Extension Bulletin E-3165 • New • November 2011

Caterpillar Pests in Cole Crops

Zsofia Szendrei and Alexandria Bryant• Department of Entomology • Michigan State University

variety of cause dam- Diamondback . are tiny, flat, A age in Michigan cole crops. The circular and cream-colored. They are three most significant species are the laid in small clusters on the leaves. imported cabbageworm ( rapae, A single can lay : ), the diamond- as many as 188 eggs over 4 days. The back moth ( xylostella, Lepi- diamondback moth is pale green doptera: ) and the or yellow-green, and by the time it’s looper ( ni, Lepidoptera: ready to pupate, it is approximately 1/2 ). These three species are inch long. The body of the commonly found on cole crops such as appears segmented and tapered at the cabbage, , , collards, ends (Figure 3). The head capsule is turnips, , , mustards and black during the early developmental other leafy greens. stages. As the caterpillar develops, the FIG. 1. The body of the imported cabbage- head capsule turns yellow-green but Identification worm caterpillar is covered with fine hairs, remains distinguishable from the rest which give it a velvety look. The color of of the body. The diamondback moth the caterpillar matches that of the leaf; Imported cabbageworm. Eggs are when scouting, look for signs of fresh caterpillar is commonly found on the yellow and conical, laid individually on damage and frass on the plants. A closer leaf surface. The adult diamondback the leaf surface and occasionally on the look will reveal the caterpillar feeding on the leaf. moth is a 1/2-inch-long tan moth with stem. An adult butterfly can lay 300 to brown, diamond-shaped markings on 400 eggs in her lifetime. The imported the wings, visible when the wings are cabbageworm caterpillar is dark green folded over the back at rest (Figure 4). with fine hairs covering the body, which create a velvety appearance (Figure 1). . Eggs are round and Large larvae begin to develop a yellow white and typically laid on the leaf stripe on the upper surface of the body, surface. The cabbage looper is a pale which is clearly visible on the final green caterpillar with a white stripe larval developmental stage. Maximum down each side of the body (Figure 5). length for the imported cabbageworm The cabbage looper reaches 1 to 1 1/2 is approximately 1 inch. This caterpillar inches in length and has a distinctive is commonly found along the veins of FIG. 2. Adults of the imported cabbage- pale green head capsule. It gets its name leaves and easily blends into the foliage. worm are commonly seen flying above from its inchworm motion as it moves plants throughout the summer. These along the surface of the leaf. Cabbage As an adult, the imported cabbage- white butterflies can be recognized by the worm is a white butterfly with black black markings on the wing tips. Female looper adults are about 1 inch long, butterflies lay eggs during the course of with wings folded over the back at rest. markings on the wing tips. The adult their life on cole crops and feed on pollen is commonly referred to as the cabbage and nectar. Adults are dark brown and gray , white butterfly (Figure 2). with a distinct figure “8” white pattern in the middle of the wings (Figure 6). Caterpillar Pests in Cole Crops

Behavioral differences between the cater- Damage pillars can help with identification. When disturbed, the imported cabbageworm will All of the caterpillars cause damage usually remain motionless. On the other by chewing holes in the leaves (Figure hand, the diamondback moth will wiggle 7). The imported cabbageworm feeds its body around. The cabbage looper will along the edges of leaves, leaving only either inch away or will try to defend itself thick veins behind. It feeds on all ages of by whipping its head toward the source of leaves but prefers the younger leaves. the disturbance. The cabbage looper will often feed on the underside of leaves, feeding only on Life Cycle the first layers of leaf tissue. After the FIG. 3. The larva of the plant’s immune system is weakened, Cabbageworm. This overwinters in diamondback moth is light green the looper begins chewing into the leaf, the pupal stage, and adults emerge in the and segmented with tapering forming holes. A good indicator of the spring. Eggs of the imported cabbageworm ends. The caterpillar spends its life feeding on the leaf. presence of loopers and imported cab- hatch in about five days. The caterpillar feeds bageworms is fresh frass (droppings) on and develops for approximately 11 to 20 leaves. days before forming a from which the adult butterfly emerges after 6 to 11 days. In Young diamondback larvae mine in Michigan, an estimated 4 to 5 generations the plant tissue and consume only the can occur. middle layer of the leaves. As larvae grow, they exit the leaves and consume all the Diamondback moth. In Michigan, the pupae tissues except the thin layer of transpar- overwinter in field debris, and adults emerge ent wax. This type of damage resembles in the spring or early summer. Diamondback windowpanes and is characteristic of moth adults can also arrive in Michigan on this pest. wind currents; eggs and larvae can be trans- ported with seedling transplants. The eggs FIG. 4. The adult diamondback of the diamondback moth hatch in 5 to 6 moth is a small (1/2 inch) brown Control insect, with characteristic days. The larvae feed for about 18 days, and diamond-shaped markings on the Cultural then they pupate in a loosely woven net. wings, visible when they are folded Finally, the adult moth emerges after 4 to over the back at rest. Heavy rainfall or overhead irrigation 15 days. In Michigan, an estimated 4 to 5 systems can remove and kill some cater- generations can occur. pillars. Removing debris from the field eliminates overwintering habitat for Cabbage loopers. Cabbage loopers migrate pests. Rotating crops can also eliminate northward on wind currents from southern overwintering populations by removing states, so they often arrive in Michigan later their food source the next season. in the growing season. Larvae emerge from Removal of weeds closely related to cole eggs within a few days and develop for 16 FIG. 5. The cabbage looper to 19 days before pupating. The pupa turns caterpillar is a pale green insect crops, including mustards such as field from initially green to dark brown, and the with a white stripe down each side mustard (Sinapsis arvensis), removes an of the body. The cabbage looper adult emerges 9 to 10 days later. The entire additional food source for these cat- reaches 1 to 1 1/2 inches in length erpillars. It also prevents them from life cycle lasts 32 to 37 days, allowing for and has a distinctive pale green one to two generations to develop in a head capsule. It gets its name from establishing populations before the typical Michigan season. its inchworm motion as it moves cultivated crops are transplanted into along the surface of the leaf. the field.

