Pine sawflies

The caterpillar-like larvae of two species of sawfly can occasionally cause considerable defoliation of pine (Pinus) trees. 

Pine sawfly larvae

Quick facts

Common name Pine sawfly and fox-coloured sawfly
Scientific name Diprion pini and Neodiprion sertifer
Plants affected Pine (Pinus)
Main symptoms Caterpillar like larvae defoliate plants
Most active Spring-Summer

What are pine sawflies?

Although they can appear fly like sawflies are hymenopteran insects more closely related to bees ants and wasps. The larvae of sawflies are superficially similar in appearance to moth caterpillars.

Adult pine sawfly (Diprion pini) are 10mm long brown winged insects, The larvae reach up to 25mm long and are pale green in colour with a brown head and black markings.

Fox-coloured sawfly (Neodiprion sertifer) is 7-9mm long and pale orange-brown. The larvae reach 25mm in length are a dirty green colour with a black lateral stripe and a black head.

The larvae of both species of sawfly feed in groups on the needles of pines during the spring and summer.

Symptoms

Heavy infestations of larvae can cause considerable defoliation of trees, whilst this is unusual in gardens when it occurs it can affect the growth of the trees.

Control

Control is only feasible on trees that are small enough to treat, on large trees these insects have to be tolerated.

Check pines frequently from spring onwards so action can be taken before a damaging infestation has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.

Non-pesticide control

  • Where possible tolerate populations of sawfly
  • Encourage predators and other natural enemies of sawfly in the garden, such as birds and ground beetles.
  • Check plants regularly from spring for the presence of larvae and remove by hand where practical
Pesticide control
  • If numbers of larvae are too high for hand picking, control may be achieved by spraying with pesticides. Spraying at dusk is likely to give the best results
  • Organic contact insecticides containing natural pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer). Several applications of these short persistence products may be necessary to give good control
  • More persistent contact insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
Follow label instructions when using pesticides.

Plants in flower should not be sprayed due to the danger to bees and other pollinating insects.

Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.

Download

Pesticides for gardeners (pdf document outlining pesticides available to home gardeners)

Biology

Pine sawfly adults are active in spring and the females lay eggs in slits they make with an egg laying organ (ovipositer) in the needles. The larvae hatch after a few weeks and feed in groups on the needles. These larvae will lift their abdomen off the plant in a group as a defensive posture when disturbed. The larvae are usually fully grown by July where they spring brown cocoons on the foliage or on the ground beneath the plant. New adults emerging in July or August can produce a second generation of larvae that feed in late summer and autumn. The autumn generation spins a cocoons in soil or leaf litter, pupating in the following spring.

Fox-coloured sawfly adults are active in late summer and early autumn and lay eggs on the needles. The eggs overwinter and hatch in spring. The larvae feed in groups which will raise their abdomens when disturbed. In late June the fully grown larvae drop to the soil to pupate in an oval cocoon. This species has one generation a year.

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