Pine sawflies
Click for larger image European pine sawfly (Hymenoptera) rearing up in characteristic defensive posture on pine (Pinus)

Conifer sawflies are a unique group of defoliating insects. The various species (see "Pine Sawfly – Species") are distributed throughout the area where their preferred hosts grow. The larvae are hostspecific and feed on old and current year foliage at some point in their development. Some species have one generation per year with defoliation occurring in the spring and others produce three or more generations with defoliation occurring on into fall. Fall defoliation has a greater impact on trees and it is for this reason that sawflies having multiple generations are considered more devastating.

Sawfly adults resemble large houseflies but are actually primitive broad-waisted wasps. While true flies have one pair of wings, the sawfly has two pairs of wings. The females are equipped with an ovipositor that is serrated, which enables them to saw little slits in the needles where eggs are laid, thus the name "sawflies".

Defoliation by sawflies is sporadic, occurring in localized or region-wide outbreaks lasting one or more years. Growth loss the year following a severe defoliation (greater than 75%) can average over 50% and mortality increases due to secondary invasion by bark beetles and pine sawyers.

European pine sawfly is the most common sawfly in Missouri.

Symptoms and Diagnosis

Sawfly larvae have the curious habit of raising their heads and tails in a threatening manner when disturbed. Overwintering eggs that have been deposited in the needles can be easily located after a heavy frost turns the egg-laying scar yellow. (See "Pine Sawfly – Species" for a detailed description of larvae.)

Life Cycle - European Pine Sawfly

In August to September, the adult European pine sawflies emerge from their cocoons to mate and lay eggs. Female sawflies emit a sex pheromone that helps the male locate females for mating purposes. Using her saw-like ovipositor to cut through the tough outer skin of the needle, the female sawfly deposits overwintering eggs in slits she makes in the needles. Each female may lay six to eight eggs in each of 10 to 12 needles, but this will vary by sawfly species. European pine sawflies overwinter in the egg stage.

The eggs hatch in April through May and the larvae may feed until mid-June. The larvae feed in groups or colonies, often with three or four larvae feeding on a single needle. Being small and having small mouth parts, they merely rasp off the epidermal cells from the needles, which removes the protective barrier against desiccation. Thus, needles become twisted and brown as they dry out and die. Dead needles are easily detectable when contrasted against the green or surrounding "healthy" needles. As the larvae grow, they remain together and feed from the tip of a needle to the base. The larvae feed on older foliage and move from branch to branch as they strip the needles. Mature larvae drop to the ground and spin tough, brown cocoons in the duff. A few larvae may pupate on the tree.

The above life cycle is for the European pine sawfly that only has one generation per year. The life cycle of those with more than one generation per year may vary slightly as some may overwinter in the ground in the pupae stage. Climate and latitude have a great bearing on the number of generations per year.

Integrated Pest Management Strategies

1. Maintain plant vigor. Dry weather and poor soil conditions encourage damage by sawflies, therefore, water during times of drought. Keep plants vigorous with a fertilizing program.

2. Natural controls. Several parasites have been introduced to control this pest. Native birds feed on the larvae. Rodents often eat the pupae in the soil. These agents are usually not adequate in urban settings.

3. Mechanical control —egg removal. If the needles containing overwintered eggs can be found before they hatch, they can be pulled off the plant and destroyed. Do not simply throw on the ground since young could still hatch from the eggs.

4. Mechanical control. Colonies of larvae can be easily removed by clipping off the infested branch. Place these branches in a plastic bag and destroy. Colonies can also be knocked off by sharply striking the infested branch. Crush the larvae or knock into a pail of soapy water. If few colonies are present, they can be controlled using these methods, but large infestations are better controlled by general spraying.

5.Insecticide. Several horticultural oils and insecticidal soaps are labeled for control of sawflies on ornamentals. These usually work well when the sawfly larvae are small, and thorough coverage of the colony can be achieved. Pesticides registered for use include acephate (Orthene), azadirachtin (Bio-Neem, Margosan-O), or carbaryl (Sevin).

Organic Strategies

Strategies 1, 2, 3, and 4 are strictly organic approaches. For an organic approach to Strategy 5, consult the Organic Materials Review Institute (OMRI™) for appropriate insecticidal soap products.

More images:

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European pine sawfly (Hymenoptera) on pine (Pinus)
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European pine sawfly (Hymenoptera) on pine (Pinus); note that new growth is untouched
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European pine sawfly (Hymenoptera) beginning to take a defensive posture on a pine needle (Pinus)
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European pine sawfly (Hymenoptera) bent over backwards in characteristic defensive posture on a pine needle (Pinus)
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Adult female European pine sawfly (Hymenoptera) on pine (Pinus spp.) L-M. Nageleisen, Departement de la Sante des Forets, Bugwood.org
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Adult male European pine sawfly (Hymenoptera) on pine (Pinus spp.) L-M. Nageleisen, Departement de la Sante des Forets, Bugwood.org
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Adult female European pine sawfly (Hymenoptera). P. Kapitola, State Phytosanitary Administration, Bugwood.org
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This pine sawfly larva (Hymenoptera) has eaten only the older needles of a Mugo pine (Pinus mugo) leaving new growth for next year's meal
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Pine sawfly eggs and larvae (Hymenoptera) on pine (Pinus)
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Pine sawfly eggs and larvae (Hymenoptera) on pine (Pinus)
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Pine sawfly eggs (Hymenoptera) and emergence holes on pine (Pinus) needles
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New growth on mugo pine (Pinus mugo) after pine sawflies (Hymenoptera) have finished feeding on the older needles.