Cruciferous vegetables are a large and important crop group to Florida agriculture. A number of insects feed exclusively on crucifers and can affect an assortment of highly valued crops such as broccoli, cabbage, cauliflower, collards, mustard, turnip, watercress, and Chinese cabbage. Two insects that impact crucifers include the diamondback moth Plutella xylostella (L.), perhaps the most serious pest of crucifers, and the yellow margined leaf beetle (Microtheca ochroloma Stål). This article will examine the biology of these two foes and the damage they inflict.
The female moth of this species will deposit her eggs on the underside of the leaf surface as either a single egg or in groups. Eggs of the diamondback moth are yellow or pale green, oval and flattened in appearance and are very small measuring 0.44 mm in length and 0.26 in width.
Within two or three days the eggs will hatch out to begin their feeding habits. The diamondback has four instars with the larvae remaining very small and abundant throughout each one. Depending on weather conditions, it can take on the average 25 to 30 days from egg to pupal stage with perhaps eight to twelve generations per year in South Florida.
The pupa stage will appear as a loose transparent case attached to the lower or outer part of the leaves, in cauliflower and broccoli, pupation may occur in the florets. The pupa is yellowish 7mm to 9 mm in length with the cocoon lasting from 5 to 15 days. In its first instar the larvae are colorless but subsequently turn green as they mature. Their bodies have a tendency to taper at both ends with a set of prolegs that form a distinctive V protruding from the posterior end of the larvae. The adult diamondback will appear as a small grayish brown moth with elongated antennae. The slender moth is about 6mm long, and distinguished with a broad cream or light brown band along its back. When identifying the adult look for constrictions on the cream band, it’s these constrictions which give it its common name diamondback.
Plant damage is caused by the small abundant larvae that will consume plant leaves but not the veins. In this stage damage will appear especially on seedlings and have the potential to disrupt head formation in cabbage, broccoli, and cauliflower.
In South Florida, increase of diamondback moth activity will occur between the months of December and March, attacking the crops anytime during the cycle. During this period fields should be scouted walking in a Z or figure 8 patterns. Either way, it is important to scout quadrants, inside and around the perimeter of the field. As part of your scouting tool box try to stock a 10X lens, note pads and pencils for reporting, and clear plastic bags with tweezers for collecting.