The Q&A Archives: Green Worms on Purple Dome Asters

Question: I have grown this type of aster for 3 years now, and each year, just before this plant blooms in the fall, I noticed that I get these green wormy type bugs. I have been using "trounce", on the plants each year. I also have noticed that the same plants, in someone else's garden have no green wormy type bugs.

Answer: It's really important to identify exactly what is on your plants, and whether they are doing any real damage, before attempting to spray something on them. You may do more harm than good. There are many green worm-like caterpillars, so I'm not sure what you have from your description.

Many beneficial insects can move in to kill off the "bad bugs," but spraying will kill beneficial insects as well. Also, it's best for the long term balance in your garden to start with the easiest, most benign method first and if that doesn't work, move on from there. Caterpillars are easy to control by handpicking. If they get out of control, try Bacillus thurengiensis or Bt. It is a biological control--a bacteria that kills only caterpillars and is safe to other insects, humans and animals. However, it will kill all caterpillers, including desireable ones such as butterfly larvae.

If you can regularly monitor and tolerate some damage to your plants, over time Mother Nature strikes a balance, with the beneficials coming in to control the bad guys. (Perhaps that's what's happening at the other person's garden.) Healthy vigorous plants will withstand insect attacks best, and it's really the best thing you can do to provent insect problems.

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