I have received many calls recently regarding rapidly defoliated shrubs and trees, with no leaf debris on the ground or signs of insects on the bare branches. If you have seen this, you might be experiencing damage caused by sawfly larvae.
Sawfly wasp larvae are plant eaters. Most resemble caterpillars in general appearance and also in damage. Many sawflies are plant pests that cause noticeable-to-destructive loss of plant foliage.
Sawflies often feed in groups and can quickly defoliate portions of their host plant. Sawflies are host-plant specific; that is, each different species of sawfly feeds on a specific host plant and does not move from one plant type to another.
For example, the European pine sawfly larvae are gray-green larvae with shiny black heads that live in clusters and eat pine tree needles in May; they will not feed on other plants. Similarly, the dogwood sawfly larvae that eat entire leaves from gray and red osier dogwood plants in late summer will be found only on dogwood shrubs.
Defoliation, which may range from spotty to complete, is not usually fatal to healthy, well-established trees and shrubs. Small, newly transplanted and stressed trees may warrant protection from severe defoliation. Otherwise, control is probably not justified.
It is typical to discover the damage after the larvae have finished feeding and dropped from the leaves. In this case, it is too late to take any effective action.
Treat sawfly larvae when they are young and half their full-grown size or less. If larvae are fully grown, the damage is done and treatment is not effective.
Most sawflies feed in groups, and it is possible to spot treat them instead of treating the entire plant. This helps reduce the amount of pesticide used.
Low impact pesticides
Insecticidal soap and horticultural oil are effective when managing small numbers of young sawfly larvae.
- These products have minimal impact on natural enemies.
- Repeat applications might be needed as the product needs to come in contact with the sawflies.
Azadirachtin and spinosad are effective for one or two weeks so sawflies that feed on treated foliage are still affected.
Bacillus thuringiensis var. kurstaki (Btk) is a common low-impact insecticide used against moth and butterfly caterpillars. It is specific to caterpillars and does not kill sawfly larvae.
Contact residual insecticides
Permethrin, bifenthrin, lambda cyhalothrin, carbaryl and acephate are most effective against young sawfly larvae. One application should usually be sufficient.
Systemic insecticides are pesticides that are transported by the plant through tissues that carry food and water to the leaves and needles. When sawflies feed on the needles and leaves, they receive a toxic dose. Common examples are imidacloprid and dinotefuran.
Caution: Be careful when applying systemic insecticides to hardwood trees and shrubs that are attractive to bees. This includes linden, crab apple and sugar maple, as well as juneberry (serviceberry), pagoda dogwood, nannyberry viburnum and many other shrubs. This does not include evergreen trees and shrubs.
Apply systemic insecticides to trees and shrubs only after flowering has already occurred to reduce pesticide exposure to bees. If sawflies are active when trees and shrubs are flowering, use an alternative method of treating them. Do not apply systemic soil drench pesticides when flowering plants are next to trees or shrubs.
Commercial tree care companies have experience in managing sawflies and in applying pesticides. When treating trees is not practical, use a licensed pesticide applicator working for a professional company.
Caution: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
For more information, visit www.extension.umn.edu. Until next time, happy gardening!
Robin Trott is a horticulture educator with University of Minnesota Extension. Contact her at 320-762-3890, or at trot0053@umn.