Preventing Slug Damage In Corn And Soybean

PRINCETON, KY.
   Is there something predictive about writing an article on slugs (Figure 1) while it is cold, rainy, and the ground is soggy? Let’s hope not.  At any rate, I have received several questions mainly concerning whether or not this past difficult winter will reduce slug problems. Though I am no expert on slugs, I do know people that are! Their answer to this question is generally a slightly hedged “no.”

Figure 1. Grey garden slug

   Apparently slugs are very good at overwintering. They have the ability to move very deep in the soil and, thus, to avoid the extremes in temperature that we have seen. In addition, during some of our very cold periods, we had significant snow coverage on the ground. Snow acts as an insulator and though the ground may (probably will) freeze, soil temperatures may not reach the very low levels that air temperatures reach.
   While most people with experience in slug biology will agree that there will probably be a greater level of over wintering mortality this year, that increase in mortality may not have a direct effect on the slug damage you may see in the field. Here is an example why.
   Note: The following numbers are made up for illustration and don’t bear any resemblance to the real levels.
   Just suppose it requires 10 slugs per square foot to cause economic damage. Also, that in most years 100 slugs per square foot go into winter and 20 percent of them survive the winter. That still leaves 20 slugs per square foot to cause damage the following spring. So, if in this past winter, the mortality of overwintering slugs goes up from 80 percent mortality to 90 percent mortality this still leaves 10 slugs per square foot to cause economic damage. Yes, winter has resulted in fewer slugs to damage the crop, but not enough fewer to prevent problems.
   In addition, winter weather, though important, is probably not as important as the weather that occurs when slugs are infesting your crops. Crops that are planted in cold, wet ground struggle to germinate and grow. So, if you will, the plants are less vigorous. However, slugs, if not enjoying cold, wet, overcast days, are at least better adapted to that weather than are our crop plants. So in these cases the same number of slugs per plant unit will cause more damage during cold, wet, overcast weather than they will during warm, sunny days, which favor the crop plants.
   I know of no pesticide control that is really useful in controlling slugs in corn and soybean. Yes, there are some available, but the chance of obtaining an economically positive response with them is diminishingly small. Your best bet is to plant into moist but well drained soil when the weather is suited for a quick emergence and sustained growth of the plants. This is especially true if you are planting into an area with a history of slug problems. Not easy to time I know, but that is about the best strategy to avoid slug damage.∆
   DR. DOUG JOHNSON: Extension Professor of Entomology and IPM Coordinator, University of Kentucky