The redheaded pine sawfly feeds on the needles, causing defoliation of the tree, which can lead to reduced growth and performance, stem deformity and mortality in small trees. The sawfly is natural to our area and its population is cyclical in nature. Currently, it appears that the occurrence of redheaded pine sawfly is increasing and could lead to landowners losing a significant amount of investment if not controlled.
Some red pine plantations or even small areas within red pine plantations are more susceptible to attack than others. Young red pine plantations, less than 4–5 metres tall, are preferred hosts. These trees tend to be even more susceptible if they are already stressed due to environmental factors, such as drought. The edge of plantations where light levels are low (i.e., adjacent to a hardwood forest) can also be “hot spots” for attack.
The best time of year to identify an attack in your plantation is late June through to September when the yellow larvae, with six rows of black spots and a red head are easy to see. Often the sawfly larvae will be found feeding in clusters of 20 to 150. Early in the year, it may be more difficult to find the larvae since they are very small when they first emerge in June to early July. Another easy way of finding larvae is by looking for defoliated branch tips where the insect has been feeding. Recently, areas with sawfly have been confirmed on Crown land in eastern Ontario (between Cloyne and Bancroft), the Ottawa Valley and near Parry Sound.
If this insect is found, there is a range of physical, chemical and biological options for controlling the outbreak. The type of control method you use may also depend on the type of outbreak and your available resources. Physical means of treatment can be effective by pruning the branches where clusters of larvae are present and where eggs have been laid by the adults. The pruned branches with the sawfly then need to be destroyed. Alternatively, if the numbers of larvae are few, they can be picked off the branches by hand and destroyed.
Chemical pesticide treatment options should only be explored after all other methods of control have been pursued, and in cases where a large amount of investment has been made in the plantation. Chemical pesticide treatments using a variety of insecticides, most of which are federally registered, usually require a licensed pesticide applicator. Due to this requirement, chemical treatments are harder to coordinate and are more expensive to apply than physical control methods. If the landowner has a certified woodlot (i.e., FSC®), check with the appropriate organizations to ensure that the application of chemical pesticide treatments is permissible under the certification system. The chemical pesticide option is a reasonable option if the extent of the infestation is large and the physical control option is not practical. One issue with the chemical control option is that if some colonies are missed, the risk of further spread after the initial treatment is high.
In cases where large infestations are present and there is a large risk to the investment made in the plantation, a biological control option involving the use of the Lecontvirus is preferable to the chemical option. The benefits of treatment with the virus are that it is a highly effective biological control of the sawfly and it is natural in our environment. From an environmental standpoint, use of the Lecontvirus is very attractive because it does not harm beneficial non-target insects. Another attractive feature of this control method is that the virus not only kills the infected larvae, but it can also be transmitted both within a colony of larvae and between colonies. The virus is transmitted from diseased sawfly by rain and anything else that may come in contact with it. When treatment with Lecontvirus has been correctly timed, retreatment of a plantation should not be required. Lecontvirus is registered under the Pest Control Products Act and is therefore subject to the same requirements as the chemical treatments described above.
The problem with Lecontvirus is that it presently is unavailable and requires time to propagate. Local companies with sustainable forest licences that operate on Crown land are currently working on a project that may involve treatment of red pine plantations with Lecontvirus in the future.
Landowners facing the challenge of a redheaded pine sawfly infestation have a few organizations they can contact for help. For a start, they can try visiting the Ministry of Natural Resources & Forestry (http://www.ontario.ca/rural-and-north/forest-monitoring) or Natural Resources Canada – Great Lake Forestry Centre (http://www.nrcan.gc.ca/forests/research-centres/glfc/13459) websites. Both the Ministry of Natural Resources & Forestry and Natural Resources Canada have field staff that can provide advice on how to best deal with a redheaded pine sawfly issue on your property. The OWA can also put you in touch with people who are involved in related projects.