Intercropping for pest control at multiple spatial scales

Background/Question/Methods

Insect feeding and damage are a major concern for many vegetable farmers. Reliance on pesticides and extensive monocropping has been linked to losses of biodiversity and ecosystem services in many agricultural ecosystems. Intercropping is a farming practice that can increase the recruitment of natural enemies, delay pest development, and influence pest behavior and feeding. However, little is known about how different spatial scales or patterns of intercropping influence the extent to which pest suppression and damage occurs. The spatial arrangement of intercrops in a field could influence pest immigration and emigration rates, and subsequently the distribution of damage seen in a field. We compared the effect of planting aromatic intercrop elephant garlic, with kale in three different spatial patterns and kale monocultures. The spatial patterns ranged from alternating individual elephant garlic and kale plants within crop rows (checkerboard), to 3-row blocks of elephant garlic next to blocks of kale within a plot. We measured arthropod abundance and diversity, damage, and yield in a subset of kale. We examined these effects at the scale of a plot and at the scale of a local plant neighborhood surrounding a focal kale individual. The experiment was repeated at two field sites.

Results/Conclusions

There were no significant effects of spatial pattern of intercropping on the total abundance of arthropods or the total abundance of predators, compared to monocultures. There was a significant increase in the abundance of the herbivore Plutella xylostella(diamondback moth) larvae on kale in crop mixtures compared to monocultures. Kale in alternating-row plots exhibited a significant increase in damage compared to monocultures, with checkerboard plots exhibiting marginally significant increases in damage compared to monocultures. These results suggest resource dilution effects on P. xylostella in monocultures, resulting in less kale damage and lower herbivore abundances in these plots. There were significant differences in arthropod abundance and damage patterns between the two field sites and over the experimental period. Damage and total arthropod abundance significantly increased over the experimental period as seasonal temperatures increased. The field site located in an intensively cultivated area had significantly lower abundances of total arthropods, in addition to significantly lower abundances of predators like spiders, and significantly higher abundances of P. xylostella compared to the other field site located near pine forest habitat. Our results highlight the importance of the surrounding habitat matrix in supporting natural enemy populations that contribute to biocontrol on farms.