Photo: Joe Boggs
Photo: Joe Boggs
When I hear a tree “looked good last week,” it’s been my experience that more times than not, it didn’t. For whatever reason, it’s not unusual for the telltale signs that a tree is in deep trouble to be missed for weeks, months, or sometimes years. However, with White-Marked Tussock Moth (Orgyia leucostigma) caterpillars, a heavily infested tree may have looked good last week. Hordes of these ravenous leaf-feeders can quickly defoliate a tree victim.
So-called “outbreaks” occur when a rapid population expansion of this general defoliator outpaces the biological suppression provided by its enemies. These include the 3-Ps: Predators, Parasitoids, and Pathogens. Usually, the 3-Ps eventually catch up meaning that the outbreaks are most commonly single-season events with no repeat performances the following season.
Highly localized outbreaks of these native flamboyant-looking caterpillars are not unusual in Ohio. Unfortunately, outbreaks can occur so quickly that they “sneak up” on homeowners and managers of landscapes, nurseries, and Christmas tree plantations.
The image below of columnar sweetgums (Liquidambar styraciflua 'Slender Silhouette') was taken last Thursday in Dayton. There’s nothing special about the foliage at the top of the tree on the left other than the caterpillars just hadn’t gotten to those leaves yet. The caterpillars were also moving onto the tree on the right.
The conspicuously colored distinctive-looking caterpillars have been recorded feeding on over 140 woody plant species including conifers. Just about all of the shade tree species commonly used in Ohio landscapes are on the caterpillar’s menu.
The caterpillars are also known to cause serious damage to conifers with outbreaks in the Canadian Maritime Provinces defoliating over 1 million acres of conifers as well as hardwoods. Research has shown that one year of severe defoliation can kill balsam fir, Abies balsamea. Canaan fir (A. balsamea var. phanerolepis) is one of the most popular conifers grown in Ohio Christmas tree plantations.
Overwintered white-marked tussock moth caterpillar eggs hatch in early spring. On deciduous hardwoods, the first instar caterpillars commonly feed in groups as leaf skeletonizers meaning they leave behind leaf veins. The young caterpillars may also spin short strands of silk to ride the wind to new horizons. This behavior is known as “ballooning” and plays a major role in the spread of white-marked tussock moth.
The literature notes that on deciduous hardwoods, older caterpillars consume entire leaves, but leave behind the midveins. However, as shown in the pictures illustrating this Alert, the caterpillars on sweetgum consumed leaves, veins, and all, and then started chowing down on leaf petioles. It's an example of insects not reading what we write about them.
As the caterpillars approach pupation, they construct silk cocoons with their hairs commonly interwoven into the silk, perhaps to provide some protection against predators. The cocoons may be found on infested trees or nearby structures. Male cocoons are smaller than female cocoons.
It’s also common for caterpillars to congregate on their plant hosts before they pupate. This produces an unsightly mass of cocoons that may include plant debris.
Male and female white-marked tussock moths look nothing like each other; a condition known as sexual dimorphism. Males are medium-sized brown moths, and they are relatively good fliers.
Females look nothing like a moth, and they cannot fly. They appear as plump light-gray to grayish-brown creatures that are brachypterous meaning they have short, vestigial, non-functional wings. This is why the ballooning first instar caterpillars are so important for the distribution of white-marked tussock moths.
The females attract the males by emitting a sex pheromone. The pheromone has been synthesized and can be used in traps to detect and monitor white-marked tussock moth populations.
Mated females lay 100-300 eggs in frothy masses commonly located near the female’s cocoons. This is the overwintering stage and also marks the initiation of a new generation.
It’s reported that the white-marked tussock moth has two generations per season in Michigan with overwintered eggs hatching at 206 GDD (base 55F) and second-generation eggs hatching at 1157 GDD (base 55F). This implies developmental synchronization.
However, I did not observe this in Dayton as illustrated by the image below. These caterpillars were photographed this past Thursday. As you can see, both early and late-instar caterpillars were present at the same time.
Indeed, I observed every life stage except adults and newly deposited eggs. Early instar caterpillars were feeding among late-instar caterpillars constructing cocoons, pre-pupal caterpillars inside cocoons, and pupae.
This Everything, Everywhere, All at Once condition can present a serious management challenge. I believe it developed from a lengthy hatch of the overwintered eggs and it implies that white-marked tussock moth development may not be synchronous.
Management
It’s commonsensical that general defoliators with multiple generations per season will cause more damage than those with a single generation. As noted above, the white-marked tussock moth has at least two generations per season; however, the generations may overlap as observed in Dayton. Thus, defoliation may occur for an extended period unless the early-season caterpillars are eliminated.
Healthy, mature trees can recover from heavy single-season defoliation. However, trees under chronic stress may suffer health-debilitating damage from acute defoliation. Thus, pest management should first focus on tree health management.
On the other hand, the overall health of recently planted young trees may be compromised by a single-season defoliation. Newly planted trees should be closely monitored for significant leaf-feeding pests as well as other pests. For example, some of the images presented in this Alert show the sweetgum was also infested with a light population of Calico Scale (Eulecanium cerasorum).
There is a wide range of effective “caterpillar insecticides” that will suppress white-marked tussock moth caterpillars if applications are required. However, it’s important to keep in mind that insecticide applications can present a risk to many of the natural enemies that keep white-marked tussock populations in check from year to year.
So-called “biorational” insecticides that present less risk to the bio-allies should be considered for front-line defense. Variants of the soil-borne bacterium Bacillus thuringiensis (Bt) have tight targets with Bt kurstaki and Bt aizawai being highly effective against caterpillars; they do not kill other insects. However, they are most effective on small caterpillars, so multiple applications may be required if moth development is not synchronous.
Azadiractin is effective against all caterpillar instar stages and has low toxicity to predators and parasitoids. The same is true for spinosad which also has translaminar activity meaning the insecticide remains effective longer than Bt products. The anthranilic diamide insecticide, chlorantraniliprole (e.g. Acelyprin), is highly effective against caterpillars and presents a reduced risk to bees and other beneficial insects.
Other insecticides include pyrethroids (synthetic) and pyrethrums (naturally occurring). Bifenthrin (e.g. Talstar) is an example of a pyrethroid insecticide. Products with pyrethroid active ingredients are commonly used for “quick-kill” rescue applications owing to their broad labels and relatively low cost. However, they may be more impactful on non-target insects including beneficials.
A Near Look-a-Like Tussock
Although white-marked tussock moth caterpillars are distinctive in their markings and coloration, other tussocks may be mistaken for them or vice versa. The caterpillars of the Definite Tussock Moth (O. definita) (I love the common name!) are often confused with the closely related white-marked tussock moth caterpillars. Their body plans are very similar; however, definite tussock moth caterpillars are definitely yellow. The caterpillars prefer willow but may also be found on oak, maple, hackberry, and birch. However, definite outbreaks are rare.
On a final note, all tussock moth caterpillars (family Erebidae, subfamily Lymantriinae) should be handled with care. That’s because many species have stinging (urticating) hairs. The hairs are not attached to venom glands like the hairs and bristles on more dangerous caterpillars. However, they can puncture the skin causing hives and skin rashes on individuals who are highly sensitive to the irritation. So, it’s look, but don’t touch.
Tags