Use of BT on Hibiscus

Asked August 11, 2015, 3:51 PM EDT

I used BT on my hardy hibiscus this spring. The results were great! No green worms/caterpillars, great growth, lots of red blooms. Do I need to use BT each spring to continue with these results or was once enough. Without this treatment, the leaves were eaten and the growth/blooms were reduced in prior years. I used a soapy solution to some success but wanted to avoid nasty chemicals. Thanks for the help...

Montgomery County Maryland

5 Responses

It was a great year for many plants this season, as there was plenty of rain and temperatures were lower than usual. We think this is why you had good results with your hardy hibiscus.

Bt is not a preventative, or is systemic. It is indicated for caterpillars, but the worms that often eat hibiscus are actually not caterpillars, but sawfly larvae. Bt does not work on them. Next year if they show up, hand pick them and put them in a bucket of soapy water.

Bt should only be needed or used if you actually have caterpillars on something, because they need to be present and eat it for it to work.

In future seasons, offer supplemental water to your hibiscus when the weather gets dry, and monitor for sawflies regularly and you should have another good year.

cm

Thanks for the info. The only difference for this plant this year was the use of BT. I read that it is not effective, but still. I have a watering system that I use in the garden and this plant gets plenty of that moisture. So, can I use one of the "natural" treatments next spring like Spinosad? I can't always manually pick off the pests. It appears that these creatures only attacked this particular plant. Thanks again...

No, we do not recommend using pesticides unless you actually have correctly identified a pest, understand it's life cycle and have the correct chemical.

Spinosad does not work as a preventative. The pest has to be there. It will work on sawfly larvae, but you should know that it is toxic to other non-target organisms like earthworms and bees (while it is wet).
Here is a page from the National Pesticide Inofrmation Center about spinosad which will inform you further: http://npic.orst.edu/factsheets/spinosadgen.html

Here is our educational page about Hibiscus sawfly and how to control them:
http://extension.umd.edu/hgic/sawflies-flowers

cm


OK. So, my use of soapy water in a sprayer when the infestation was beyond a "handful" was as good a solution (pun intended!!!) as any in the past. Will hope next year is as good as this. Thanks...

You are welcome.
We'd keep monitoring and then if there are too many to hand pick (they tend to feed in gregarious groups together and you could consider clipping a whole branch) you could try either horticultural oil or insecticidal soap while young. Be careful with homemade soapy solutions, especially on hot days as they can sometimes burn foliage.

cm