CORVALLIS - Mosquito season is here. There is a relatively new "least toxic" mosquito control method now easily available in many garden centers, called mosquito disks or dunks, explained Amy Dreves, Master Gardener program assistant and entomologist at Oregon State University.
Mosquito disks are little doughnut-shaped, time-release rings that can be floated in a pond or water feature. They slowly release B.t.i. (Bacillus thuringiensis israelensis), a soil bacteria tested and found lethal only to mosquito larvae, black flies and fungus gnats. B.t.i. is active over a 30-day period. All other organisms should be unaffected.
To discourage mosquitoes, the first course of action should be to eliminate standing water, said Dreves. But in a backyard pond, birdbath, ditch, tree hole, unused swimming pools, flower pots, rain barrels or garden water feature situation, B.t.i. mosquito disks are a less toxic alternative to killing mosquitoes than most other insecticides.
Like other strains of B.t., B.t.i. acts as a stomach poison, damaging cells in the mid-gut of mosquito larvae that have eaten the spores. B.t.i. crystals dissolve in the intestine of susceptible insect larvae. They paralyze the cells in the gut, interfering with normal digestion and triggering the insect to stop feeding. B.t.i. spores can then invade other insect tissue, multiplying in the insect's blood, until the insect dies.
Studies show that B.t.i. acts quickly - in experiments, a moderate to high concentration killed half the mosquito larvae within 15 minutes and finished off the remainder after about an hour.
B.t. pesticides are unlike many of the more broad-spectrum pesticides, in that they only kill certain groups of insects. Michigan State University researchers tested B.t.i. over a three-year period in the field and laboratory for possible impacts on "non-target organisms," other aquatic organisms besides mosquitoes, black flies and fungus gnats. They observed no negative impacts on other aquatic insects including stoneflies, mayflies, dragonflies, caddis flies or other aquatic flies such as crane flies.
As with all pesticides, it is imperative that users follow label instructions carefully.
There are different strains of B.t., each with specific toxicity to particular types of insects. Besides B.t.i, there are B.t. aizawai (B.t.a), used against wax moth larvae in honeycombs and B.t. kurstaki (B.t.k.) controls various types of moths and butterflies, including the gypsy moth and cabbage looper. A newer strain, B.t. san diego, is effective against certain beetle species and the boll weevil. To be effective, B.t. must be eaten by insects during their feeding stage of development, when they are larvae. B.t. is ineffective against adult insects.
For more complete information, visit OSU's Extoxnet, Extension Toxicology Network's web site section on Bacillus thuringensis.
Extoxnet is a pesticide information project of Cooperative Extension Offices of Cornell University, Oregon State University, the University of Idaho and the University of California at Davis Institute for Environmental Toxicology and Michigan State University.