2 Caterpillar Pests in Cole Crops

Planting floral sources near cole crops Tachinid , such asVoria ruralis and provides pollen and nectar sources concinnata, are common for beneficial and can increase of the cabbage looper. As in biological control. Buckwheat around the case of the , The the edges of fields can provide a good caterpillar can feed and develop, but it nectar source. cannot become an adult, and it cannot go on to reproduce. Intercropping can also contribute to pest population reduction. Intercrop- Predators: In addition to parasitoids, ping is planting two or more crops predators also reduce caterpillar popu- side by side in a field, such as potato FIG. 6. Cabbage looper adults are about an lations. Predators that consume cater- inch long, with wings folded over the back and broccoli. Intercropping often at rest. Adults are dark brown moths with pillar eggs and larvae include the spined reduces pest abundance because it de- a distinct figure “8” white pattern in the soldier bug, minute pirate bug, spiders, creases the concentration of the food middle of the wings. harvestmen, ground , lacewing source within a field, making the field larvae and lady beetles, such as the less attractive to colonizing insects. pink-spotted lady (Figure 9).

Natural enemies Chemical Parasitoids: Cotesia rubecula is a small, The beneficial insects described above dark parasitic wasp that attacks young can be conserved by avoiding the use of imported cabbageworm caterpillars. broad-spectrum , especially This wasp lays an into a caterpil- early in the season when pest pressure lar, where it hatches into a larva and is low. Selective insecticides, such as feeds on its host. The kills products containing Bacillus thuringi- the host caterpillar when it reaches ensis (Bt), are safe to natural enemies its final developmental stage; then it and therefore recommended for use. emerges and forms a white pupa on Bt products are most effective when the outside of the caterpillar (Figure applied to small caterpillars. These FIG. 7. Caterpillar damage on cabbage: 8). Although the parasitized caterpillar holes chewed into the leaves and head. insecticides have a one-day preharvest can feed on the plant until it reaches interval. There are Bt-resistant popula- its final larval developmental stage, it tions of diamondback moths, but at dies before it can develop into a repro- this time we have not confirmed any ductive adult. cases from Michigan. The other two caterpillar species are not known to de- Diadegma insulare is another small velop resistance to Bt. Other parasitic wasp that attacks diamond- options that allow natural enemies to back moths. They also develop inside survive include products that contain their caterpillar hosts, and the adults spinetoram, novaluron or methoxyfe- emerge from the diamondback moth nozide. Spinetoram has a broad-spec- pupae. The caterpillar can feed and de- trum knockdown action; it controls velop, but it cannot become an adult, multiple growth stages and provides and it cannot go on to reproduce. residual control.

FIG. 8. White parasitoid pupa attached to its imported cabbageworm caterpillar host.

3 Caterpillar Pests in Cole Crops

Novaluron and methoxyfenozide are The development of insecticide resis- both insect growth regulators: they tance can be avoided by rotating insec- stop insects from developing through ticide classes between applications and their normal life stages. These insecti- by using appropriate application rates. cides are more effective on small cater- One of the cornerstones of insecticide pillars. Methoxyfenozide has a one-day resistance management is to avoid and novaluron a seven-day preharvest using rates that allow some insects to interval. Broad-spectrum insecticides survive and pass their ability to over- are also available for use in cole crops. come toxic effects to the next genera- Many of these are applied to control FIG. 9. Pink-spotted lady beetle adult is a tion. Regular scouting and the use of other pests but will also be effective predator commonly found in Michigan cole treatment thresholds can lower the against caterpillars. crop fields. These insects feed on the eggs number of chemical applications while and small larvae of cabbage pests. maintaining clean and healthy plants.

Scouting thresholds for caterpillars in cole crops

Crop Stage Percent plants infested Imported cabbage- worm, Diamondback cabbage looper moth CABBAGE Transplant to cupping 50% with more than 30% 5 larvae per plant

Cupping to early head 50% with more than 20% 5 larvae per plant

Early head to harvest 10% with more than 1 larva per plant BROCCOLI, Seedbed 10% 10% CAULIFLOWER Transplant to first curl 40% 20% First curl to harvest 10% 10%

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