar need an alternate means of digesting soybean proteins.  Most animals rely on trypsin-like serine proteinases.  Insects that feed on soybeans often cannot avoid the inhibition of their serine proteinases.  However these soybean-feeding insects are capable of producing large amounts of cysteine proteinases which are not inhibited by soybeans.

A recent article in the Journal of Pesticide Science by a group of Brazilian scientists (J Pest Sci (2012) 85:117–123 DOI 10.1007/s10340-011-0403-0) presents evidence that the velvetbean caterpillar is compensating for the inhibition of its serine proteinases by using cysteine proteinases instead. Cysteine proteinases are much more common in beetles than in caterpillars. However, many insects that feed on soybeans have adapted by relying on cysteine rather than serine proteinases. The use of cysteine proteases is an adaptation that several groups of insects have evolved independently.