|Forest tent caterpillars can be recognized by their dark-|
gray to brownish body color, with pale-blue and yellow lines
extending along each side, and a chain of distinct,
whitish shoeprint-shaped spots running down the middle of
Forest Tent Caterpillar
Forest tent caterpillar, Malacosoma disstria, is one of the most widespread and abundant of the tent-making caterpillars in the U.S. Like its close cousins the eastern and western tent caterpillars, forest tent caterpillars feed primarily on trees; but unlike their cousins, and the also abundant fall webworm, the forest tent caterpillar doesn't make an actual tent. Instead it aggregates between feedings on a silken mat which is spun on some area of the trunk or on large branches of the host tree. After completing its larval development in two or three weeks, the caterpillar pupates and eventually turns into a handsome, but obscure, brown moth.
|Forest tent caterpillars rest between feedings on a |
silken mat where they may be easily seen
and treated. Photo via NBCDFW Channel 5 TV.
Forest tent caterpillars appear but once a year, usually in April in Texas, and sometimes in very large numbers. They then "disappear" for a whole year until the cycle begins again. Some years caterpillar numbers are very high, but most years they may be noticed only by the sharpest-eyed observers. The cycles of up and down appear to be driven by a combination of environmental and natural control factors, like birds and parasitic insects. This may be one of those abundant years, at least for some areas of Texas. Sam Houston Electrical Cooperative reported dozens of power outages this month from masses of tent caterpillars covering electrical transformers, causing fuse overloads. I have been receiving emails over the past few days from homeowners concerned about the caterpillars massed on their silken mats.
|Spring cankerworm can defoliate entire trees and cover plants|
and sidewalks with their webbing. This tree is losing its leaves
as quickly as they can emerge.
Cankerworms belong to the moth family Geometridae. Spring cankerworms range in color from light green to brown. Like all geometrids, spring cankerworms have fewer than normal caterpillars. Where normal caterpillars have three to five abdominal feet, spring cankerworms have only two. The result is a distinctive looping walk, giving these caterpillars their common name "inchworm". Click here to see a video of this walk.
Eastern and Western Tent Caterpillars
|Tent caterpillars make their nests in the crotches of trees in|
the rose family.
|Fall webworm tents extend over the tips of|
branches. Feeding then occurs inside the
Non-Chemical Control Options
With fall webworms and tent caterpillars, non-chemical control might include physical removal or destruction of the nest(s). A pole pruner can be used to cut out webs from high branches, or it can be used to physically disrupt the nest and knock caterpillars from the tree. This is only practical for early infestations restricted to a few nests or superficial branches.
To Treat or Not to Treat
Most trees can withstand significant leaf loss without any significant reduction in sugar production and storage (a measure of tree health). A rough rule of thumb is that 20% loss of foliage (for deciduous trees) should not be harmful to a tree. Over 20% defoliation and trees may suffer slowed growth and stress. An otherwise healthy tree can lose all its spring leaves and will re-leaf; however this does reduce vitality. Defoliation in combination with other stresses, such as root compaction, drought, disease, or other insect attack, can push a tree into decline or die back. For this reason, it may sometimes be worthwhile to tree a tree for caterpillars.
Other reasons to treat a tree for caterpillars include aesthetics (preventing unsightly webs, or temporary leaf loss) or prevention of nuisance factors from droppings and caterpillars falling from trees.
Every customer faces a few decisions before having trees sprayed for caterpillars.
- Is the cost of the spray worth it for aesthetic options?
- Is the tree in a location where it can be treated without drift falling into a swimming pool or neighbor's yard? If drift is inevitable, can it be mitigated by covering the pool, or getting the neighbor's OK to proceed?
- Can the spray treatment be done quickly enough to be worthwhile? This is often the critical question, because most tree infestations are not noticed until caterpillars are nearly fully developed and the damage is mostly done.
Chemical Control Options
If you and your customer determine that a spray is justified, you must determine the best active ingredient. There are many options for caterpillar control. Spinosad and Bacillus thuringiensis are two of the better low-impact insecticides effective against caterpillars. However they are most effective against smaller caterpillars. Newer options include clothianidin (Arena®), chlorantraniliprole (Acelepryn™), and the combination product spinetoram+sulfoxaflor (Xxpire™). Pyrethroid insecticides are fast and tend to provide longer residual; however they are more toxic to non-target organisms such as beneficial insects. You should note that many of these products are toxic to bees and pollinators, so it is best to avoid spraying trees in bloom.
Lastly, it may be possible to save your customer some money and reduce drift potential and damage to beneficial insects through spot treatments. Forest tent caterpillar aggregations are easily spot-treated with a variety of insecticides including insecticidal soap and horticultural oil. Likewise, individual nests of tent caterpillars or fall webworms can sometimes be spot treated even with a pump sprayer using a pinstream nozzle. Such applications, being very targeted, should control the caterpillars with little impact on other beneficial insects.