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Tuesday - October 06, 2009

From: Austin, TX
Region: Southwest
Topic: Diseases and Disorders
Title: What is the sticky stuff coming out of the trees in Austin?
Answered by: Barbara Medford


Hello - I live in Austin TX, and in the past half month or so, have been noticing much sticky, sweet sap being exuded from trees. In a normal year I only notice the crepe myrtles doing this, but this year we've seen copious sap dripping from our pecans, and also elms and mulberries around town. Car windshield are getting quite sticky! Is it an aphid or other insect? A friend thinks it may be the tree itself but I suspect that a tree would be unlikely to toss out such a resource, and especially across such a breadth of different species. Thanks for any help.


There has been a lot of conversation about this problem this year, and it is most likely the result of aphids. There are a few other insects that can cause similar problems, but you are right, those trees are not likely to be squirting liquid. We don't know if the very extreme heat and drought has caused this, with the opportunistic insects attacking weakened trees, or if the insects are having an extremely good year. Ordinarily, when there is disease actually present in the trees themselves, you will see exudations from the bark, or signs of rot around the base, or leaves and branches dying.

There are several insects that can create the honeydew, that is, excretions from the insects that can cause this sticky rain. The honeydew, in turn, can be infected with a sooty mold, which is not particularly dangerous to the plant, but is unsightly and, if it gets too plentiful, can actually shade the leaves from needed sunlight. In this area, the most likely candidates for mess-making bugs are aphids, whiteflies, mealybugs and soft scale. We found a website that pretty well covers it all in terms of what can be done about these problems from the University of California Integrated Pest Management on Sooty Mold. Generally, the solutions are pretty low-tech: (1) ignore it, the leaves will be off the trees soon anyway, (2) give it a good spray of water and (3) try a good spray of soapy water. 

We'd like to say there was a permanent cure, but there isn't. They will all be back next year. Just hope we start getting enough rain to wash the little monsters off the trees without our having to use our precious water rations.


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