Janet Mackey – Halton Master Gardener
Sometimes its exciting to see caterpillars in our gardens. They are signs of life and balance in the garden. There’s hope for colourful butterflies such as the Monarch or the amazing Clearwing Hummingbird moth. However, sometimes it can be concerning that there are so many on a newly planted tree or favourite flowering shrub. Should we take action? Before beginning to consider caterpillars as pests, it is important to consider that caterpillars are an essential source of protein in spring for birds. The protein and soft bodies of caterpillars make them ideal food for young hatchlings. Dr. Douglas Tallamy an etymologist from the University of Delaware, has documented the activity of chickadees to determine the enormous amount of caterpillars (up to 9000) that are required to raise the young of a single nest! In addition, adult birds have been observed gathering silk from tent caterpillars, which may be used in nest construction. Let’s consider some types of caterpillars that mass together on a single plant: Eastern Tent Caterpillars, Forest Tent Caterpillars and Gypsy Moth Caterpillars.
Gypsy Moth, Lymantria dispar
Gypsy moths originally came from Europe & Asia during the 19th century. Despite efforts to control them with predators and sprays they continue have a significant, negative impact by defoliating hardwood forest canopies. Host trees can include: Oak (Quercus), Birch (Betulus) and Aspen (populus) in the north, to various hardwoods such as Sugar Maple (Acer saccharum) American beech (Fagus grandifolia) and softwoods such as Eastern White pine (Pinus strobus) and Colorado blue spruce (Picea pungens) in southern Ontario.
Adults (the moths) are most active during mid-summer laying eggs.
- Females – wingspan of 5 cm, Light coloured with dark markings and CAN”T FLY!
- Males – wingspan of 2.5 cm; brown in colour and DO FLY.
Caterpillars– have 5 pairs of blue dots and 6 pairs of bright red dots. Early instars look very similar but the dots are less visible (see photo). Last photo is the pupa of the gypsy moth.
Lifecycle: This includes: egg, larva (several instars), pupa and adult
- If you see caterpillars venturing up your trees, install a burlap wrap around tree trunk. Remember to remove and dispose of the caterpillars daily. Here’s an instructional video.
- If you have a large forest canopy on your property you may want to install pheromone traps to catch and confuse male moths.
- Search your property for for egg masses: The best time to search for egg masses on trees is after the leaves drop off the trees in fall as they’re easier to find. However you can look for them at anytime from early summer through to the following spring. Look for them in sheltered locations such as:
- under branches or patio furniture;
- on rocks, firewood, fences or windowsills,
- under the eaves, near bird houses or mail boxes.
NOTE: Do not simply scrape the egg mass off and let it drop to the ground as the caterpillars may still hatch. Here’s how it’s done:
- Spray the mass with water to stop the egg mass from crumbling.
- Scrape the mass with a stick or any object with a flat surface such as a knife or paint scraper into a container (i.e., brown paper bag) and then place it into a bucket of soapy water.
- Soak the egg mass for a few days to destroy the eggs.
- Dispose of the water responsibly
There is one natural threat to gypsy moth control and that is a fungus (Entomophaga maimaiga), that is active during damp spring weather. If a dry spring occurs, gypsy moth infestations are more widespread because the fungus does not reproduce.
Eastern Tent Caterpillars, Malacosoma americanum
Eastern Tent caterpillars are native to our region in Ontario. Populations fluctuate from year to year with outbreaks occurring every several years. The egg masses are found most commonly on small branches of cherry, apple, and crabapple trees but are also sometimes on hawthorn, maple, cherry, peach, pear and plum. The newly hatched larvae build small silken tents over the egg mass and the surrounding branch. The tent gradually increases in size as the larva mature. Staying together in the tent allows the caterpillars to conserve heat, elevate their body temperature in order to grow more quickly during cool spring weather. The adults emerge as moths from their cocoons in late spring or early summer and then mate.
These caterpillars are actually ‘social’. During the heat of the day or rainy weather, they stay inside their tent, heading out to feed early in the morning, evening or at night, if it’s not too cold. As they move along the branches, the caterpillars secrete silk from a spinneret wherever they go, and frequently-used pathways soon have noticeable silk trails. They lay down pheromones along the trails by dragging their abdomens which leads other caterpillars to the food source.
Adults: The body and wings are warm, fawn brown; the forewing has some white
- Female: wings are paler and more yellowish
- Male: The wings of the male are darker and more brownish
Caterpillars: are black with a white stripe down the back, brown and yellow lines along the sides, and a row of oval blue spots on the sides.
Lifecycle: egg, larva (several instars), pupa and adult
Egg Mass in early spring found in the crotch of small branches on favoured host trees; Silk Tent full of Eastern Tent Caterpillar larvae; Adult Stage: Female & Male
Damage: While Eastern Tent Caterpillars can completely defoliate a tree, most trees will usually recover and put out a 2nd crop of leaves. Since these caterpillars only have outbreaks every 6-8 years, most well-established trees can withstand the defoliation. However, if you have a tree that is newly planted, stressed or a favourite tree; it is hard to stand by and hope for the best. According to the University of Kentucky’s Entymology Department, insecticides are largely ineffective but there are some control measures available:
- Natural enemies play a large role in control of the Eastern Tent Caterpillar. They are often parasitized by various tiny braconid, ichneumonid, and chalcid wasps.
- Prevent an infestation by checking ornamental plants and fruit trees during the winter: removing and destroying the egg mass (as directed above for Gypsy moths).
- Larger tents can be pruned out or wound around the end of stick.
- Young caterpillars (early instars) can be controlled through the use of BTK (Bacillus thuringiensis var kurstaki)
If you want to protect your prized trees from defoliation by tent caterpillars and assist your resident birds, simply prune out the infested branches, tents and all, and relocate them to a suitable host tree. They will likely establish a new nest and hopefully become bird food.
Forest Tent Caterpillar Malacosoma disstria
This caterpillar (which doesn’t create tents) is found throughout North America and particularly in eastern North America. Forest tent caterpillar outbreaks tend to recur at reasonably regular intervals, every decade or so, usually lasting two to four years. The adult moths are active from April to August and the caterpillars are most commonly found April to June. The adults raise just one generation per year, leaving silken egg masses on favoured trees (Species include: alder, basswood, birch, cherry, oak, poplar, willow). Once hatched the larvae spin silken masses to congregate which helps them to regulate their bodily temperature, ensuring rapid growth.
Caterpillars: are black, dark brown, or gray, with broad long, blue stripes and thin yellow stripes, along each side. The back of each segment has a white spot that is wider toward the head end. The sides are partially covered with fur-like long setae.
Adults: are stout-bodied; forewing light brown with two darker, thin parallel lines extending across mid-portion, the area between often being dark and appearing as a single broad, dark band
Forest Tent Caterpillars congregating on a tree trunk; Male (left) & Female adult moths with the larva; Large Flesh Fly is a parasite of the larva; Egg Mass with Adult Female Moth.
Damage: If an outbreak occurs, it can affect trees reaching millions of acres and heavy infestation can permanently damage trees or branches, particularly those that are stressed through drought, disease or other insects.
Control: Again, BTK (Bacillus thuringiensis var. kurstaki) can be used to protect tree health during outbreaks. Predators include: ants, birds, vespid or yellowjacket wasps. Parasites are also important in controlling this pest, such as the Large Flesh Fly (Sarcophaga aldrichi), Tachinidae and Sarcophagidae.or the nuclear polyhedrosis virus (NPV).
Further Reading & Photo Sources:
University of Kentucky: College of Agriculture, Food and Environment https://entomology.ca.uky.edu/ef423