MSU Extension

A recent press release from the Michigan Department of Agriculture has generated some concern among residents who think they might have seen this insect. Certainly nobody wants or needs another invasive forest pest in Michigan or the United States for that matter. But there are some important aspects of the Sirex noctilio capture in Michigan we need to keep in mind before we get too excited about this capture.

First, horntails, including Sirex noctilio, are related to wasps. Horntails can look an awful lot like a wasp and many people will likely mistake the bluish-black wasps that they see around their home for a horntail.

Second, there are 23 different species or subspecies of horntails that are native to North America. Some species colonize pines or other conifer trees. Other horntail species colonize hardwood trees – beech, for example, is a common host. Virtually all horntails are some combination of brownish, yellowish or black in color and they all pretty much look alike. Even entomologists have a difficult time trying to distinguish one species from another. Native horntails are not considered a problem. Because they colonize dying or recently dead trees, they play an important role in decomposition and nutrient cycling.

In countries where Sirex noctilio is invasive, most problems have occurred in large plantations of non-native pines. The tree that was most affected was Monterey pine, a North American species that was planted in extensive plantations in places like Australia, New Zealand, Chile, Argentina and Brazil. When drought occurred, many pines became very stressed and were subsequently infested and killed by Sirex noctilio. Keep in mind, however, that when extensive monocultures of non-native pine trees are planted, an outbreak of a plant pest is not especially surprising.

Whether Sirex noctilio will become an important pest of pine in Michigan or other areas of North America is yet to be determined. We have many insect species that colonize stressed, dying or recently killed pines. Sirex woodwasps will have to interact with and compete with those native insects for that same pine resource.

In addition, we do not yet know if Sirex noctilio is established in Michigan or if the insect collected in the trap was simply dispersing from an infestation in Ontario. Wood wasps are very good fliers and it’s quite possible this particular insect originated in Ontario. Many traps were set in Michigan this year for Sirex noctilio following the identification of the infestations in Ontario last year. After horntail flight ends (late summer) and once all the horntails collected from traps or trap trees have been identified, we’ll have a much better idea of whether Sirex noctilio is actually established in Michigan. Forest entomologists, resource managers and regulatory officials will then meet to determine what, if anything, will be done about Sirex noctilio in Michigan. That meeting will likely occur this fall.

Another aspect worth noting is that Sirex noctilio will face native natural enemies in North America and possibly an introduced biological control. There are at least two groups of native parasitoid wasps that will attack the horntail larvae. In addition, an active biological control program was developed in Australia that makes use of parasitic nematodes. Methods have been developed to inoculate pines and introduce the nematode into Sirex noctilio populations. Research is underway in the United States to assess potential non-target effects of the nematode. If results indicate that the nematode can be safely released, the methods developed in Australia will likely be used to establish the nematode here.

There are several good web sites with information about the biology of Sirex noctilio woodwasps and photos of the insect and infested trees. A few that might be of interest include: