Chisago County Master Gardener
It’s got to be the most common question I’m asked at this time of year. One day our favorite plants look fresh, healthy and full of vigor. Then, overnight, something comes along and begins to defoliate them.
The most likely culprit is the sawfly larva, one of the most efficient eating machines in the insect world.
Sawflies are related to wasps and bees. Their name is derived from the saw-like ovipositor the adult female uses to lay eggs.
Adult sawflies are inconspicuous wasp-like insects that do not sting. The plant-eating larvae look like hairless caterpillars, the immature stage of butterflies and moths. The main distinguishing character between sawflies and caterpillars is the number of prolegs: fleshy, leg-like projections on the abdomen. Caterpillars have 2 to 5 prolegs, while sawflies have 6 or more. Sawflies often feed in groups and can quickly defoliate portions of their host plant.
There are many different species of sawflies. Each prefers specific plants or groups of related plants.
If you have Mugo Pines in your landscape, you are probably already very aware of the damage the European Pine Sawfly can do.
I recently had to prune out two well-established shrub roses on which every single leaf was skeletonized by Rose Sawflies. Fortunately, they didn’t eat any branch tissue, so the shrubs will re-leaf this season. They may not bloom again until late summer.
Controlling sawfly larvae infestations can be difficult, so early detection is very important.
Mechanical control methods include hand-picking larvae from plants and physically dislodging them with forceful water sprays.
If chemical control measures are needed, these should be directed toward the young larvae, which are much more susceptible to chemical applications.
This is especially true when using insecticidal soap. If the larvae are nearly full-grown, chemicals will not be as effective and most of the plant damage will have already been done.
Because sawflies feed in groups, chemical applications should be directed only to the areas they are feeding on. Entire tree sprays are usually unnecessary.
The best control is prevention. Correct plant selection, proper site selection and good care will ensure that plants are in excellent health. The better condition a plant is in, the more damage it can tolerate.