The effect of intercropping onion with fenugreek and onion with safflower on population density of the onion thrips, Thrips tabaci, biodiversity of its predators, and crop yield

Strip cultivation is one of the effective methods to reduce the population of pests and increase the abundance and diversity of natural enemies in any habitat. Onion thrips, Thrips tabaci Lind., is a pest of onion (Allium cepa L.) worldwide. Larvae and adults of thrips feed on cell sap and cause silver spots on the surface of leaves of onion plants that consequently result in a significant reduction in onion yield. Control of onion thrips is difficult due to the short generation time, high fecundity, and the emergence of insecticide-resistant genotypes. Predators in each region, such as hemipterans, ladybugs, syrphid flies, carabid beetles, mites, and spiders, are effective in the biological control of the onion thrips population in fields. Intercropping systems can affect the biological control of pests by providing food and habitat resources for natural enemies. Therefore, designing effective and appropriate methods to reduce the population of onion thrips and increase the diversity and abundance of natural enemies can effectively manage T. tabaci in fields.

In this study, the effect of intercropping four rows of onion (Allium cepa L.) with two rows of fenugreek (Trigonella foenum-graecum L.) (4O:2F) and four rows of onion with two rows of safflower (Carthamus tinctorius L.) (4O:4S) was investigated on densities of the onion thrips, biodiversity of predators, and crop yields in 2022 and 2023. The experiments were accomplished in a randomized complete block design with two intercrops of 4O:2F and 4O:4S along with the sole crops. The density of onion thrips and predators per plant was counted and recorded on weakly sampling dates by direct counting using a magnifying glass. The predatory species that we suspected in their identification were transferred to the laboratory and carefully identified through morphological characteristics under a stereomicroscope. The average annual abundance of each predator species and the abundance of predators per plant in each plot were calculated during the onion growing season. In addition, Shannon's diversity index (H') and Pielou's evenness index (J') were calculated for the complexity of predators in each plot and in each tested year to compare the cropping systems. The onion, fenugreek, and safflower yield per square meter were estimated in strip intercropping systems and single crops. Then, the land equality ratio (LER) was calculated for both intercrops to estimate the yield advantage regarding the monoculture of each crop.

Three intercrops led to a significant reduction in the densities of larvae and adults of onion thrips compared with sole onion. Further, the leaf damage index was lower in intercrops than in single onion in two seasons. A significant coefficient of determination (R2) was calculated between the leaf damage index and the dry weight of onion per plant in two intercrop and sole onions. The total abundance of predators was higher in intercrops, especially 4O:2F than in the sole crop. More values of the Shannon diversity index and the Pielou's evenness index for predators were recorded in intercrops compared with the sole crop in two seasons. The dry yield of onions per square meter was more significant in both strip intercropping systems compared to single onions in 2022 and 2023. On the contrary, the dry yield of fenugreek and safflower per square meter was insignificant among intercrops and single crops in two years. The highest land equivalent ratio (LER) was calculated for 4O:2F.

Overall, the intercropping of onion and fenugreek (4O:2F) and onion and safflower (4O:2S) demonstrated a reduction in the population of onion thrips. Both intercrops, particularly 4O:2F, contributed to an increase in the abundance and diversity of predators and the values of LER when compared with sole crops. As a result, farmers could consider implementing both intercrops, particularly 4O:2F, as an environmentally conscious approach to managing T. tabaci in the field.