Hotline, Summer 1998
Thankfully, we have not seen an overabundance of gypsy moths recently. Not too long ago many of us were tying burlap skirts around our oak trees to trap the hungry critters before they had a chance to munch on the canopy of leaves above. But whom or what should be thanked for the culling of this multitude of intrepid moths? If you say the spraying of the bacteria known as Bt, (Bacillus thuringiensis), you’d be mostly wrong.
But lets start at the beginning. The beginning for most of us was when we noticed our deciduous woodlands were being devoured. In 1981 this amounted to 13 million acres over 12 states.1 Where had this pandemic of gypsy moths come from and why were they overwhelming the forest? Actually the gypsy moth had been around since it was imported from Europe in 1869 intended to be bred with silkworms to increase hardiness. The gypsy moth was imported out of its native environment without the accompanying pathogens, poisons and predators that most flora and fauna have in their native land. The moth was to be used in a laboratory so this was not a worry (except… guess what?) some moths escaped when a cage was accidentally knocked over, and so began the wild gypsy moth in the United States.
As early as 1910, biocontrol was attempted by importing a controlling enemy, a fungus (Entomophaga maimaiga). It was known to infect a similar moth species in Japan. However, the growing gypsy moth population seemed unaffected and E. maimaiga was presumed a failure and lost as a control in the US. Synthetic spraying was attempted with little success, and after the publication of “Silent Spring” in 1962, there was pressure to once again find a biocontrol. Bt was actually already on the scene in the 1950’s but was used mostly in agriculture. Because it had a short half life (36 hours) and was not toxic to mammals, birds, reptiles, fish or worms, it was seen as a very acceptable solution to the gypsy moth problem. However, Bt, contrary to what most of us thought, was not targeted uniquely to the gypsy moth, but also killed some other moths and butterflies.
At this point, even those in the know must have thought it was a worthwhile tradeoff for serious aerial spraying took place in communities all over the northeast. The following year saw some decrease in the gypsy moth population. Was the Bt working? Yes, but there were other factors afoot.
Trees themselves after stress will often produce more toxic leaves as well as the fact that there was a decreased amount of foliage due to the previous year’s assault. For these reasons alone the gypsy moth population typically peaks and crashes, rising again over approximately a four year cycle. The greater the peak, the deeper the fall. Thus in 1985, the peak was somewhat diminished over 1982 and strangely during the 1989 peak, the caterpillars grew to half their usual size and died. If this was Bt in action , why didn’t they die during the earlier spraying?
One proposed answer (and there are others) is as follows: 2 Slowly the public sentiment was going against the comprehensive aerial spraying. People were questioning the wisdom of such an all encompassing strategy and fewer municipalities were spraying. Thus it was discovered that the gypsy moth was succumbing not to the reduced Bt spraying but to E. maimaiga, the fungus believed extinct in the US, as well as a virus that is able to move in when the moths are in weakened condition. The fungus, (E. maimaiga) had possibly been moving through the generations in other similar organisms, and the reduced spraying allowed not only the gypsy moth population to grow but other susceptible insects and their pathogens to thrive as well. The fungus, E. maimaiga, may have adapted over the years to the American environment, being hosted in other similar species, and finally becoming effective against the spreading gypsy moth population.
So what can one conclude about the gypsy moth from this scenario? Perhaps that in nature, most life forms are kept in balance through various mechanisms from disease to poison to predator and the balance is not always easily understood. Killing seemingly unimportant groups of organisms can in fact be detrimental in ways we can’t foresee. The Bt was likely slowing the better control mechanisms of the fungus by diminishing the other butterfly and moth populations where it was possibly carried.
This tale suggests that in our own landscapes, pesticide use is not always the best way to go. If we can provide a rich variety of plants supporting both prey and predator and accept sometimes less than perfect foliage/flowers, we have a better chance of having a truly healthy, balanced landscape, i.e. one that has little need for pesticides. There are many books that give guidance towards plantings that support beneficial insects as well as plant varieties that are themselves resistant to the typical problems that might cause pesticide or fungicide spraying.
1 Noah’s Garden, Sara Stein, Houghton Mifflin 1993, p. 99
2 loc. cit.
by Sue Seppi, GASP Vice-President