How to Get Rid of Corn Borers
What are Corn Borers?
The corn borers that we find in our gardens are European Corn Borers (Ostrinia nubilalis). They are not native to North America. They were brought here, unintentionally, prior to 1917 in broom corn (Sorghum bicolor), which was imported to make brooms.
Corn borer larvae overwinter in the plant debris in your garden. They hatch into pale brown moths with zigzag markings in June. The female moths lay eggs from the end of June to the middle of July in clusters of 15 to 20 on the undersides of leaves of their favorite vegetables. The eggs hatch within a week, and the larvae feed for an additional three to four weeks. The young larvae feed on leaves, tassels, and under the husks while older larvae burrow into the stalks of your corn or the stems of your other crops.
Once burrowed into the stalks and stems, the larvae pupate and hatch into a second generation of moths which begin the cycle again. In warm climates, thanks to the longer growing season, it is possible to have up to three generations of moths in one growing season.
Kill Them by Squeezing the Corn Stalks
If you find a corn borer larva burrowing into the stalk of your corn or the stem of one of its other favorite crops, you can kill it by squeezing the stalk. This is a great solution if you are like me and don't like touching the larvae. Not only do you not have to actually touch it, there is no mess to clean up afterwards. The squashed larva remains inside of the stalk.
Attract Beneficial Insects
There are a number of beneficial insects that eat corn borer eggs and larvae including lacewing larvae and trichogramma wasps. Invite them into your garden by planting their favorite herbs and flowers. They prefer plants that have tiny flowers. Herbs such as yarrow, dill, fennel, and mint attract many beneficial insects.
A word of warning: if you choose to grow mint, be sure to grow it in a container. It is incredibly invasive. If you plant mint directly into your garden, it will take over, crowding out more desirable plants. Fortunately container grown flowers attract beneficial insects just as well as flowers grown in the garden.
Use Lady Bugs
Lady bugs love corn borer eggs and can eat up to 60 eggs per day. If you don't already have lady bugs in your garden, you can buy them through mail order and release them in your garden where they will gorge themselves on corn borer eggs and the aphids on your roses. Just bear in mind that when they have eaten all of the available food, they will fly away in search of more. They don't stick around.
Bt, or Bacillus thurengiensis, is a naturally occurring bacteria that is found in most organic pesticides. It is safe for use on vegetables and can be applied right up to the day of harvest. It is safe for humans and animals.
Bt is usually applied in the form of a spray. Limit your spray to the plants that are infested with the larvae. The larvae have to eat it for the Bt to get inside them and kill them, so be sure to spray both the tops and bottoms of the plant leaves. Bt will kill butterfly larvae and the larvae of other beneficial insects, so you don't want to haphazardly spray it around your garden.
Be patient. Bt can take a few days to work. It doesn't kill on contact. Also be aware that it will only last about a week on the plants, because sunlight degrades it. You may have to re-apply it if more larvae appear.
Another good organic insecticide that you can use is pyrethrum. It is an extract from chrysanthemums. It works by paralyzing the insects' nervous systems, thereby killing them. It works almost instantly. Pyrethrum is applied as a spray. If you get any of the spray on your skin, there is no toxicity for humans, because it is difficult to absorb through the skin.
Pyrethrum lasts a much shorter time than Bt, only about 12 hours. So it is considered safe for use on vegetables. Like Bt, you need to be careful where you apply pyrethrum, because it is toxic to all insects including bees. So only apply it to the areas where the borers are located and not all over your garden.
Do a Thorough Garden Clean-up in the Fall
Prevention is always preferable when it comes to controlling garden pests. Clear away all plants and weeds in the fall and then till your garden. Removing garden waste robs the borers of places to overwinter and the tilling kills any that are left.
© 2014 Caren White