Diamondback moth is usually considered a relatively insignificant pest. However, at high densities they can become troublesome to Brassica crops. The damage is caused by the larvae that dig tunnels in the leaf tissues or scrap the surface of the lower surface of the leaf blade. Irregular patches are visible (even though the upper leaf epidermis) can occasionally be left intact, creating a windowing effect. Older larvae are voracious and during severe infestations, the whole leaf can be eaten up, except the veins (leaf skeletonization). The presence of larvae on the florets can disrupt the formation of heads in broccoli or cauliflower.
The damage is caused by the larvae of the diamondback moth, Plutella xylostella. Their main hosts belong to the Brassica family and include broccoli, cabbage, cauliflower, radish, and turnip as well as a number of weeds. Adults are small and slender, about 6 mm long and with pronounced antennae. Their body is dark brown with a characteristic light brown band along the back. They are not good fliers but can nevertheless be transported over long distances by the wind. Each female deposits on average 150 eggs under the leaf, usually in small groups of up to eight and close to the leaf veins. Young larvae have a leaf-mining feeding habit while older ones feed on the lower surface of the leaf, resulting in irregular patches. Rainfall has been identified as a major mortality factor for young larvae.
Enemies of the diamondback moth include the parasitic wasps Diadegma insulare, Oomyzus sokolowskii, Microplitis plutellae, Diadromus subtilicornis and Cotesia plutellae. Besides parasitoids, solutions containing entomopathogenic fungi or nuclear polyhedrosis virus can be used to control populations. Insecticide solutions containing Bacillus thuringiensis are also useful even though a rotation of products is recommended to avoid the development of resistance.
Always consider an integrated approach of both preventive measures together with biological treatments if necessary. Resistance to insecticides is widespread, and includes most classes of products (including some biological ones), so rotation of active ingredients is highly recommended. Products containing pyrethroids start to fail already after heavy usage in the decade of the 80s.