The rhododendron gall midge (or tip midge), Clinodiplosis rhododendri (Felt), is a native pest of Rhododendron catawbieuse Michaux., R. maximum, R. ponticum and their many hybrids during the May to October growing season. The larval feeding damage from this fly can cause discoloration and distorted foliage. This damage can appear as in-rolled, twisted leaves that have turned a yellow color. This can be seen on young leaves that have separated from the bud scale. Leaves attacked in the bud stage may die if the injury is severe. Care must be taken to distinguish this pest damage from similarly manifested damage caused by leafhopper injury, chemical injury, leaf-roller activity and aphid feeding.
LIFE CYCLE
This insect overwinters in the soil as a pre-pupa, with complete pupal development taking place in the spring. The adults are very small (less than 1/16″) and may be difficult to detect. Eggs are laid on the undersurfaces or rolled edges of the leaves as soon as they are free from the bud but before the leaves have fully separated from each other. Within three days, the eggs hatch and the larvae, which are small white maggots, start to eat leaf tissue within the moist, protected area of the inner surface of the leaf roll. The larvae mature in about seven days, drop to the ground and pupate in the top one inch of soil inside a flimsy, silken cocoon.
Work done in Connecticut showed that most midge populations will complete three generations, but depending on growing conditions, this could reach four to five generations per year.
Early season damage, if present, should be visible in early June. It appears that infestation of the first growth flush is considerably less (20% infestation) than the second and third growth flushes (95%infestation combined). This is an important fact to take into consideration in order to protect the crop. Early detection and treatment will greatly reduce the pest population and, thus, the damage in later growth periods.
CONTROL
Recommendations are based on timing applications of insecticides with the bud break stage of rhododendron development. Repeat applications may be needed when Rhododendrons experience a second and third flush of growth which may result in additional generations of the gall midge.
At this time, there is still little information available on nonchemical alternatives for control. A cultural practice that may have some value as a control strategy is to disturb the soil around the plants so that the pupae are exposed to harsher environmental conditions. This may not be practical due to the time involved. Other cultural practices would be to maintain healthy plants so that they could survive any damage caused by an infestation or to prune newly-infested foliage that contain larvae.
For additional information regarding insecticides and management strategies of the Rhododendron Gall Midge, contact your local Cooperative Extension center. Or visit the on-line guides of Cornell University and Penn State. This information may not apply to your state.
References:
Baker, J.R. (ed.). 1980. Insects and Related Pests of Shrubs: Some important, Common and Potential Pests In the Southeastern United States. North Carolina Agricultural Extension Service.
Brand, M.H. 1989. The Rhododendron Tip Midge–A Relatively Unknown Pest Gains Momentum. The Connecticut Nursery Newsletter. 1(1):6-8.
Hanula, J.L. 1991. Seasonal Abundance and Control of the Rhododendron Gall Midge, Clinodiplosis rhododendri (Felt), in Container Grown Rhododendron catawbiense Michaux. J. Environ. Hort. 9(2):68-71.
Johnson, W.T. and H.H. Lyon. 1988. Pages 470-471. In: Insects that Feed on Trees and Shrubs, 2nd. Ed. Cornell University Press, Ithaca.
Lamb, E. (ed.). 2011. 2011 Cornell Pest Management Guide for Commercial Production and Maintenance of Trees and Shrubs. Cornell University Cooperative Extension.
Updated by: Mary Concklin, IPM, University of Connecticut. 2011
Prepared by: Tim Abbey Extension Educator, Nursery
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