Pine false webworm

Written by J.P. Brandt Revision 2024

• French common name: Pamphile introduit du pin
• Other common names: False pine webworm, pine tree web-spinning sawfly
• Scientific name: Acantholyda erythrocephala (L.)
• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Insecta
• Order: Hymenoptera
• Family: Pamphiliidae
• Partial list of synonyms:
  • Lyda erythrocephala Linnaeus

General information and importance

Pine false webworm is an introduced sawfly species native to Europe and Asia. When first introduced into Canada, it was only a serious pest in young pine plantations, Christmas tree farms, and pine ornamental landscape plantings. In the 1990s, outbreaks began to occur in older red (Pinus resinosa) and white (P. strobus) pine plantations in Ontario and New York state, causing serious damage.

The common name ‘sawfly’ refers to the female adult’s saw-like ovipositor, which she uses to deposit her eggs. Adult sawflies resemble their winged bee and wasp relatives with membranous wings. They differ with a broader “waist” between the thorax and abdomen.

Distribution and hosts

The first record of the insect in North America was in Pennsylvania in 1925. Since then, it has spread through much of the northeastern United States and Lake States. Pine false webworm was first found in Canada in Ontario in 1961. It is currently found throughout much of southern and central Ontario, and at a few locations in the northern part of the province. In Canada, specimens of this insect have also been found in Edmonton and Grande Prairie, Alberta, St. John’s, Newfoundland, and southern Quebec. The insect is native to Europe and Asia, where it feeds on several 2- and 5-needle pine species. In Canada, pine false webworm feeds on several native pine species: red pine, white pine, and jack pine (P. banksiana). Both lodgepole pine (P. contorta var. latifolia) and ponderosa pine (P. ponderosa) have also been defoliated when growing in association with the other pine species listed above. The sawfly also feeds on several introduced pine species in Canada: Japanese red pine (P. densiflora), mugo pine (P. mugo), Austrian pine (P. nigra), and Scots pine (P. sylvestris).

The insect pupates and overwinters in the soil beneath its host, so the movement of pine seedlings and young ornamental trees with soil is a likely mechanism of spread to previously uninfested areas of the country.

Tree parts affected

Needles

Symptoms and signs

The adult stage of this winged insect is known as a sawfly with a broader “waist” between the thorax and abdomen. Adult males are entirely black or bluish-black except for a yellow face (i.e., the frons) and yellow forelegs. Adult females are larger than the males. They are also black or bluish-black. They have red heads with black eyes and reddish forelegs. During the mating period, adult sawflies are commonly found flying in the crowns of host trees, especially if populations are high. Eggs are deposited end-to-end along needles closest to the buds by females. Eggs are yellow when first laid, and slowly darken to a green-brown before hatching. Larvae are olive green to green-brown. They have orange-yellow heads with brown spots, and longitudinal purplish-red stripes running the length of their abdomen. The larvae have three sets of legs and two short appendages on their last abdominal segment to spin silken threads and assist in movement. Larvae are about 15 to 20 millimetres in length when full grown. The larvae construct silken tubes or nests along the branch from within which they feed by attaching silk strands to needles, cutting them off, and pulling them into the webbing. The tubes fill with partially consumed needles, frass, and cast skins from the moulting larvae. If larvae are disturbed while outside their silken shelter, they immediately withdraw back inside. Depending on food supply, older larvae disperse and feed singly from within other individual silken tubes that they have built. The larvae feed primarily on old foliage. If all the old foliage has been consumed, larvae will feed on current-year needles.

Life cycle

Pine false webworm has one generation per year. This insect overwinters as larvae in earthen cells in the soil beneath the host. Pupation occurs in early to mid-May. During mid to late May, adults emerge from the soil, mate, and females lay 30 to 40 eggs in groups of 4 to 10. The eggs are laid end to end along the surfaces of the previous year’s needles, near the current year’s expanding buds. Eggs hatch about 10 to 14 days after being laid. Newly hatched larvae move to the base of the needles nearest the developing bud and create silken tubes. Larvae cut nearby needles and often pull them into the tubes that they have created to feed. Feeding is usually completed in about 21 to 28 days, from mid to late June. Mature larvae drop to the ground, form earthen cells in the upper 10 centimetres of soil, and overwinter. A proportion of the larvae have a delayed development, with pupation and adult emergence after two winters.

Damage

For many years, in Canada and the United States, pine false webworm was generally considered a pest of relatively small, young, or immature trees in plantations for Christmas tree production, field-grown ornamentals, and afforestation. This period ranged from its first introduction into North America until around 1993. For Christmas tree production and field-grown ornamentals, the main damage consists of negative aesthetics associated with defoliation and the often unsightly appearance of webbing and partially consumed red needles that are difficult to remove.

The insect’s feeding behaviour appeared to change around 1993. It began attacking larger, semi-mature red pine, white pine, and the non-native Scots pine in Ontario, Quebec, and the northeastern United States. The reason for the change in behaviour is unclear. Even light to moderate levels of feeding on young trees can result in unsaleable Christmas trees and pine ornamentals. In forestry settings, a single year of severe defoliation in semi-mature pine plantations was correlated with tree mortality. This would only increase if defoliation was to occur in multiple consecutive years.

In the insect’s native range in Europe and Asia, pine false webworm is not considered a serious pest. However, there have been a few localized outbreaks that have been documented.

Prevention and management

Pest management strategies for a particular pest vary depending on several factors. These include:

• the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
• the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
• an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
• how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
• the value of the host(s) versus the costs of pest management approaches; and
• consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

In Europe, there are several natural enemies and parasitoids that generally keep populations of this insect in check. In North America, there has been some work on investigating these various enemies and parasitoids for release for classical biological control efforts against this invasive species. Although the parasitoid complex is relatively rich in Europe, the most promising species for biological control is the tachinid fly Myxexoristops hertingi.

