Weed Control Efforts

Insects Can Be Your Friend
Gary Sager
Spring, 2009
Updated Summer, 2014

The size and terrain of our property makes it impractical to control noxious weeds by the most common methods: hand removal, mowing and spraying. These methods are suitable for smaller properties (perhaps up to 10 acres) and for large properties consisting primarily of flat, open meadows, fields or pasture. Our property is relatively large (196 acres) with about half being dense forest; it has many slopes ranging from moderate to steep. The size of the property suggests use of mechanized mowing and spraying, but the terrain prohibits use of large machinery. In fact, some places are steep or dense enough to prohibit use of backpack equipment. An additional complication for us is a year-round stream running through the property; many of the most effective herbicides are not approved for use near streams.

After some study, we decided to enlist an army that would not be subject to these prohibitions. In 2004, 2005 and 2006, we purchased and released varieties of insects to attack three specific noxious weeds: Leafy Spurge, Musk Thistle and Canada Thistle. Results so far have been mixed, ranging from “excellent” to “time will tell” and “maybe not.”

The good news is that the most effective insects are likely to spread to our neighbor's property over the years and help reduce weed problems for them as well. In fact, some of the insects are now so widespread that they are no longer sold. Those concerned that the insects might attack other plants should be reassured that the insects are selected and tested by the USDA to be specific to the invasive species they attack before they are allowed into the country. Readers may want to follow links on the insects we describe below to see if they are already present on their own property and consider obtaining some if they are not.

When using insects for weed control, an important part of the process is to regularly visit release sites and identify what species have survived and are attacking the weeds (see pictures below). To facilitate this process, we have marked the GPS coordinates of known sites and taken yearly pictures of some of the worst infestatations.

Leafy Spurge

Leafy Spurge is one of the hardest weeds to control because of its deep and extensive root system. New infestations (less than three years old) can be killed with herbicides or regular hand removal before their root system is well developed. Mowing is likely to speed up the spread unless properly timed.

Leafy Spurge has had a long presence on our property. George Christie told us how his father had tried to control the infestation on this property for many years using herbicides. Thus, we knew when we moved here the weed had been established for decades and herbicides were having little or no effect.

(click on pictures to view larger version)

This gall caused by Spurgia Esulae inhibits seed production and weakens the spurge plant.

Oberea Erythrocephala admiring her work (note broken stem). This insect lays an egg in the stem and the larvae eat the plant from the inside.

Aphthona Nigriscutis adults feed on leafy spurge foliage and flowers; larvae feed on roots. This insect and its sister flea beetles are the most promising insects for spurge control.

We found this caterpillar in 2004. This insect was released in Bridger Canyon decades ago, but it does not thrive well enough to be a factor. It is easily preyed upon by birds and mice.

Click beetle found on spurge plant. It must be lost since this beetle doesn't attack the plant. A good eye and a good reference book are needed to properly identify desirable insects!

Eggs laid by Hyles Euphorbiae (spurge hawkmoth) on a spurge plant. These will turn into caterpillars like the ones pictured at far left.

We released bugs to control Leafy Spurge at several sites in 2004 and 2005. In 2007 and every year since, we identified well established colonies of 3 of the 4 varieties we released. There are additional varieties of insects released by others over the years that have found their way to our area; obviously, those have had very limited success.

We located and marked 7 leafy spurge outbreaks on our property with a GPS system. We photograph two sites periodically to track the progress of spurge spread or control. We do not use any other controls (e.g., herbicides) on the photographed sites so we can judge the effectiveness of the insects year to year. The photo at right (click to see a large version) is a montage of pictures taken June 1st 2006 through 2009 at one of the sites. It seems to indicate that there is progress at this site. Variations in weather from year to year make it difficult to judge; 2008 saw a very late Summer, while 2009 saw very warm weather in May. It may take 10 or more years to determine conclusively that our colonies control the weed.

We stopped taking pictures at the other site in 2009 (after 4 years) because few plants sprouted and the spurge no longer showed up in photos. There has been a marked decrease in the number of plants found at many of the smaller infestations.

We have applied herbicides to several sites that appear to be relatively new and less well established in hopes that we could retard their growth and make them more susceptible to the insects that do manage to gain a foothold over the next few years. While spot spraying does seem to have reduced the density (plants per square meter), spurge can still spread through its deep and extensive root systems, increasing the area affected. Lately, it appears the insects are getting the upper hand at these sites and we have stopped spraying them.

