Pine sawflies

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

Pine sawfly larvae

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?

Adult pine sawflies (Diprion pini) are 10mm long brown winged insects, although they can appear fly like sawflies are more closely related to bees ants and wasps. The larvae of sawflies are similar in appearance to caterpillars.

Pine sawfly larvae reach up to 25mm long and are pale green in colour with a brown head and black markings.

Fox-coloured sawfly (Neodiprion sertifer) larvae also reach 25mm in length are a dirty green colour with a black lateral stripe and a black head.

Both species 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 infestations have to be tolerated.

Non-pesticide control

Larvae that can be reached can be removed by hand.

Pesticide control

  • If infestations are too heavy for hand picking, control may be achieved by spraying with pesticides. Spaying at dusk is likely to give the best results
  • If numbers of larvae are too high for hand picking, control may be achieved by spraying with pesticides. Spaying at dusk is likely to give the best results
  • Organic contact insecticides containing natural pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug 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 Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug 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
  • Do not spray near plants in flower due to the danger to pollinating insects
  • Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Download

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to 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.


Gardeners' calendar

Advice from the RHS

Find out what to do this month with our gardeners' calendar

Advice from the RHS

Did you find the advice you needed?

RHS members can get exclusive individual advice from the RHS Gardening Advice team.

Join the RHS now

Get involved

We're a UK charity established to share the best in gardening. We want to enrich everyone's life through plants, and make the UK a greener and more beautiful place.