The last two years have turned out to be some of the toughest of the last half-century on our central Texas trees. Last year’s drought outright killed an unusually high number. Ironically, many that have survived, especially pecans, are now breaking apart due to drought-weakened wood supporting (or more accurately, not supporting) excessively heavy branch tips and nut crops due to recent rains. For every tree service I have spoken to, 2012 has been the year of dead tree and broken branch removal.
But now what’s really making the phone ring are calls about “tree sap” making every thing sticky. These calls are mostly about pecan trees, but people with crepe myrtles and oaks are calling too. But it is not tree sap that is making a sticky mess of everything. It is a variety of insects.
Tree insect pests are very roughly divided into three categories; those that suck, those that chew, and those that bore. The class of insects causing so many problems now (summer) are the suckers, particularly aphids. They generally feed on the underside of the leaf and can be seen with the unaided eye (although accurate ID usually calls for magnification). And the sticky stuff? Well, to put it bluntly, it’s poop.
You see, aphids are kind of like many humans. They love sugar and don’t know when to stop eating. An aphid just inserts it’s sharp little drinking straw of a mouth-part into a nice, juicy leaf and keeps sucking long after the sugary tree sap is flowing out its other end. So in a way, it is tree sap. It’s just “processed”.
The one causing the most problems for pecans now is the black pecan aphid, which is common every year in Austin, but is in record numbers this year, probably in part due to the very mild winter and all the lush foliage caused by heavy spring and summer rains.
Many deciduous oaks I have looked at are having aphid problems also, including wooly aphid on post oaks, but others are having problems with another type of sucker – the lacebug. Not to be confused with the lacewing, a beneficial insect, the lace bug is a common pest of oaks and sycamores.
Other insects in the sucking family that often cause problems in Austin are spider mites, plant bugs and psyllids. Many of these sucking insects have characteristic damage patterns. The lace bugs and spider mites, for example, cause what is called stippling; a mottled fine spotting of the top surface of the leaf. This is caused by the removal of the leaf’s chlorophyll. The pecan aphid causes dark brown dead patches on pecan leaves. Other aphids and pysillids cause a curling of the leaf margin or other distortion of the leaf.
Of course, what everyone wants to know it what to do about these little suckers. The vast majority of the time I recommend doing nothing, especially if the infestation comes late in the growing season as it is this year. These deciduous trees will be losing their leaves in a few more months, and have already done a large part of their work (photosynthesis or sugar production for the plant – but that’s another blog). In fact some experts say that photosynthesis is greatly reduced in most American deciduous trees when temperatures are routinely hitting 100 degrees F.
Just like years in which we have heavy caterpillar damage (when there are no leaves left at all), a healthy tree will recuperate from this. At least with sucking insect infestations, some leaf surface is retained. Sucking insect damage does often cause premature leaf drop, but again, healthy trees will recover from this.
Another problem with aphid “honeydew”, as it is called, is that because it is so high in sugar it is a perfect substrate for mold. This frequently causes a secondary mold infestation which can turn leaves black, especially on crepe myrtles, and this can reduce photosynthesis further.
If your tree was not healthy before the infestation, or you can’t live with the mess, you may wish to treat the insects. There are a variety of things that can work. It is important to use the least toxic substance possible, not just for human safety purposes, but because you don’t want to kill the beneficial insects as well. If you wipe out the bugs that eat the pests, like ladybugs and lacewings, it can lead to a cycle of dependency on pesticides. Another irony of some high toxicity pesticides is that those used to control some sucking insects have been shown to cause an increase in populations of others, especially spider mites, which are a totally different class of bug (arachnid). Low toxicity treatments often include insecticidal soaps.
Finally, I recommend bagging and disposing of fallen leaves from infected trees. It won’t cure the problem, but it can reduce reinfestation, and can suppress certain fungal leaf diseases, which I am also seeing a lot of this year. But if your trees are healthy, fallen leaves are a valuable part of the nutrient cycle and should be mulched and left under the tree.
If your tree has not been seen by an arborist in the last year or two you should see my contact page to arrange a consultation.