Feature Article: Gypsy Moth Slow-the-Spread in Minnesota

Susan Burks, DNR Forest Health Specialist

This last year, the Slow-the-Spread (STS) program action zone boundaries were expanded westward to include Winona and Houston counties in southeast Minnesota. This signals a number of changes for the Minnesota program. One is that Minnesota is now a full voting member in the STS foundation so that the MN Dept. of Agriculture (MDA) will have an increased level of input into regional STS management decisions. Another involves changes in population monitoring protocols to allow MN data to be integrated into the national program. Changes in management strategies, while coming, won't occur until much later. More than anything else, the inclusion in the STS program indicates just how much closer the moth is to moving into Minnesota.

STS - what does it mean?

So what is the STS program anyway? It is a state and federal cooperatively funded national program that provides management guidance during the transition stage of gypsy moth population establishment. The transition stage is the stage in which an area goes from being uninfested with no permanent infestations to being generally infested with well-established populations through out the area. The STS program was designed to address survey and treatment issues within this stage of establishment. Minnesota is currently in an uninfested area. Michigan is considered generally infested, while Wisconsin has all three stages of infestation within its borders (see Figure 1.)

Each stage of population establishment is associated with different characteristics as well as differing monitoring and treatment strategies. During the uninfested stage, an area may see frequent gypsy introductions via the movement of goods, vehicles and household belongings. But most introductions fail to become established. Detection and monitoring of these introductions is done through the use of pheromone traps, cardboard structures baited with the female moth sex attractant. Occasionally, these introductions begin to reproduce, creating small isolated populations. Within the uninfested area, the management strategy is to eradicate these populations, which is fairly easy to do because of their small size.

Figure 1. Map of STS action boundaries across the Lake States. Note the one-moth line across Houston and Winona counties, indicating the edge of the transition area.

Map of STS action boundaries across the Lake States

As the front of gypsy moth spread gets closer (picture a nebulous curtain moving across the landscape), small, established populations become more and more common, and more and more difficult to eradicate. As their number increases, eradication becomes no longer feasible and management strategies shift, signaling the transition stage. Rather than attempting eradication, it becomes important to manage hot spots of infestation to slow the rate at which they build and coalesce. At the western edge of the transition area, management strategies may still resemble eradication. But at the eastern edge, infestations have often coalesced into large areas of contiguous infestation. When the number of moth catches and the presence of alternative life stages, such as eggmasses, suggest widespread permanent infestation, United States Department of Agriculture (USDA)-Animal and Plant Health Inspection Service (APHIS) declares the area generally infested. At that point, survey methods shift from male moth detection to counting the number of eggmasses present and the management goal becomes protection, through chemical treatment, of high value forestland, commonly urban and recreational areas.

STS - how does it work?

One of the greatest difficulties within the STS action zone is monitoring low-density gypsy moth populations. To address the issue, a mathematical model was developed and tested by the USDA Forest Service during the mid 1990's. The model is based on a regular grid of detection traps that monitor population densities over time. The model uses annual trap catches to create a 3-dimensional map of catches with hot spots represented by peaks in the virtual landscape. The peaks are defined not only by the number of moths caught in a particular set of traps, but the relative distance between these "positive" trap catches and the distance from the advancing front of infestation (see Figure 2). The model identifies which of these hot spots or peaks of infestation are likely to persist, grow and eventually coalesce and then recommends action.

Diagram of the three stages of gypsy moth establishment

Figure 2. Diagram of the three stages of gypsy moth establishment. Note, gypsy moth spread in the United States is east to west, and not west to east as suggested in the diagram.


Which hot spots are treated is determined by a number of factors, chief among them being the available funds (in addition to site specific environmental and social factors). Funding is determined in part by the STS Foundation, a non-profit corporation who's Board includes the State Plant Regulatory Officials from each participating state. Now that Minnesota is within the STS action zone, we become a voting member of the Foundation. That means we help decide how much federal funding each participating state will receive. While each state matches the grants they receive from the Foundation, the amount varies and depends on local need. The concept is a "share-the-wealth-for-the-greater-good" approach that leaves state funds in the state and distributes federal funds where it is most needed. To date, the Foundation has functioned very well. However, overall programs goals are not being met in all states involved in the STS program. In Wisconsin in particular, the mathematical model on which STS decisions are based does not seem to be adequately describing established populations, so treatments are not slowing the rate of gypsy moth spread as much as intended.

STS in Wisconsin - what's different?

Like Minnesota, the uninfested stage of gypsy moth management began in Wisconsin in 1970 with the capture of their first moth. They implemented a detection survey and over the next 20 years, treated a total of 4,693 acres. In 1990, population numbers began to climb and so did the number of acres treated. By 1992, Wisconsin had treated 47,130 acres. Prior to the establishment of the STS program in 1999, Wisconsin had treated a total of 355,000 acres. Last year, Wisconsin treated 319,700 acres within the STS program and another 28,000 acres within their new gypsy moth pest suppression program, for a total of 347,700 acres treated in one year. While treatment costs vary widely across the landscape and vary year by year, the average cost last year was $27.00 per acre.

