Feeding by adult beetles and larvae may result in extensive damage, causing poor seed set or seed filling, reduced yields, and delayed maturity.
Sunflower beetles feed on native and cultivated sunflowers.
Adult sunflower beetles (Zygogramma exclamationis) (Figure 1) are about six to eight millimetres (1/4-3/8 inches) long. The head is reddish brown and the thorax (area behind the head) is pale yellow with a reddish brown patch at the base. Each wing cover has three dark brown stripes that extend the length of the back. The fourth stripe ends at the middle of the wing in a small dot that resembles an exclamation point.
Sunflower beetles overwinter as adults in the soil. Usually, their emergence from the soil in the spring coincides with the time that sunflower seedlings begin to appear in late May. The beetles feed throughout the day on the emerging seedlings. Eggs (Figure 2) hatch about a week after they are laid and the young larvae feed on the leaves at night. They hide among the bracts of the flower bud and in the axils of the leaves during the day. The larvae feed for about two weeks but, because of the long egg laying period, larvae may be present in the field for about six weeks.
The mature larvae (Figure 3) drop to the ground, enter the soil, and pupate in earthen cells. The pupal stage lasts about two weeks. Adults of the new generation emerge and feed for a short period in late August and early September. They feed on the uppermost leaves or bracts of the plant before re-entering the soil to overwinter.
May June July August September Overwintering adults become active, start feeding Adults continue feeding, lay eggs Larval feeding on leaves Larval feeding ends, pupation in soil begins, new adults emerge New adults emerge and feed, enter soil to overwinter
Noticeable damage is often first seen on plants near the margins of sunflower fields. Both the larvae and adults are easily seen and counted on seedlings and young plants. Monitor in at least two locations on each side of a field with one or two sampling sites near the centre. Twenty plants should be examined at each sampling site.
Insecticides are available to control sunflower beetle. Consult the Guide to Crop Protection for product information. The threshold is one to two adult beetles per seedling at the two to six leaf stage or ten to 15 larvae per plant during the summer. Severe leaf damage may occur to plants in the two to six leaf stage when adult beetles are numerous. Control may be necessary if defoliation caused by either the adults or the larvae reaches 25 to 30 per cent, especially if more defoliation is expected. If the majority of the larvae have reached maturity at about 25 per cent defoliation, control should not be necessary.
Natural controls usually keep sunflower beetle populations below damaging levels. Sunflower beetle eggs are eaten by the thirteen spotted lady beetle and the convergent lady beetle. Larvae of the common green lacewing consume both eggs and larvae. Damsel bugs and the two spotted stink bug may also prey on larvae of sunflower beetles. Parasitoids attack sunflower beetle eggs, larvae and adults.
For further information, contact your GO representative.