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Feature Article

Gypsy moth update:
A veritable smorgasbord of information

Most folks have heard of the gypsy moth and have some vague sense that they are bad news for trees. But whether or not they will be bad for MN forests is far from given. It's greatest impact may be on the developing areas where public pressures to treat defoliating moth populations coincide with an increasing desire to protect the few remnants of native habitat left in these areas. Chemical use on these tracts is likely to be hotly debated. Tourism and import/export industries are also likely to experience noticeable impacts. Businesses involved in importing raw materials such as wood and pulp are already being looked at very closely and are being asked to carry out specific control measures at their expense. For the first time last year, male moth trap catches in some areas were not correlated with people movement and tourism. The pattern of catches has raised new questions for those managing the gypsy moth.

1999 Moth Catches
Over the past few years, moth catches have steadily increased in the south eastern portion of the state (see map) where the majority of our oak forests are found. As the front of gypsy moth infestation advances across Wisconsin, that pattern is what we would expect. The number of moth catches would continue to increase while the pattern of catches would tend to disperse as a number of small isolated infestations become established in MN. Yet in 1999, overall trap catches were well below the levels seen in 1997 and 1998 (see map). The same pattern was seen across most of the gypsy moth range, from Wisconsin to North Carolina.

The most likely answer seems to be reduced gypsy moth survival as a result of the dry open winter and heavy wet spring in 1999. Along the north shore of MN however, moth catches dramatically increased in 1999. More importantly, the catches were scattered almost uniformly up and down the lake shore in areas where tourism is sparse to nonexistent. Tourism and demographics just don't adequately explain the pattern. So what is going on?

The national Slow-The-Spread (STS) Program
A new federal cost-share program called Slow-The-Spread (STS) has been initiated across the length of the advancing front of gypsy moth infestation. MN is just outside that front, so is just now beginning to phase into the program. The program is an outgrowth of research that indicated treating peaks in the gypsy moth population within the sparely populated area along the advancing front slows the rate at which the front expands. Doing so delays establishment of the moth ahead of the front and gives those states added time to prepare. Yet within the front, the states involved are moving from a detection/eradication mode to a suppression mode, where treatments are only implemented on very high-value properties. There is little incentive for them to slow the spread of gypsy moth infestation because the moth is already well established within their respective state. The STS program is thus designed to monitor population peaks and give the states involved the incentive to treat in order to help states not yet infested..

To effectively monitor gypsy moth population dynamics, the STS program has developed a set of survey protocols and mathematical models that utilize a system of permanent plots. The 1/4 mile plots are located on a one km grid. The grid acts like a series of pixels, painting a dot matrix of moth catches across the landscape. Once fully in place, the matrix can be analyzed to define peaks and valleys in moth populations and thus can effectively identify critical areas for gypsy moth management.

Trapping Plans in 2000
Starting in the year 2000, MN will phase in the STS protocols. This will change the way the program is run and how the traps are placed. An interagency working group called the Gypsy Moth Advisory Council will design the trapping procedures and define the respective roles of each participating agency. This year, the MN Department of Agriculture (MDC), the lead agency for gypsy moth exclusion will trap the eastern portion of the state on a system of permanent plots. DNR will trap high use areas (primarily parks) in the western portion of the state. All gypsy moth trap locations will be geographically verified and digitally mapped.

The permanent grid and digitized trap locations will allow effective monitoring over time of changes in gypsy moth population numbers. The objectives are different from simply detecting a possible introduction, which has been the purpose of past trapping procedures. With the advent of the STS program, the philosophy behind trapping is changing from detection to population modeling. This shift implies a shift in our thinking about the status of gypsy moth in MN. The front of gypsy moth infestation is now close enough that infestation of MN is evitable and just over the horizon. As such detecting a new introduction becomes less important and monitoring those populations becomes more important, even though we don't "officially" have a permanently established population within the state.

Forest Management in MN
A parallel shift in our thinking should occur involving the way in which we manage our forest lands in the face of inevitable gypsy moth infestation and defoliation. We are currently in the early stages of an forest tent caterpillar (FTC) outbreak. Because FTC outbreaks have been occurring every 7-10 years for as long as we have been monitoring them, most MN forests have adjusted to their presence. Weak, suppressed or dying individuals cannot withstand repeated defoliation and they drop out of the system, leaving room (usually) for more vigorously growing trees. The rate of tree mortality is fairly low, because this process has been going on for years. But our forests have never seen gypsy moth defoliation and the first major outbreak is likely to take out more trees than our forests and dependent wildlife are accustomed to.

Once gypsy moths are permanently established in MN, population numbers will likely take five or more years before visible defoliation is noted. After that it may take another five or so years before we see outbreak numbers of gypsy moths. If we assume we are five or more years from becoming permanently infested (this will vary greatly across the state), we have about 15 years in which to work with the forests under our care to prepare them for the inevitable. Fortunately, that is just enough time, given the way forests respond to management practices, for the stands to respond with increased vigor, growth and regeneration.

Check with your local forester to see if your stands are at risk of high rates of tree mortality at cross purposes to your management objectives. And plan your work soon. Stands most at risk of defoliation-related mortality include those dominated by oak or aspen on poor soils in areas which experience wide fluctuations in precipitation (which described large portions of the state). Well-mixed stands and those on good sites or buffered soils will be better off. Of course, not all dead trees are "bad" trees. Management plans should take into consideration wildlife and natural communities dependent on dead wood and snags as well as those dependent on young growth and forest openings. Your local DNR forester or private consultant can help you with the prescription best suited to your needs. But don't wait! The moths definitely aren't waiting for us.