Fall Cankerworms

Winged male and wingless female Fall Cankerworm Moths

     It may not look like it, but both insects pictured above are the same species. They are Fall Cankerworm Moths, Alsophila pometaria, and in this species, the adult females are wingless. This is one of the very few moths who remain active through the winter, at least during warmer days. These Geometrid (inchworm) moths mature late in the year and thus avoid many potential predators, since many birds have migrated and most bats are either dormant or have also left. These are the most likely moths you will find this time of year.

Male Fall Cankerworms can be very variable in color and can hold their wings from either tightly wrapped to spread wide over their backs. They fly from Fall through the Winter. 

     The females of course cannot fly, sacrificing wings in order to be able to commit more resources to egg production. They send out pheromones to attract the winged males and then lay their eggs in rows encircling small tree twigs. It is believed that in some populations and under certain circumstances, females can even reproduce without mating.

Wingless female Fall Cankerworm Moth

     The emerging caterpillars "balloon" in the spring by sending silk out to be carried and spread by the wind to the various trees and shrubs they feed on. They are unique among "inchworm" or "looper" caterpillars in that they have a reduced third pair of prolegs on their fifth abdominal segment. Other inchowrms have only two pairs of prolegs near the rear of their bodies. Otherwise, cankerworms can be somewhat variable in color, from light green to dark brown. They feed on a wide variety of deciduous trees and shrubs, with oaks, elms, and maples being their favorite caterpillar host plants.

Fall Cankerworm caterpillars can be variable in color, but always are unique in having a 3rd pair of reduced prolegs as noted here by the arrow. Other Inchworms only have two sets of prolegs. 

     The dark color forms are much more common during over populated conditions when outbreaks of this native caterpillar occur. That is another unusual trait of this species, they periodically have population explosions where they can severely defoliate the forest. These outbreaks can have some serious effects, even killing trees. This seems to be especially true to stressed trees, with red and black oaks being the most likely to suffer.

Fall Cankerworm caterpillar in dark color form, typical of over crowded and population outbreak conditions. 

     Healthy trees can recover, leafing out after the caterpillars have gone through their fifth molt and pupated under the leaves and loose soil. But those trees that are stressed by lack of water, along ridge lines, or repeated defoliations, can succumb and perish.

Common Grackle and a beak full of multiple Fall Cankerworms to feed its young. 

     Normally, natural predators such as birds, ground beetles, and certain parasitic wasps (Telenomus alsophilae being the most important) keep the populations in check or quickly control population outbreaks. Caterpillars of course make up the vast majority of the food items that almost all terrestrial birds feed their nestlings. But these natural controls can be interrupted and are not always able to keep a check on caterpillar numbers. That is when the debates about whether or not to try and control or manage those numbers (and how to do so) come in to play. In places with the resources to do so, sticky bands around tree trunks and other methods are used to survey (and control) female cankerworm population densities. The females are caught as they try to climb the trees. Some jurisdictions that find high numbers of caterpillars (over 91 or so per sticky band) use that as the level to start artificial control of their numbers.

A cankerworm monitoring trap, here luckily filled mostly with other insects and spiders rather than cankwerworms.

     As some of you may be aware, this is something that has caused consternation and public debate for many years now, and is again happening. Those of us who try to manage our natural resources are often in difficult situations and often have to make tough decisions about what to do to try to achieve a balance of wildlife and what the environment can support. When we make recommendations for population controls of any creature or plant, we do not do so lightly. We, hopefully, use whatever knowledge or data we can to make those decisions and depend on our own expertise and/or that of others. We should not make these decisions without a lot of thought as to the consequences they entail or base them on emotion. It also can sometimes be difficult for us to then be questioned as to our motives or to have the public voice their own opinions opposing our decisions based on emotion or incomplete knowledge of the facts. At the same time, I do encourage folks to be informed, ask intelligent, non-emotional questions, and provide input, but to also understand that we sometimes end up having to pick the lesser of two evils or give in to ill-informed public pressure. It is for that reason that I am not questioning or second guessing the decisions of my likely better-informed colleagues. 

