Plant Answers

NATURE'S COOL, REFRESHING "MIST" OR "SQUIRT" FROM ABOVE

Calvin Finch and I have been doing a horticulture, radio call-in show for several years on KLUP-AM (AM 930). We receive many telephone calls (210-308-8867 or 1-866-308-8867) on Saturdays and Sundays from 12:00 p.m. to 2:00 p.m. from people who live all around South Texas and the U.S. (People can listen online all over the world by clicking on the Listen Live Now icon at the top of the page at: http://www.klup.com/ ) After six or seven years of doing these programs EVERY weekend, one might think we would get tired of answering the same old questions asked by the same callers. Interesting enough, that is not the case on the KLUP show. A case-in-point occurred several weeks ago when a person from Kerrville telephoned to ask us about "squirting" sprays of moisture from her crape myrtle trees early in the morning. I am well aware of crape myrtle and pecan "misting" but never have had "squirts" reported. I revealed the source of the "misting" from trees in a column I wrote in August, 1986. It reads:

As I sat beneath a majestic pecan tree gently swinging with my lovely bride and enjoying the cool late evening breezes, I was pleased by a gentle mist which steadily fell upon my exposed flesh. After struggling with the turmoil of life's adversities, I considered this cooling mist from above as a much-deserved manna from heaven. Then the sudden jolt back to the world of reality! There were no clouds in the sky and trees don't "manufacture" rain. Alias, the day ended with a disgusting experience - - I had been honey-dewed upon! Some folks may not mind being dewed upon but maybe that is because they do not understand what honeydew is - - read on!

When honeydew from a tree begins to fall, one should quickly realize that there are insects above doing naughty, unmentionable things. "Honeydew" is a classy word meaning "a sweet substance secreted by aphids and other juice-sucking insects." "Secrete" means to be "excreted as a waste" and the last thing that I will tolerate is to be excreted upon by aphids - - I don't care if the secretion does have a fancy sounding name like honeydew!

Aphids build to high populations in mid-summer. Although the actual severity of damage from this pest is widely debated, the normal level of leaf performance can be impaired. The primary mode of damage is from leaf shading by a mold that grows over the upper leaf surface of affected leaves. As the aphid feeds on the undersides of leaves, "honeydew" or sugar water is excreted by the pest. This sticky liquid falls down through the tree and is deposited on the upper leaf surface of leaves over which the aphids fed.

An aphid population on roses can cause distorted blooms which do not open completely

This honeydew is a perfect substrate for sooty mold fungus. Under high humidity, the fungus mold grows on the honeydew to form a dark gray to black covering over the upper leaf surface. This effectively blocks the sunlight from the leaf, reducing photosynthesis.

Sooty Mold (bottom) and aphids on Crapemyrtle

Those of you who do not appreciate being "dewed" upon should stay away from the free "misting" experience as long as it lasts since aphids and other sucking insects are practically impossible to control. It seems that a Neem oil product or a summer oil (Volick) application is as good as anything but, sometimes because of tree size, an application of an insecticide is not practical.

But all of this explanation of the "misters" does not explain the Kerrville "squirters". The answer was provided by an astute listener-the "misters" are aphids and the "squirters" are leaf-hoppers (sometimes called Sharpshooters). You may have seen some of these elusive devils scurrying around to the opposite side of limbs of okra, hibiscus or Turk's Cap as you approach the plant. They are extremely hard to get a good picture of. John Farris has taken the best digital image I have ever seen of a leaf-hopper-one image actually has the drop of honeydew accumulating on its behind, ready to be squirted. This leaf-hopper is Oncomotopia orbono, a mainly eastern species that is also known to vector Pierce's Disease (PD).

Leafhopper preparing to squirt honeydew

I think most folks know and recognize aphids but some of us are not as familiar with leafhoppers (Sharpshooters). To find out more about leafhoppers, you can read the leafhopper webpage at:
http://www.inhs.uiuc.edu/~dietrich/Leafhome.html

A complete key to leaf-hopper identification can be found at:
http://beaumont.tamu.edu/research/agroecosystems/grapes/keytoleafhoppers.htm

The main reason for the special interest in leafhoppers is that these little suckers spread virus and diseases to a multitude of plant materials. In grape producing states such as Texas, leafhoppers are the main vectors (that which spreads a disease) which distribute the bacterium Xylella fasitiosa which causes Pierce's Disease (PD) and the slow decline of most wine grape varieties. The primary vector of PD is the glassy-winged sharpshooter (Homalodisca coagulata) and is also pictured on the web-version of this column. The PD problem is completely explained at: http://piercesdisease.tamu.edu/ and http://www.cnr.berkeley.edu/xylella/

Top View of the Glassy-winged Sharpshooter

Side View of the Glassy-winged Sharp Shooter which is the main vector for PD of grapes.

Whether you are being "misted" upon or "squirted" upon, when honeydew begins to fall, you now know that there are insects above doing naughty, unmentionable things on you. Another word of caution-I think the term "honeydew" is derived from the stickiness of the secreted plant sap on surfaces and NOT from the sweetness of the excrement. The person from Kerrville confirmed that the taste was bitter rather than sweet. This could be the difference between aphid and leafhopper secretion-try some of each and let me know!!

Leafhoppers can produce larger droplets of honeydew than aphids.