Cutworms and Spanworms in Blueberries
Early in the growing season, cutworms (Lepidoptera: Noctuidae) and spanworms (Lepidoptera: Geometridae) are occasionally found in blueberry plantings. These insects apparently are not restricted to blueberry, but feed on a wide variety of plants (Forbes 1948). Adults of both species are medium-sized, grayish-brown moths with a wingspan of 20-32 mm (0.75 – 1.25 inches).
A fully-grown spanworm larva is about 25 mm long (1 inch) with a brownish-gray body and a light-colored head speckled with dark spots. Like other members of the family Geometridae, immature spanworms (also known as measuringworms) have only two pairs of fleshy legs near the end of the abdomen. One species (genus Eupithecia) has two distinctive V-shaped stripes (chevrons) midway along the upper surface of the body.
A fully-grown cutworm caterpillar is 25-32 mm in length (1.0 – 1.25 inches) with a light brown body and a dark brown head. The abdomen bears five pairs of fleshy prolegs. In one species (genus Rhynchagrotis), the upper surface of the body is marked by a series of dark brown triangles on each segment.
Both of these species apparently overwinter as adults in North Carolina. Since the moths fly only at night, they are seldom seen by growers (Dyar 1901, Cook 1920). Eggs laid on warm days in late winter may hatch into caterpillars during February or early March. The larval stage lasts about two months, and pupation usually occurs in the soil during April.
Significance and damage
Blueberry plants that have been attacked by spanworms and cutworms usually have large irregular holes chewed in the flower buds. Most of these buds turn brown and die. The insects continue feeding as the buds expand and begin to bloom in early spring. They can consume entire flower clusters prior to fruit set.
Although spanworms can sometimes be found by examining buds or flower clusters in early spring, they are cryptically colored and very hard to see. Cutworms hide under debris or in the soil during the day and are found on the plant only at night. Infestations are usually recognized by the characteristic feeding damage to buds and flowers.
Numerous biological control agents have been found attacking cutworm and spanworm larvae (Phipps 1927). These natural enemies include birds (e.g. crows, jays, robins, bluebirds), predatory insects (e.g. ground beetles and soldier bugs), a fungal pathogen (Empusa virescens), a parasitic wasp (Meteorus vulgaris — family Braconidae), and four dipteran parasitoids: Phorichaeta sequax and Phorocera claripennis (family Tachinidae);Anthrax alternata (family Bombyliidae); and Psilocephala haemorrhoidalis (family Threvidae). Overall, this natural enemy complex seems to do a good job of suppressing cutworm and spanworm populations during most growing seasons.
Neither species is usually numerous enough to warrant control measures. However, severe infestations may be managed with a single insecticide applicaiton just before bloom. See the North Carolina Agricultural Chemicals Manual for specific management information. To avoid impacting pollinators, do not apply any insecticides during bloom.
- Cook, W. C. 1920. Cutworms and armyworms. Minn. Dept. Agr. Ent. Circ. #52. 8 pp.
- Dyar, H. G. 1901. Life histories of American geometridae. XXV. Psyche 9: 250-251.
- Forbes, W. T. M. 1948. Lepidoptera of New York and neighboring states. Cornell Univ. Agr. Exp. Sta. Memoir #274. Parts II and III.
- Phipps, C. R. 1927. The black army cutworm — a blueberry pest. Maine Agr. Exp. Sta. Bull. #340 pp. 201-219.