     Having said all of that, and though this makes this a very long Blog post, I'd like to share some information on what I know about the decisions to spray for control of what is after all a native insect that can be a valuable food source, and at times can also pose disastrous damage to our remaining stressed forests. First of all, these are insects that have evolved with our native trees and so they do not under normal conditions cause the death of their food source.

     But we do not have normal, balanced ecological systems and conditions in most situations. Predator such as birds, beetles, and wasps are often in low numbers and in fragmented habitat islands. Our forests are already stressed and altered by invasive plants, changes in water tables, over browsing by overpopulated herbivores, lack of pollinators, low diversity and missing native plants, and so many other factors. This probably allows for some outbreaks in Fall Cankerworms that otherwise might not have taken place, and if let unchecked by natural controls, can be extremely damaging to the small amounts of fragmented forests we have left. 

Though the darker morph is often a sign of overpopulation, you can have both light and dark caterpillars out at the same time and even on the same leaf.

     Even if the outbreak is allowed to run its course, we have a lot fewer trees left to recover or to begin anew. If the leaves are consumed by cankerworms, then they do not provide food for insects such as other caterpillars or cover/shelter for nesting birds and other creatures. The leaves that grow back also do not supply the same quality forage the original leaves would have had. That means that some wildlife or plants may not recover. Noted entomologist and author Doug Tallamy, of Bringing Nature Home fame, has even been quoted as saying that in our jurisdictions some spraying is better in the long term than no spraying at all. But he also adds that it needs to be a measured integrated pest management approach, carried out as conservatively as possible, since eruptive populations eventually crash on their own anyways. 

     The pesticide sprayed is normally Btk, which is naturally occurring, breaks down over a month or so, and targets only caterpillars, but it kills any type of caterpillars that feed on it. That can mean not only that most of the food is killed off just when the birds and some other creatures really need it most while breeding, but that for some localized populations, they may be locally extirpated, completely wiped out. Some though would argue that when all the leaves have been consumed by cankerworms, you get the same results, as well as no nesting cover and perhaps the death of a centuries old tree that cannot be replaced. 

     But with our fragmented forests, sometimes other caterpillars, invertebrates, and their predators cannot recolonize or do so at a much slower pace. This includes such important natural controls as the aforementioned wasp species, meaning that we may then have to be locked into cycles of having to spray all the time, never allowing natural predator numbers (and other species) to recover. Even many birds do not recover, despite being able to fly in. Perhaps this is due to not having the same numbers of various caterpillars and other prey items available, or perhaps this is due to "nest fidelity" (returning to the same sites as they have had success nesting; so if not nesting one year, they may not return again for quite a while until new birds try again). Regardless, there can be long term or permanent effects one way or another.

     So there's the problem as to what to do, if anything: gamble that natural predators and disease will control the numbers (and they will, eventually), or spray to prevent perhaps even longer term affects from taking place. So what is my personal position in this dilemma? I do not think we should take a potential weapon out of our arsenal to combat these creatures if we need to do so someday. We should not wholesale decide that we cannot use Btk or spray to control cankerworms.

     Having said that, we need to be very careful and measured when we do decide to use this option as it will have definite effects on the ecology of the forests. It should only be used if we are reasonably sure of the consequences if it is not used. We need to take into account the population numbers, the color form of the caterpillars, the species composition and location of the trees (we know that black/red oaks along ridge lines and experiencing water shortage or other stress are most susceptible for example), and how much of an area we target. This takes resources though that we often don't have to get accurate measures, or studies to either prove or disprove that our actions have the intended consequences and not others. We really don't have many of these resources in sufficient knowledgeable staff, surveyors, and long term studies. 

     I think that it is very likely that we do spray too much, trying to play it safe so as to not lose whole forests. When my own neighborhood was scheduled to be sprayed last year, I opted out, knowing that in my small piece of land we did not have too many cankerworms and that spraying would cause more harm than good. But what about other locations? Luckily, Arlington did not seem to have over populations of this species, though we don't have a budget to do much if we were to find extremely high numbers anyways.

     So, there are no easy answers. I would not vote to no longer do any type of spraying, because some day we may really need to. But I also think that we need to do some studies and have better resources, perhaps including citizen scientists, to help collect population data, and trained staff to be able to make those tough calls better.