Pine false webworm poses some control challenges in its larval stage because the silken tube or nest in which it feeds for the majority of the time affords it some level of protection. Contact insecticides can be ineffective during later larval stages when the nests are relatively large and dense. If populations warrant control, insecticides with some residual activity are best applied immediately before or 1 to 2 days after eggs hatch. Monitoring for the distinctive and conspicuous adults in May will aid in both the need for and timing of insecticides, especially for Christmas tree production and field-grown ornamentals. A decision on whether a pesticide application is necessary can be based on high numbers of easily visible eggs on needles. If there is at least one batch of eggs on the needles of most twigs or branchlets, moderate to heavy defoliation can be expected.

Research trials using chemical insecticides run in the 1980s and 1990s indicated that insecticides with the active ingredients permethrin or carbaryl were most effective. The insect growth regulator, diflubenzuron, was also effective. Insecticides based on the active ingredient azadirachtin, which itself is derived from the natural extract of the neem tree, were also effective. Pesticides registered for use against pine false webworm under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against pine false webworm. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Photos

Male and female adults of pine false webworm mating.

Six pine false webworm eggs on a pine needle: the two on the left have not hatched; a small larva is emerging from the third egg from the left; and the three eggs on the right have already hatched.

Pine false webworm larva outside of its webbed nest. Note also the frass from the larva and partially eaten needles.

Selected references

Asaro, C.; Allen, D.C. 1999. Biology of pine false webworm during an outbreak. The Canadian Entomologist 131(6): 729–742. https://doi.org/10.4039/Ent131729-6

Eidt, D.C.; McPhee, J.R. 1963. Acantholyda erythrocephala (L.). New in Canada. Department of Forestry, Forest Entomolgy and Pathology Branch. Bi-monthly Progress Report. 19(4): 2.

Kenis, M.; Kloosterman, K. 2001. European parasitoids of the pine false webworm (Acantholyda erythrocephala (L.)) and their potential for biological control in North America. Pages 65–73 in A.M. Liebhold, M.L. McManus, I.S. Otvos, and S.L.C Fosbroke, editors. Proceedings: integrated management and dynamics of forest defoliating insects. 15–19 August 1999, Victoria, British Columbia. United States Department of Agriculture, Forest Service, Northeastern Research Station. Newtown Square, Pennsylvania. General Technical Report. NE-277.

Lyons, D.B. 1994. Development of the arboreal stages of the pine false webworm (Hymenoptera: Pamphiliidae). Environmental Entomology 23(4): 846–854. https://doi.org/10.1093/ee/23.4.846

Lyons, D.B. 1995. Pine false webworm, Acantholyda erythrocephala. Pages 245–251 in Forest insect pests in Canada. J.A. Armstrong and W.G.H. Ives, editors. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.

Lyons, D.B. 1996. Oviposition and fecundity of pine false webworm (Hymenoptera: Pamphiliidae). The Canadian Entomologist 128(5): 779–790. https://doi.org/10.4039/Ent128779-5

Lyons, D.B. 1999. Phenology of the native parasitoid Sinophorus megalodontis (Hymenoptera: Ichneumonidae) relative to its introduced host, the pine false webworm (Hymenoptera: Pamphiliidae). The Canadian Entomologist 131(6): 787–800. https://doi.org/10.4039/Ent131787-6

Lyons, D.B. 2013. Acantholyda erythrocephala L., pine false webworm (Hymenoptera: Pamphiliidae). Chapter 7. Pages 54–56 in Biological Control Programmes in Canada 2001–2012. P.G. Mason and D.R. Gillespie, editors. CABI Press. Oxfordshire, United Kingdom.  

Lyons, D.B.; Jones, G.C.; Scarr, T.A. (editors). 2000. Proceedings of a workshop on the pine false webworm, Acantholyda erythrocephala (Hymenoptera: Pamphiliidae). Natural Resources Canada, Canadian Forest Service, Great Lakes Forestry Centre. Sault Ste. Marie, Ontario. 49 p.

MacQuarrie, C.J.K.; Lyons, D.B.; Seehausen, M.L.; Smith, S.S. 2016. A history of biological control in Canadian forests, 1882–2014. The Canadian Entomologist 148(Supplement S1): S239–S269. https://doi.org/10.4039/tce.2015.66

Mayfield A.E., III; Allen, D.C.; Briggs, R.D. 2007. Site and stand conditions associated with pine false webworm populations and damage in mature eastern white pine plantations. Northern Journal of Applied Forestry 24(3): 168–176. https://doi.org/10.1093/njaf/24.3.168

Syme, P.D. 1985. Pine false webworm. Canadian Forestry Service, Great Lakes Forest Research Centre. Sault Ste. Marie, Ontario. Leaflet.

Thompson, D.G.; Mickle, R.E.; Lyons, D.B.; Helson, B.V.; Robinson, A.G.; Chartrand, D.T.; Buscarini, T.M. 2003. Deposition of azadirachtin following ultra-low volume aerial applications to red pine plantations for control of pine false webworm, Acantholyda erythrocephala (L.) (Hymenoptera: Pamphiliidae), in Ontario, Canada. International Journal of Pest Management 49(1): 9–15. https://doi.org/10.1080/713867833

USDA Forest Service. 1985. Insects of eastern forests. United States Department of Agriculture Forest Service. Miscellaneous Publication No. 1426. 608 p.

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