THE SPRING ritual of phone calls and visits to the AgriLife Extension Office in addition to urgent e-mails submitted by county residents this time of year is proof that insect pests and disease problems on landscape plants can be expected to return with warmer weather conditions.
The following is a sampling of questions asked by local residents:
Q: My oak trees have been invaded by an army of caterpillars that are very hairy and quite colorful. Any ideas on the name of this caterpillar? Is this a stinging caterpillar? Should I treat my oak trees?
A: This year is shaping up to be an active year for caterpillars across Texas. Populations of tent caterpillars have been enormous in some areas of Texas. Tent caterpillars have caused dozens of blackouts in the Dayton, Texas, area when caterpillars moved from tree limbs onto high voltage transformers and caused electrical shorts. Galveston County residents are fortunate in that tent caterpillars rarely occur in this area.
The caterpillar that has alarmed local residents is commonly known as the Live Oak Tussock Caterpillar (scientific name is Orgyia detrita). The Live Oak Tussock Caterpillar does not form tents. While this caterpillar is known to occur in low numbers in most springs, residents in the League City and Friendswood area are reporting major outbreaks of this insect pest.
The good news is that a healthy tree or shrub can generally tolerate a total defoliation without suffering permanent damage. By the time homeowners notice the leaf damage on their oak trees, most caterpillars have stopped feeding and are entering the non-feeding pupal stage.
A mature caterpillar is about 1.25 to 1.5 inches long, with two long tufts of black hairs projecting forward from the head and a similar tuft of black hairs projecting backward from the rear of the body. Four dense patches of hair occur midway on the topside of the caterpillar’s body.
While Live Oak Tussock Caterpillars are not regarded as being stinging caterpillars, it is advised that people do not handle them. Some people aren’t bothered by the caterpillar but others could have a reaction that ranges from a mild to fairly severe rash. Parents are advised to keep an eye on their toddlers when outdoors to ensure that they do not make contact with the caterpillars.
I suspect that the abundance of Live Oak Tussock Caterpillars is correlated with the mild temperatures over last winter.
Q: The leaves on my oak trees have developed numerous bulges or bubble-like structures that eventually turn black. Some trees have lost most of their leaves. What causes this?
A: Our office has received numerous visits, e-mails and phone calls reporting similar symptoms on leaves of oak trees across the county. Your oak trees have a disease known as Oak Leaf Blister which is caused by a fungal pathogen.
Oak Leaf Blister causes small, rough (concave-convex) spots as leaves expand in spring. The spots turn pale green and become somewhat thick, then turn brown or greenish brown and eventually becoming black. Leaves with numerous spots will fall prematurely.
Control of this disease requires an application of a fungicide during early spring before new growth starts. It should be noted that once diseased foliage is present, fungicides will not “fix” the damage that has been done. Diseased leaves will simply fall naturally. Thankfully, it looks worse than it actually is. Since the tree’s health is not in danger, don’t be alarmed by this pesky fungus.
Once the blisters appear, the application of fungicide will be a wasted effort. Fungicide applications (with products containing chorothalonil or mancozeb as active ingredients) used to prevent this disease would have to be applied in late winter before the tree buds begin to grow. If Oak Leaf Blister is serious enough to cause leaves to fall, rake them up and dispose in curbside trash pick up. Otherwise, go fishing!
I am not surprised by the widespread occurrence of Oak Leaf Blister this spring as this disease is favored by high rainfall during late winter and early spring when leaves are emerging. In contrast, not a single case of Oak Leaf Blister was reported during the spring of 2011, which was a record drought year.
Photo by William Johnson
This year is shaping up to be an active year for caterpillars across Texas. While the Live Oak Tussock Caterpillar is known to occur in low numbers in most springs, residents in the League City and Friendswood area are reporting major outbreaks of this insect pest.
William Johnson is a horticulturist with the Galveston County office of Texas A&M AgriLife Extension Service. Visit his website at aggie-horticulture.tamu.edu/galveston/index.htm