Characterization of digestive proteolytic activity in Lygus hesperus Knight (Hemiptera: Miridae)

The tarnished plant bug, Lygus hesperus Knight, is a pest that causes considerable economic losses to vegetables, cotton, canola, and alfalfa. Detailed knowledge of its digestive physiology will provide new opportunities for a sustainable pest management approach to control this insect. Little is known about the different protease class contributions to the overall digestion of a specific protein. To this end, the proteolytic activities in female adult L. hesperus salivary gland and midgut homogenates were quantified over a range of pH's and time points, and the contribution of different classes of proteases to the degradation of FITC-casein was determined. In the salivary gland, serine proteases were the predominant class responsible for caseinolytic activity, with the rate of activity increasing with increasing pH. In contrast, both aspartic and serine proteases contributed to caseinolytic activity in the midgut. Aspartic protease activity predominated at pH 5.0 and occurred immediately after incubation, whereas serine protease activity predominated at pH 7.5 after a 9 h delay and was resistant to aprotinin. The salivary serine proteases were distinctly different from midgut serine proteases, based on the tissue-specific differential susceptibility to aprotinin and differing pH optima. Collectively, the caseinolytic activities complement one another, expanding the location and pH range over which digestion can occur.