As fast as the leaves and needles grew this spring on our trees and shrubs, they already are starting to disappear. Not fall off -- but seemingly vanish into thin air.
This trick is most likely the work of a sawfly, a feeding insect larvae. These seasonal pests seem to be eating more than their fare share of plants this year. If leaves and needles or your landscape plants are starting to thin, here is what you need to know about sawfly larvae feeding:
Contrary to the name, sawflies are not flies. They are nonstinging wasps. The females of most species have a saw-like apparatus at the tip of their abdomens, which accounts for the name "sawfly." The saw is used to slit or cut plant tissue and aids in the insertion of eggs into these slits.
There are two major types of sawflies in our area. Some attack evergreens such as mugho and Scots pines. Some consume leaves of broad-leaved trees such as oaks, birch, sycamore, ash, dogwood and elm. The larvae of all species look much like caterpillars of moths and butterflies. However their leg pattern gives them away. Sawfly larvae have more than five pairs of prolegs on the abdomen, each of which lack the hooked spines typical of caterpillar larvae. Caterpillars have two to five pairs.
European pine sawfly is the most common species found in our area. It overwinters as eggs inserted into the needles of host plants during August and September. Warm temperatures in early spring stimulate egg hatch in late March and early April. Newly emerged larvae immediately begin their feeding activities on "old needle growth." By mid-May, larvae will be fully developed. While some larvae form cocoons on the host plant, most drop to the ground and form cocoons beneath surface debris (in fact, they may use bits and pieces of soil and debris to cover/coat their cocoons). By mid- to late-August, adult sawflies will emerge from their cocoons. Female sawflies emit a sex pheromone, which help the males locate them for mating purposes. Using her saw-like ovipositor, the female sawfly will deposit overwintering eggs into the slits made in needles. There is only one generation per year.
Pine sawflies are feeding now on Scots and mugho pines. When the worms were small, they could not consume a complete needle, so they rasped off the top layer of cells turning the individual needles brown and twisted. The larvae are gregarious, so a number of larvae can be found close together. Now that the larvae are maturing, they will consume whole needles and can virtually strip a tree. This happens before new needles expand, so the tree is rarely killed.
All species of sawflies can be controlled with products that contain "natural" insecticidal materials. Azadirachtin, a bioinsecticide derived from neem trees, works as an insect growth regulator. Horticultural oils and horticultural soaps effectively control the soft-bodied sawfly larvae. But with oils and soaps, what you spray is what you kill. Once the spray dries, they do not provide residual control. So if initial spray treatment coverage was not thorough, an additional treatment might be necessary to "clean up" larvae that were untouched and have continued feeding. The rotenone and pyrethrin plant derivatives have very brief residual properties. Chemical insecticides such as Orthene, cyfluthrin (Bayer Lawn and Garden Multi-Insect Killer), malathion, esfenvalerate (Ortho Bug-B-Gon Multipurpose Insect Killer and Monterey Bug Buster) and Sevin can be used as well.
- Bruce Chladny is horticulture agent at K-State Research and Extension-Douglas County. For more information, call him at 843-7058 from 8 a.m. to 5 p.m. weekdays.