Every summer we see insects munching on sweet potato foliage, but this year there seems to be a bit more feeding than normal. In addition to the usual suspects – flea beetles, grasshoppers and Japanese beetles – we are also seeing more activity from two other insects, the spotted cucumber beetle and the golden tortoise beetle.
Several species of tortoise beetles like to feed on plants in the morning glory family. The most common species found on Ontario sweet potatoes is the golden tortoise beetle, Charidotella sexpunctata bicolour. Adults and larvae of this species feed on sweet potato foliage, making large round holes all over the leaf. Young larvae initially eat only the outer layers of the leaf, but as they grow they begin to eat their way through. Adult tortoise beetles are oval and slightly flattened, with the edge of the body very extended, covering most of the head and legs and looking somewhat turtle-shaped. The adult is a brilliant metallic golden colour, but becomes orange with black spots when disturbed (Figure 1). The larvae are broad and flattened, about 9 mm long, with spiny sides and a hook along the abdomen that they use to cover themselves in feces, which helps hide them from predators.
Tortoise beetles overwinter as adults under bark or in leaf litter, emerging from May to June to feed on morning glory weeds and moving to crops when they are planted. Eggs are laid on the undersides of sweet potato leaves. Larvae hatch in 7- 10 days and feed for 3-4 weeks before pupating and emerging as adults in early August. These beetles can have multiple generations per year in southerly regions, but may have only one in cooler climates.
Both adults and larvae feed exclusively on the foliage of sweet potato roots, and typically the defoliation does not have a significant effect on yield for healthy sweet potato plants. As a result, they are generally considered a minor pest and usually only become a problem in plant beds, or on very small transplants when beetle populations are very high. There are no chemicals registered against this pest on sweet potatoes in Canada, however control is not typically required. Keep plants healthy, with sufficient fertilization and irrigation. Controlling weeds from the Convolvulaceae family (e.g. morning glory and bindweed) can also be helpful in keeping populations low.
Spotted Cucumber Beetles
Adult spotted cucumber beetles will chew irregular shaped holes in sweet potato foliage but, as with the tortoise beetles, adult damage to foliage is not typically sufficient to impact yields. However the larval stage of this insect will bore into the outer skin of maturing sweet potato roots, rendering them unmarketable.
In the U.S. sweet potatoes, cucumber beetle larvae eat small holes in the skin of roots, causing scars 1-3 mm wide with shallow cavities under the skin. Holes are generally smaller than that caused by wireworms, and cavities shallower. Damage can occur throughout the season, but appears to increase as the season progresses.
There are no established thresholds for this insect in Ontario, however in Louisiana a threshold of 2 adult cucumber beetles per 100 sweeps with a sweep net is indicative of a potential problem. There are no insecticides registered for use against cucumber beetles on sweet potatoes in Ontario. Applications of Matador for control of flea beetles on sweet potatoes may also have some effect on spotted cucumber beetle adults. Organic management methods mentioned for this insect on other crops include use of row covers and/or growing nearby cucurbits as a trap crop, but it is not known whether this would be effective for managing this pest in sweet potatoes. Some sweet potato varieties are less susceptible to spotted cucumber beetles than older varieties such as Beauregardd.