Sawflies of ornamental trees and shrubs

By Joan Allen - UConn Home & Garden Education Center
Featured Article - posted Wed., Jun. 20, 2012
Contributed
- Contributed Photo

Many sawfly larvae are actively feeding at this time of year. They often feed in groups and will sometimes rear back in unison in response to a perceived threat, making them quite entertaining.

Sawflies are insects that are closely related to wasps and bees. They get their name from the saw-like ovipositor that females use to create egg-laying sites. The larval stages of these insects are plant feeders and can cause damaging levels of defoliation. They resemble caterpillars which are the larvae of moths and caterpillars. There are sawflies that occur on conifers, such as pine and spruce, and hardwoods, including ash, birch and other hosts. The amount of damage caused depends on host plant health, sawfly population levels, the time of year and the sawfly species. Heavy defoliation of deciduous plants is usually less serious than on evergreens. This is because most evergreens keep their needles or leaves for more than one season, so losing a large portion of them will affect their health for more than the current growing season.

The European pine sawfly was introduced from Europe and its preferred hosts are mugho, Scotch, red and jack pines. Other pines may also be fed on if they are located near the preferred hosts. The larvae feed in groups on the previous year’s needles. Once they have devoured all these needles on a single branch, they move on to another one. These larvae never eat new needles, so this sawfly seldom kills trees. Repeated defoliations can slow growth. Larval feeding begins around mid May and continues through June. They overwinter as eggs and there is one generation per year.

The introduced pine sawfly has the potential to cause more damage. The favored host is white pine, but they also may feed on Scotch, jack and red pines. These sawflies are more damaging to their hosts because there are two generations per year. The first generation larvae feed on the previous year’s needles but the second generation larvae feed on both old and new needles. Defoliation is usually heaviest in the upper portion of the tree. The first generation larvae feed from late May/early June to early July, and the second generation feeds from late July through early September. This sawfly overwinters in the soil as a prepupa.

The red-headed pine sawfly feeds on many species of pine but usually attacks trees less than 15 feet tall. The larvae feed in groups and can completely defoliate a tree. There are two generations per year and the winter is spent as a pupa in the soil.

The white pine sawfly prefers white pine and will also feed on red pine. These larvae feed in groups and eat both old and new needles, so entire trees can be defoliated. These also overwinter in the soil as prepupae. There is usually one generation per year and the larval feeding period is late June to early August.

The blackheaded and brownheaded ash sawflies have similar life cycles and both feed on species of ash. These sawflies overwinter as prepupae in the soil and have one generation per year. You would notice the sawfly larvae feeding from early May through June. Trees can be completely defoliated in a short time when heavily infested.

Mountain ash sawflies feed in groups and eat entire leaves, leaving only the midveins, and will defoliate entire branches, then moving on to another one.  Trees usually survive a single defoliation, but if heavy feeding occurs for more than one season consecutively, the tree will weaken and possibly be killed. This sawfly has two generations per year.

The introduced pear sawfly, or pear slug, was introduced from Europe and feeds on hosts including plum, cherry, cotoneaster, pear, mountain ash and hawthorn.  Feeding is on the upper leaf surface, leaving the veins. Heavy feeding gives the tree a scorched appearance and may cause premature leaf drop. There can be two generations per year.

Rose is host to the roseslug sawfly. Populations of this pest are generally low enough that damage is minimal. The single generation of larvae feed from mid-May through June.

Control measures for sawflies should depend on plant health, sawfly population, sawfly age, time of year, sawfly species and the type of host (deciduous or evergreen). If damage is confined to one branch, the branch can be pruned out along with the sawflies and destroyed. Control should not be attempted if sawflies are full grown, as they will already have completed most of their feeding.  Late season (second generation) feeding on deciduous plants will usually have little impact on tree or shrub health, as it will have already stored most of the reserves it needs for the next spring. The sawfly species characteristics such as number of generations per year and feeding habits (i.e. new or old foliage eaters) will also affect the degree of harm a specific sawfly infestation will cause.

If control is needed, there are mechanical and chemical options. Mechanical controls include hand removal of larvae from the plant, dislodging them with a strong spray of water, or other physical means of eliminating them. Chemical controls include biorational options such as insecticidal soaps and conventional insecticides. BT (Bacillus thuringiensis) will not control sawflies. If considering chemical control, read and follow all label instructions carefully. Make sure the product is labeled for both the intended host and pest.

For more information on sawflies, contact the UConn Home & Garden Education Center, toll free, at 877-486-6271 or by e-mail at ladybug@uconn.edu.

Photo credit: kentcoopextension.blogspot.com
 


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