The calculated rate of spread in Wisconsin over the last three years has been comparable to pre-STS program rates, unlike that seen in the other STS states where the program has dramatically reduced the rate of spread. So what could explain the high rates of spread in Wisconsin? Scientists don't really know. But there are a number of theories beginning to take shape, some of which have prompted new research.

Because the STS model was designed to identify peaks of infestation across a landscape of relatively few trap catches, the higher the "background noise", i.e. the baseline number of trap catches across the entire landscape, the more difficult it becomes for the model to identify population peaks. If the peaks are not identified and thus not treated, they increase in size and become more difficult to manage. There have been several instances where the virtual landscape of trap catches in parts of Wisconsin was relatively flat one year and then mushroomed the following year to an unmanageable size, creating a "bulge" in the front of gypsy moth spread. What factors are allowing these bulges to build are yet unknown.

One factor may be the distribution of habitat types. Gypsy moths have been established in the upper peninsula of Michigan since the late 1980s. Although they successfully reproduce there, gypsy moth population numbers rarely reach defoliating levels, likely because of winter mortality. However, Sharov et al, 1999, found that the rate of spread in the upper peninsula of Michigan was faster than that found in the southern peninsula, even though winter temperatures were lower. Winter temperatures were not correlated to the rate of spread, while susceptible forest types were. In Wisconsin, where habitat ideal for gypsy moth survival exists in more or less contiguous forests, these findings suggest that the rate of spread could be much faster in Wisconsin than elsewhere.

One factor may be the movement of male moths in and out of the area. Some managers have theorized that "moth blow" or moth dispersal from adjacent areas is confounding the trap data by raising the level of background catches, thus masking established populations. If true, the background noise could mean the model fails to detect some infestations, allowing them to spread unchecked.

Another theory relates to the level of predation. Tcheslavskaia et al, 2002, found that predation rates were much lower in Wisconsin than in Virginia where similar work had been done. As a result, adult females seemed to survive longer and were successfully mated more often in Wisconsin than in Virginia. They also found that male moths dispersed over a greater distance as measured by recapture of male moths released at regular intervals.

Another Wisconsin study, not yet published, found that predation rates are strongly influenced by forest type and negatively correlated to gypsy moth population growth. Small mammals contributed to more gypsy moth mortality than invertebrates and where their populations were low, gypsy moth populations were building more rapidly.

Temperature may be affecting moth behavior and/or pheromone dispersal. Past studies conducted by the USDA -APHIS indicate that male moths fly less frequently when summer temperatures drop below 70?. Although studies in southern Wisconsin produced results similar to Virginia, a recent study in northern Wisconsin found that on a few of the plots, a sizable proportion of the females were successfully mated even though there were no male moths caught on those study plots. That suggests that in colder climates detecting reproducing populations may be more difficult and that male moth captures may be more rare. If that is true, a positive trap capture in a cold climate might indicate a much larger population than it would in a warm climate. As a result, trap catches in colder climates may be underestimating low-density populations numbers.

A pilot study in northern Minnesota looking at the effect of temperature on moth dispersal and recapture rates, found that male moths appeared to live longer and dispersed further than typically seen in warmer climates. If true, flight distances might contribute to higher mating success due to the better chance of finding a mate. That plus the difficulty of finding populations in cold climates might contribute to an increased rate of population build up and spread across the landscape.

STS - where is it going?

So how is the program addressing these issues? The STS program is guided by a technical steering committee made up of scientist from across the country. They regularly review the trap data against actions the model has recommended and discuss the implications. They have already modified the model to some extent based on the situation in Wisconsin. The STS program statistician is beginning to analyze Wisconsin weather patterns and forest structures within the area of those bulges mentioned earlier in the hopes of describing those factors that seem to have the greatest influence on gypsy moth population dynamics. Others are looking more deeply at the affect of temperature on survey results. The discussions are ongoing, so other areas of needed research will be identified and implemented as resources allow. In the meantime, the statistician has revised the predicted rate of spread into Minnesota (see Figures 3 and 4). Figure 3. is based on the average rate of spread across Wisconsin over the last ten years. Figure 4. is based on the fastest rate of spread seen over the last few years. Based on these predictions, the moth will become a permanent resident of a small portion of SE Minnesota sometime between the years 2007-2009.

map showing spread across Wisconsin map showing spread into Minnesota

Figures 3. and 4. Predicted spread rates into Minnesota based on the 10-moth line, i.e. where the average number of moths caught in the area per trap is ten moths.

STS - how do we prepare?

The efforts of the MDA to keep the gypsy moth from becoming permanently established for as long as possible can delay the inevitable and thus give us more time to prepare. That delay can save the state millions of dollars in lost business and treatment costs. So it's important that state residents do what they can to support that effort. Cooperate with MDA trappers when asked. Inspect your vehicles and belongings when traveling in and out of infested areas. And assess forestland under your care for the risk of future damage when the moth does become established. Forest management practices take time and forest stands need even longer to respond once management practices are implemented. If you are a landowner, talk to your local forester about management activities appropriate for your site and to obtain the DNR Silvicultural Tipsheet #11 Minimizing Gypsy Moth Damage. It can also be found on this site: Minnesota DNR - Minimizing Gypsy Moth Damage