Eggplant-garlic intercrops reduce the density of Tetranychus urticae on eggplant and improve crop yield

Diversification of cultivation systems plays a critical role in ecosystem functions such as pest control, diversity of beneficial arthropods, and soil fertility. Repellent plants (releasing volatile organic compounds as arthropod repellents and providing alternative prey for natural enemies) may be used in an intercropping system for pest control; however, little is known concerning their relative priority. In this study, the impact of intercropping eggplant (E) (Solanum melongena L.) and garlic (G) (Allium sativum L.) in three designs - with row ratios of 4E:4G, 4E:8G, or 4E:12G - was assessed on the density of the two-spotted spider mite (TSSM) (Tetranychus urticae Koch), the abundance and diversity of predators, and crop yields, compared with the sole crops over two growing seasons of 2019 and 2020. In three intercrops, a significant decrease in the density of TSSM egg and mobile stages was observed compared with the sole eggplant at each growth stage of eggplant. The damage index by TSSM on eggplants was lower in intercrops (8.1-11.5%) compared with sole crop (37.0-40.3%) in the two seasons. The abundance of Stethorus gilvifrons was lower in the three intercrops than in the sole crop on the blooming-initial fruit set and the fruit development stages of eggplant. The density of Orius niger was higher in the three intercrops compared with the sole crop on vegetative growth and flowering to initial fruit set stages of eggplants. The abundance of Chrysoperla carnea was not significant among treatments. Greater values of the Shannon diversity index and the Pielou's evenness index for the construction of TSSM predators were recorded in the three intercrops compared with the sole eggplant over both growing seasons. In addition, a significant improvement in the yield per unit area of eggplant and garlic was obtained in intercrops compared with sole crops. These results indicate intercropping eggplant and garlic was a practical solution for reducing TSSM on eggplant, promoting the abundance of predators, and improving the crop yields of eggplant and garlic compared with the sole crops.