Dr. Ajay Nair
Assistant Professor, Department of Horticulture
Iowa State University
As the season winds down for summer crops (sweet corn, pepper, tomato, etc.) we should not let our guard down on fall crops such as broccoli, cabbage, brussels sprout, kale, etc. These crops are on still actively growing and need attention when it comes to pest management. Several insects feed exclusively on cole crops but there are also few generalists that can reduce crop yield, vigor, and quality. Below are some common insect pests of cole crops and strategies to manage them effectively.
|Fig. 1 Imported cabbageworm larvae|
Imported Cabbageworm (Pieris rapae): The imported cabbageworm is the larva (caterpillar) of a yellowish-white butterfly. The butterflies have several black spots on their wings and they fly around cabbage plants during the day. The butterflies lay eggs singly on either side of the leaves. Eggs are yellow, oblong, bluntly pointed at the ends, deeply ridged lengthwise and attached to the leaf by one end. The larvae have a narrow orange stripe down the middle of the back and a yellowish stripe along each side of the body (Fig. 1). The damage caused by imported cabbageworm is similar to cabbage looper injury, however, they do not limit feeding to areas between leaf veins but chew through leaves indiscriminately.
The caterpillar of imported cabbageworm can be effectively controlled using Bacillus thuringiensis (B.t.) product, spinosad or pyrethrin. Many organic and sustainable growers are familiar with B.t. which is an OMRI certified a microbial insecticide that contains spores of these bacteria and is used to control caterpillars when they feed on leaves containing the spores. Bacillus thuringiensis works best while the caterpillars are small. Spray B.t. early or late in the day and make at least two applications with 2-3-day intervals. In addition to spraying, an effective tool is to use floating row covers that provide a physical barrier to imported cabbage worm loopers in small cole crop plantings. Row covers could also help increase temperatures and enhance crop growth, however, they can also reduce light available to the plant.
|Fig. 2 Cabbage looper larva|
Cabbage Looper (Trichoplusia ni): The cabbage looper is a one of the common pests of cole crops and could be very destructive. It is the larva of a medium-sized grayish brown moth. The moths have a figure-8-shaped silver spot near the middle of each of the front wings. They are most active at night and lay greenish-white eggs singly and mainly on the lower surfaces of the outer leaves of the plants. The eggs are smaller than a pinhead and are round. Newly hatched larvae (caterpillars) have dark heads and almost clear bodies. They later become pale green and have several white lengthwise stripes. The caterpillar moves with a looping motion and hence the name ‘looper’. Larvae initially feed on small areas on the undersides of leaves but later move to the center of the plant, eating through the leaves between the veins (Fig. 2). Large larvae are heavy feeders and may cause serious damage to the plant and should be quickly controlled.
Management strategy for cabbage looper is very similar to the imported cabbageworm. The caterpillar of imported cabbage looper can be effectively controlled using Bacillus thuringiensis (B.t.) product, spinosad or pyrethrin.
|Fig. 3. Turnip aphids (gray) and parasitized aphids (swollen brown)|
Aphids: Three primary species of aphids attack cole crops: the cabbage aphid (Brevicoryne brassicae), the turnip aphid (Lipaphis erysimi) (Fig. 3), and the green peach aphid (Myzus persicae). Plants in all stages of growth are frequently covered with dense clusters of whitish-green aphids. They suck plant sap from the leaf resulting in curling and crinkling of leaves. Affected plants are dwarfed, grow slowly and do not produce marketable heads. Aphids infestation is rampant under cool and dry weather conditions. On a smaller scale, as in a vegetable garden, spray foliage with soapy water, then rinse with clear water or use insecticidal soaps (M-Pede®). Neem oil-based sprays (Azatin XL®, Trilogy®) are also affective.
On a larger scale, two or three insecticide treatments at five-day intervals may be needed to clean up plants. Effective insecticide classes include pyrethroids, neonicotinoids, and spinosyn. When two percent of the plants are infested with aphids, an insecticide application should be made with high spray volume and adequate pressure to thoroughly wet foliage. It is essential to add spreader-stickers to the spray mix to break the surface tension of the spray droplets.
The take home message is to actively scout fields weekly throughout the season for damage and take appropriate measures to limit the damage. Check plants carefully, even if no feeding damage is apparent, to look for eggs that will hatch into small caterpillars several days to a week later. Examine the lower leaves of plants for the larvae of each pest. Although feeding damage and fecal material are signs of activity, it’s better to rely on larvae counts to determine the level of infestation.