Effects of Different Host Plants on Digestive Enzyme Activity and Nutrient Content in Megalurothrips usitatus

Megalurothrips usitatus can successfully develop and reproduce on host plants such as cowpea (Vigna unguiculata), green bean (Phaseolus vulgaris), and white bean (Vigna cylindrica), with cowpea being the most suitable host. In contrast, its development and reproductive capacity are significantly limited on wax gourd (Benincasa hispida).

The objective of this study is to determine the digestive enzyme activity and nutrient content in M. usitatus after feeding on different hosts, and to explore the adaptation mechanism of M. usitatus to different host plants.

F₁ adults of M. usitatus reared on cowpeas for 3 d were transferred to petri dishes containing cowpea, green bean, white bean, or wax gourd, with four replicates per treatment. Sampling was conducted at 0, 3, 6, 12, 24, 48, and 72 h after feeding. The dynamic changes in the activities of key digestive enzymes (including α-amylase, trehalase, trypsin, and chymotrypsin) were systematically measured. Concurrently, changes in the content of major nutrients, such as glycogen, soluble protein, and fat, were analyzed.

Significant changes were observed in the activities of digestive enzymes and the content of nutrients in M. usitatus after feeding on different host plants. The activities of various digestive enzymes in M. usitatus fed on cowpea, green bean and white bean were generally higher than those fed on wax gourd. Specifically, the α-amylase activity of M. usitatus fed on cowpea was the highest (1.33 mg·min^-1·mg^-1 protein), along with high trehalase activity (469.80 nmol·min^-1·mg^-1 protein). Trypsin and chymotrypsin activities reached 20.42 and 24.86 U·mg^-1 protein, respectively. M. usitatus fed on green bean had the highest α-amylase activity (1.49 mg·min^-1·mg^-1 protein), but the activities of trehalase (304.81 nmol·min^-1·mg^-1 protein) and two proteases were slightly lower than those of cowpea group. Trehalase activity in white bean group was significantly decreased (175.61 nmol·min^-1·mg^-1 protein), while trypsin activity was increased (21.15 U·mg^-1 protein). In contrast, all enzyme activities of M. usitatus fed on wax gourd were at the lowest level, the trehalase activity was only 152.89 nmol·min^-1·mg^-1 protein. Regarding nutrient content, glycogen content (1.61 mg·mg^-1 protein) and fat content (5.54 μg per individual) were highest in M. usitatus fed on cowpea, while the lowest values were observed in the wax gourd group (glycogen content 0.79 mg·mg^-1 protein; fat content 3.37 μg per individual).

Different host plants significantly influenced the digestive enzyme activities and nutrient contents of M. usitatus. M. usitatus rapidly adjusted these physiological parameters after feeding on cowpea, green bean, and white bean, but exhibited slower adaptation on wax gourd. These findings indicate that M. usitatus has higher adaptability to cowpea, green bean, and white bean, whereas its adaptability to wax gourd is comparatively poorer.