Musk Thistle

We released Trichosirocalus Horridus and Rhinocyllus Conicus for Musk Thistle. The results have been dramatic. We used to see Musk Thistle all along our driveway, but now the plants are uncommon. In Spring, we can observe these insects in their mating frenzy on newly sprouted plants; we now leave these early plants to serve as incubators for a new generation of bugs. Trichosirocalus Horridus attacks the plant stem, weakening the plant and causing it to sprout many weak stems. Late in Summer, we sometimes see what appears to be a patch of Musk Thistle only to find it is a single plant reacting to Trichosirocalus Horridus attacks and in such a weak state that it is unlikely to spread. Rhinocyllus Conicus is especially satisfying, since we can take people on hikes in early Summer, spot a thistle, break open the flower and show how larvae have eaten the seeds. Rhinocyllus is now so widespread and successful that it is no longer available for sale. However, they have a single generation per year and do not eat the seeds that appear in late Summer. We help things along by pulling the heads off thistles that put out late flowers; this is much easier than trying to pull up complete plants, and probably just as effective.

Canada Thistle

Releases of Urophora Cardui and Ceutorhynchus Litura for Canada Thistle have produced less dramatic results; the plant is difficult to control since it propagates through its root system as well as by seeds, so pulling up plants or collecting seed heads is only marginally effective. A well-timed mowing can help subdue this weed. Pulling, mowing and spraying helps weaken the "colony" of plants by reducing the energy they can put into their root systems. A combination of spot spraying and mowing has made an impact on a couple of our sites, but we are losing this battle in areas where mowing is impossible and spraying is difficult. We may resort to goats to clear those problem areas.

Bull Thistle

We have some problems with Bull Thistle, which does not appear to be attacked by the insects. However, the extent of infestation is small enough to control with spot spraying and pulling. Adventurous souls may want to try eating it themselves!

There are native thistle species, so those who wish to allow the natives to continue on their property need to do a detailed study of how to recognize the natives.

Hound's Tongue

A very annoying noxious weed we would like to control is Hound's Tongue, but there are no insects approved for release in the United States. Insects have been approved and released in Canada, and are starting to be found in northern Montana. The reason these insects have not been certified for release in the United States is that they have been observed to attack closely related native species in southern states.

In large, relatively flat and open spaces, mowing and spraying can help control this plant, but it grows well on the steep slopes and among the trees on our property so we cannot use those techniques. We take occasional hikes with a backpack sprayer along game trails to kill Hound's Tongue. This is like the proverbial drop in the ocean, but seeds spread primarily by attaching themselves to animal fur, so game trails are prime areas to pick up and spread seeds. Late in the Summer, we collect the seeds of plants that have flowered and burn them. It is good to pull up the entire plant, but collecting only the seed stalk is still quite effective in preventing a new generation of plants and is much easier to manage when one has to collect plants by hand over a large area. Plants can put out another seed stalk, so timing is important.

Spotted Knapweed

We had a very small patch of spotted knapweed on the property, but eradicated it with a combination of weed pulling and spot spraying over the course of two years. There are insects specific to knapweed we could have used if the problem had been extensive. We are always on the lookout for this weed since there is an extensive population elsewhere in Bridger Canyon.


Cheatgrass is also a problem for us, but we have not yet taken action beyond trying to plant some competitive species in an attempt to keep it from completely taking over certain areas. Fertilizer is likely more effective than herbicide since the fertilizer benefits competitive plants more than the cheat grass and herbicides that kill cheat grass are very likely to kill competitors.


Of course, one should always opt for the ounce of prevention when possible. In our case, we made certain that topsoil was reserved and restored to construction sites. We reseeded disturbed areas as soon as possible. We monitor them carefully and spot spray weeds as they make an appearance. As a result, most disturbed areas are reasonably weed-free and there is little or no need to continue spraying. Similarly, we reseed and monitor areas where logging has been done or slash has been burned.


This posting has only touched the surface of what one should know about weed control with insects. One should follow the links above and do more research on the web to learn more about the life cycles of the weeds and insects in order come up with an effective plan. Knowing the life cycles will help determine when to look for insects and how to time the use of other measures (mowing and herbicides) so as not to interfere with them. The Montana Department of Agriculture has a web page that is an excellent start. The Montana War on Weeds web site is also a good source of information.