Friday, April 10, 2015

Spring caterpillars

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
their back.
As every PMP knows, bluebonnets aren't the only thing that emerges each spring. The annual symphony of ringing phones in pest control offices this time of year is proof that insect activity returns with warm weather.  This year is shaping up to be an active year for caterpillars in Texas, so I thought that a review of some of the more common spring caterpillar pests might be appropriate.

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
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.
Another caterpillar that can completely defoliate trees in the spring is the spring cankerworm, Paleacrita vernata. When cankerworm outbreaks occur, they can produce some of the most spectacular tree defoliation events seen in this part of the country. If you have a chance to walk through an infested forest you will see millions of tiny caterpillars hanging from tree branches and blowing on the wind. After a week or two of feeding, trees can be largely stripped of leaves. Fortunately, these outbreaks normally pose little danger to trees, especially trees in woodlots and forests. Healthy trees on good soil can usually withstand total defoliation without significant damage. Trees that lose their leaves to spring cankerworms generally re-leaf and show no signs of long-term damage. However, trees that are under stress from drought or transplantation may benefit from a timely insecticide application, before the leaves are stripped. This species also has only one generation per year, so late treatment of a tree with an insecticide, or follow up sprays, are not necessary.

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.
The eastern and western species of tent caterpillars are in the same genus, Malacosoma, as the forest tent caterpillar.  The eastern tent caterpillar, Malacosoma americanum, feeds mostly on trees in the rose family, which includes most fruit trees (apples, cherries, plum, crabapple and hawthorn).  These species, however, go beyond silk mat construction and produce protective tents in branch crotches.  These tent caterpillars are common sight throughout the country.  In addition to chewing leaves, the tents themselves, when abundant, detract from the appearance of the trees in urban landscape settings especially.  Again, these caterpillars produce only one generation per year.

Fall Webworm
Fall webworm tents extend over the tips of
branches. Feeding then occurs inside the
Even though it is called the fall webworm, in Texas Hyphantrea cunea can be found throughout the growing season, starting in the spring.  Recorded from over 400 different species of trees, fall webworm webs are a common site both in cities and along rural roadsides.  Unlike the tent caterpillars, fall webworms live full-time within their webs.  As leaves from the tree are devoured, the webworm colony simply expands its web to cover new leaves.  Silk from tent caterpillar and fall webworm "nests" likely serve to protect the insects from bird and, to some extent, insect predation.  In some years whole trees can be stripped of leaves and become covered with the unsightly webbing.  Unlike the other tent-making caterpillars, fall webworms produce 3-4 generations per warm season, and may need repeated treatment.

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.