Ask Mr. Smarty Plants

The Texas Mountain Laurel (Sophora secundiflora) frequently is attacked by the genista caterpillar during the summer. Stephen Gowin, Texas County Extension Agent – Agriculture for Rains County has an informative factsheet online about webworms that you might read.

He says, “Unless infestations cover a tree, they are usually not that damaging to tree health. An otherwise healthy tree will withstand up to 40% defoliation during the summer months, and will often re-leaf after being stripped by caterpillars. So if you don't treat in time, don't worry. Chances are your tree will survive, although you may not be happy with its "webby" appearance.”

In the future, he suggests, “Low impact pesticides tree-feeding caterpillars include insecticide soap, horticultural oil, Bacillus thuringiensis, or spinosad insecticide sprays. Pyrethroid insecticides will also provide fast control of most caterpillars.”

And if you want more information on the Genista caterpillar that is particularly happy to feed on Texas mountain laurel, the Texas A&M AgriLife Extension website has information by Bastiaan Drees and Carlos Bogran, Extension Entomologists. Here is some of what they write: The genista caterpillar, Uresiphita reversalis, is the immature stage of a moth (Lepidoprera: Pyralidae) that occurs on Texas mountain laurel, Sophora secundiflora, a small native tree sometimes used in the landscape.

Genista caterpillars occur commonly in central Texas landscapes and throughout the Gulf coast. They form loose webbing on the foliage and feed on leaves causing unsightly damage.

Plant health is generally unaffected by feeding unless large numbers of caterpillars cause heavy defoliation (leaf loss). In these cases, plant growth may be slowed and aesthetic damage may be significant. Control is generally unwarranted, but in some situations, such as commercial plant production in nurseries or heavy infestations in high-value landscapes, pest control or suppression may be desired.

In the case of the Genista caterpillar, winter is spent in the pupal stage and adult male and female moths emerge in early spring. They are active at night when they mate and females lay overlapping clusters of cream colored eggs.

Larvae are active during the day, but may feed at night under moderate temperatures and develop through 5 instars before they pupate. Young larvae feed in groups but become solitary as they grow. They produce loose silk webbing around the leaves where they feed. Several overlapping generations can occur during the warmer months of the year, as evidenced by the presence of many caterpillars of different sizes and at different stages of growth on a single host plant. Pupation occurs inside silk webbing or loosely spun cocoons among the older leaves or in secluded sites around the plants.

Leaves of Texas mountain laurel contain many secondary plant chemicals that protect plants against generalist herbivores. Genista caterpillars are immune to these chemicals and even benefit from them. Larvae can store and concentrate secondary chemicals in their bodies which makes them toxic to the most common natural enemies of other caterpillars. However, a few species of predaceous arthropods, such as spiders and assassin bugs, may feed on young caterpillars. These and parasitic insects (parasitoids) may help keep their populations in check.

Plants, especially small Texas mountain laurel trees, should be checked frequently during the spring to detect egg masses. These are not easily found but are visible. Look for small batches of eggs in overlapping clusters. Monitoring efforts should be focused on the newest growth because young leaves are preferred by larvae. Light infestations on a few plants or small plantings may be controlled by hand removal, using high-pressure water sprays or by pruning infested terminals. Control or suppression of heavy infestations on a large number of plants may require insecticide applications. For best results make foliar spray applications (liquid sprays to leaves) when small larvae first appear.

Insecticide products containing Bacillus thuringiensis var. kurstaki (Bt) are best used when caterpillars are small (½ inch or less) and actively feeding on leaves. The active ingredient in Bt products is a caterpillar-specific toxin (deltatoxin) that acts as a stomach poison. Therefore it must be ingested by the larvae to be effective. As with any contact pesticide thorough spray coverage is essential, especially targeting both the upper and underside of new leaves on which caterpillars feed. Repeated applications may be necessary during periods of new growth flush because Bt products have short residual activity. As with any insecticide, read and closely follow all instructions provided on the product label.

And finally, the Central Texas Gardener website has some good cultural information about growing Texas Mountain Laurels that may help with your question about fungus-like signs. Full sun is best but can take part shade. Adapts to most soil types but wants good drainage. Do not over water once it is established. It has a very long tap root, which makes it drought-tough (but also harder to move!).