Cabbage white butterflies are medium-sized white butterflies in the family Pieridae. They have black spots on their wings, and a black band on the top of their forewings (top wings), distinguishing them from other species. The butterflies themselves don’t do any damage, however their caterpillars do significant damage to plants in the Brassica family, including cabbage, broccoli, cauliflower, and collard greens.
Life Cycle (Figure 1)
Cabbage white butterflies overwinter as pupae on their host plants (this may include weeds in the Brassica family) and emerge in the spring to mate and lay eggs on the underside of the leaves of their host. Their eggs are oblong and yellowish, with many vertical ridges on the outside, and laid individually. The eggs hatch after approximately one week, and their larvae, small green caterpillars with a yellow stripe along their back, will feed and grow for two to three weeks until they reach their fifth and final instar. They will then attach to the stem of their host plant near the underside of a leaf and molt into a light yellow-green speckled pupa, usually on the underside of a leaf. After a week or two the adults will emerge to mate and lay eggs once again. This process occurs three to five times a year.
Figure 1. The various life stages of cabbage white butterflies, consisting of egg (top left), larva (top right), pupa (bottom left), and adult butterfly (bottom right) (Photos by John Obermeyer).
Damage
Cabbage white caterpillars feed in a circular pattern on leaves, resulting in round spots of damage. Their dark brown and green excrement leaves discoloration and contamination (Figure 2). The impact of feeding damage depends on the crop in particular. Broccoli and cauliflower can withstand damage to the outer leaves without compromising floret production, and any feeding on collards and cabbage can reduce yield.
Figure 2. Feeding damage (left) and frass (right) on cabbage leaves (Photos by John Obermeyer).
There are three other caterpillar pests of Brassica, including the diamondback moth, the cross-striped cabbage worm, and the cabbage looper. All these caterpillars have some green coloration but vary in size and distinguishing features. The diamondback moth caterpillar has pointed ends and wriggles violently when disturbed on the plant (Figure 3, bottom right), the cabbage looper has white lines defining their body segments and moves like an inchworm (Figure 3, top right), and the cross-striped cabbage worm is the only one whose eggs are laid in clusters, and therefore found in clusters when they first hatch. This caterpillar has horizontal black, white, and yellow stripes along its back (Figure 3, bottom left). All four of these species produce similar feeding damage and can often be managed using the same techniques. The only one that flies during the day you may see fluttering about in the garden is the cabbage white.
Figure 3. Comparison of Brassica-feeding caterpillars, including the cabbage white (top left), the cabbage looper (top right), the cross-striped cabbage worm (bottom left), and the diamondback moth (bottom right) (Photos by John Obermeyer).
Management
Cabbage white caterpillar populations are naturally controlled via parasitoid species, including several small wasp species and a few species of tachinid flies. Depending on the species, these insects target various life stages of the caterpillar, including the egg, larval, and pupal stages. While these natural enemies are present, they do not manage populations at a level that will reduce economic damage.
However, numerous other methods of pest management can be implemented against cabbage white and all the other caterpillars mentioned. One of the easiest control methods to execute is cultural control, such as the management of weeds in the Brassica family, preventing the caterpillars from increasing their population on separate host plants and migrating over once the crop is planted. Additionally, deploying exclusion netting immediately after planting or transplanting crops prevents the adults from accessing the leaves to deposit eggs. Exclusion netting should be secured as soon as the crop is planted, as hesitation may result in pests being trapped under the netting with the crop. Bacillus thuringiensis is a common bacterial biological control agent used against caterpillar pests in general; it must be applied directly onto the plant’s leaves once a week after the first larvae appear on the plants. Other conventional pesticides may be applied; however, there is a chance that the insecticides will kill off natural predators in the process. For more information on pest control methods and their implementation, visit the Midwest Vegetable Production Guide.