Question: The top leaves on my rhododendron are deformed. Any ideas of what is the problem?
Answer: Some common insects that attack rhododendrons are lace bugs, scales, root weevils, rhododendron borer and rhododendron tip midge. If detected early most of these pests will not reach large numbers and control is easy and quick.
With early detection, while numbers are small, less toxic pesticides such as insecticidal soap or horticultural oils can be effective. Pest control is not difficult but it is important to first correctly identify the pest and then learn about the biology and life cycle of the pest. After that, management techniques are most effective.
The damage you are seeing is most likely the result of a native pest known as the rhododendron tip midge. You are not alone. Last season the helpline had multiple calls on this issue. This pest was first identified in 1939 and has been reported in New York, New Jersey, Pennsylvania, North Carolina, and is possible on most of the East Coast. Rhododendrons growing in these areas are the only known host of this pest.
The rhododendron tip midge is a light-brown small fly. The pest overwinters in the soil as a pupae. The adult flies emerge and lay eggs in the spring on the undersides or rolled edges of the leaves as the new growth emerges from the buds. Within three days the eggs hatch and the larvae which look like small, white maggots start to feed on this new growth. The larvae mature in approximately seven days, then drop to the ground and pupate in the top inch of soil in a cocoon. There can be multiple generations per year.
Young-infested leaves less than 5 cm long will develop rolled inward swollen greenish-yellow tissue. Lightly infested leaves have pale-green bulges covering most of the surface and are stunted and distorted.
Damage can first be seen in late May or early June, and then can reoccur later in the season. Early damage is usually less severe than damage that occurs from subsequent late season infestations.
Control should begin by looking for the white larvae inside the curled edges of the new growth. Heavy infestations in the home garden are not commonplace and are usually sporadic. Home gardeners can prune out and properly dispose of the infested foliage. This will help control infestations for a year or two. Disturbing the soil around the plants and exposing the pupae to a harsher environment can help in controlling infestations.
If chemical control is necessary apply carbaryl, neem oil or spinosad in May when new growth appears and in early August. And with all infestations, maintaining a healthy vigorous plant will help the plant fight off the pest and recover quicker.
Mona Bawgus is a certified master gardener and consumer horticulturist with Rutgers Cooperative Extension of Atlantic County. Write to her c/o Rutgers Cooperative Extension, 6260 Old Harding Highway, Mays Landing, NJ 08330. Email: