|
Table of Contents:
Keywords: Insect conservation, biological control, biodiversity, parasites
|
 Photo by ©Virge Kask, Uconn-Storrs |
I am often asked how I could study insect conservation and insect control, both seeming disharmonic. Many readers could rightly suggest that people who work on insect conservation have very different values and attitudes from those who work on pest control. In fact, it would seem that they are working toward opposite goals. Readers may feel that while the conservationists are rearing endangered species and preserving habitat, integrated pest managers are struggling between spraying deadly chemicals and releasing deadly biological agents (such as Bacillus thuringiensis or parasites). From my perspective, however, solutions to conserving insects and controlling insect pests are both rooted in the same applied population biology and ecological management science. From a population biology perspective, consider the following two examples:
1. As a conservation biologist trying to save an endangered butterfly, you know that each female lays 500 eggs in her lifetime. How many caterpillars need to survive to adults in order to maintain a healthy population?
2. You work for a pest control company and are trying to stop a new, invasive moth that eats crops. Each female lays 500 eggs in her lifetime. How many caterpillars can you allow to survive to adults from each set of eggs?
In both examples, if we allow two caterpillars to survive (one male and one female), the population will remain stable. If three caterpillars survive (two females and one male, assuming that the males can mate twice), the population will double every year. Therefore, in the first conservation example, our goal is to ensure that the number of insects dying is maintained at less than 99.6% (stability) so that the population continues to increase. If we maintain the death rate at 99.4% (three survivors -- two female and one male) or 99.2% (four survivors – two female and two male), the population will continue to double every year. At some point in the future the butterfly will likely become a pest, and the public may start complaining that there are too many. (Note: in population dynamics “males” are considered unimportant.)
In the second conservation example involving pest control, we need to maintain mortality between 99.7% (starting to decline) and 100% (local extinction) or we will soon be fired or replaced. With species such as the gypsy moth which can outbreak at millions of caterpillars per hectare, it might look and feel as if we are killing a lot of caterpillars by averaging 90% mortality; however, if the population has a 50/50 sex ratio, it is increasing 25 fold per year. In other words, if we maintain 90% mortality in both examples, in five summers the population could be nearing 10 million critters! The “big difference” between insect conservation and insect control in the two examples is a mere fraction --1/2 of one percent -- of total mortality. Both insect conservationists and insect pest control workers are walking a very thin, green line around population stability.
The bottom line for conservation managers is that it is very normal for the bulk (>99+ %) of an insect’s population to die. (Note to the animal rights crowd-and “Bambie® lovers”: stick with mammal issues, insects will really break your heart). We can’t manage insects as if they were slowly-reproducing whales, protecting every individual. Megadeath in the insect world is normal. That is not to say that we shouldn’t be concerned when we see, as happened last winter, dead monarch butterflies piled several feet deep in the over-wintering colonies of Mexico, but more importantly, we should keep insect mortality in perspective. Luckily, the monarch colony began the fall season of 2001 at near-record numbers. If winter of 2001/02 was a fluke, and barring any other major catastrophes, then we should expect monarchs to recover over the next few years (see recent monarch surveys in New Jersey and Iowa). We can view the 2002/02 experience as a wake up call to better monitor the effects of logging and weather on the monarch colonies, but we have to imagine that prior to our limited number of years of monitoring, monarchs have been through numerous historic crashes, and have managed to persist for thousands of years.
It is difficult for us, as mammals, to understand insect population mortality. I have been studying insects that were released in Massachusetts over the last hundred years for biological control of gypsy moth and brown-tailed moth. Classical biological control involves reuniting parasitoids (parasitoids are parasites that kill their hosts) with their hosts in a new location. Nearly 350 species of insects have been accidentally introduced to Massachusetts during the last century. In addition, more than 150 other insect species have been released in Massachusetts over the past one hundred years to biologically control a few of the accidental introductions. Our lab found (Boettner et al. 2000), that one species of introduced flies, Compsilura concinnata, released to control gypsy moth and browntail moth, caused an 81% mortality rate in one of the native silk moths, H. cecropia, in Massachusetts forests. In 2001, Shelly Kellogg, Linda Fink and Lincoln Brower found similar results with luna moths in Virginia. Some biocontrol workers correctly argue that even with an 81% mortality rate from the introduced fly, cecropia moths can persist, because females lay 300 eggs and 99.4% mortality is likely the norm for the species. In addition, because C. concinnata is a forest loving species, cecropia moths may still be able to find enemy-free space in more open habitat. Nevertheless, we cannot deny that the introduction of C. concinnata may have a huge impact on silk moths or on biodiversity. Nor can we be assured that a new agent that causes so much mortality is benign.
Only twenty-four species of native silk moths exist east of the Mississippi River, and these moths support close to 100 native parasitoids (Arnaud 1978, Piegler 1996, Tuskes et al. 1996). These parasitoid species include many tachinid flies, ichnumonid wasps and hyper parasitoids (parasitoids that specialize in killing other parasitoids, such as chalcid wasps), and even one hyper-hyper parasitoid (that specializes on killing only hyper parasitoids). Before C. concinnata was brought to N. America, H. cecropia alone had a suite of 11 native parasitoids that depended upon it as a host. To replace a portion of this diverse system with one efficient generalist, C. concinnata, could be a huge setback for North American biodiversity. C. concinnata develops very fast, killing its host in seven days, because the female larviposits a maggot directly into the host and, therefore, may be able to outcompete native parasitoids, particularly flies. Have we already lost some species of native parasitoids without anyone noticing? C. concinnata has been recovered from 180 other species of butterflies, moths and sawflies; what has been the impact on them and all of their associated parasitoids? If I were to study one host species of C. concinnata a year, looking only at the direct effects on known hosts and ignoring competition with hundreds of native parasitoids, it would take me nearly four lifetimes to understand the impact of this one introduced fly.
Estimates suggest that up to five species of parasitoids exist for every insect herbivore (Hochberg 2000). It would not be a huge stretch to guestimate that at least 2-4 parasites live on almost every bird, mammal, insect and animal. If the goal of preserving biodiversity is to save the bulk of the gene pool, saving biodiversity should really mean preserving parasites (see discussions by Hochberg 2000, Windsor 1995 a & b). Because parasites sap energy from their hosts and prevent their hosts from reaching their reproductive potential, we could also argue that parasitism makes room in nature for various species to coexist; therefore, parasites may be one of the key reasons we have so much biodiversity in the first place (Windsor 1995 a & b).
However, the “little things” that E. O. Wilson claims “run the world”, are rarely studied in conservation biology. Donald Windsor (1995a & b) has repeatedly called for “Equal Rights for Parasites” in conservation, but he has yet to be taken seriously. A look at back issues of Conservation Biology yields very few references to parasites. Nearly all of the studies of butterfly and moth conservation involve monitoring adults from year to year, (annual adult butterfly counts, Pollard counts, transect counts, black light trapping of adults), but virtually absent are any reference to caterpillar survival or the role of parasites and disease. Imagine studying birds, but ignoring survival of nests, eggs, juveniles or sub adults, or any of the things that cause mortality at these stages. There is simply a shortage of people studying parasites. While bird watchers in North America number in the millions to observe 750 species of birds, the 2002 Tachinid Times lists only 46 North American members to study approximately 1,400 North American tachinid flies (Arnaud 1978). As funding becomes tighter at museums, several of these positions are lost through attrition (This shortage of members may also partially explain why there isn’t a rush to produce a Peterson’s guide to the tachinids or parasites).
A shortage of researchers also means that very few insect extinctions – and even fewer parasite extinctions -- are noticed. Since 1600, only 64 species of insects are known to have become extinct (Stork and Lyal, 1993). Scientists have no idea how many parasites have become extinct. Many bird lice are thought to be host specific, so it has been suggested that the loss of each bird species may cause the co-extinction of its associated lice (Stork and Lyal, 1993). The authors suggested that two species of louse (Columbicola extinctus and Campanulotes defectus) likely vanished when the passenger pigeon, Ectopistes migratorius, became extinct. Recently, C. extinctus was rediscovered on band-tailed pigeon and C. defectus was found to be a likely case of misidentification (see full story at Robert Dunn’s website). However, as Dunn points out, even though the passenger pigeon lice story has a happy ending (i.e. rediscovery), it is uncertain that other co-extinctions of other parasites, even on passenger pigeon, have not occurred.
Should we be concerned over the loss of insects and parasites? With an estimated 3-5 million species of insects in the world (of which only about 1 million have even been named), surely there is room for numerous insect extinctions and, carrying this viewpoint further, many more parasite extinctions. Paul Ehrlich, however, views extinctions differently. Ehrlich pointed out that species are like the rivets on an airplane, on our “spaceship earth.” If we were traveling in a plane and looked out and saw a single rivet missing, it is not likely that we would refuse to fly. As more and more rivets disappeared, each one of us would reach a point when the number of missing rivets would cause us to become concerned about flying. If we treat the big animals (birds, mammals, herptiles, fish etc.) as the missing rivets we are likely to notice, then parasitoids would more likely be the unseen tangle of nuts and bolts that are hidden from view. We can comfort ourselves by not looking under the hood of our spaceship earth, but that doesn’t mean we may not have lost some key engine bolts along the way.
A classic example of the significant role of parasites in “the web of life” was demonstrated during an effort to save rhinos by creating a safe reserve from hunting. To maximize the health of the rhinos, veterinarians removed all of the animals’ parasites, including big, species-specific rhinoceros ticks, before putting the rhinos into the reserve. Over time, ornithologists realized that oxpeckers (birds that sit on the backs of rhinos and catch insects) had stopped breeding. It turned out that oxpeckers need huge blood-engorged ticks to feed their young. If the oxpeckers disappeared, the rhinos would have little protection against other biting insects. Ticks needed to be reintroduced to the system to save the oxpeckers. This demonstrates how little we know about the complex role of parasites. (Photo above by Dr. Jon Atwood, Antioch College, New Hampshire; Botfly (Cuterebra fontinella), a parasite of white-footed mice (Peromyscus leucopus)).
How do we straddle the thin line between protecting biodiversity and controlling pests? Not doing any control of a pest is still making a decision. If we choose not to treat an area with gypsy moths, native caterpillars will still have to compete for leaves, and multiple-year defoliations can kill the host trees.
Pest control can take two forms: chemical pesticides or biological controls. Pesticides can be, and are, appropriately used in many cases, but are often difficult to sell to a public scared by books such as Rachel Carson’s Silent Spring. Biological controls have had a host of successes, but some scientists have questioned whether the negative sides have been thoroughly explored (Howarth 1991, Miller and Aplet 1993, Simberloff and Stiling 1996, Follett and Duan 2000, Boettner et al. 2000). On the positive side of pesticide use, Frank Howarth has pointed out that when we make a mistake with a pesticide, we have to deal with half-lives (or in other words, figuring out how many years until half of the pesticide is gone and then the next half and so on…) whereas, when we make a mistake with a biological control agent, we are dealing in doubling times (how many years till the population doubles, then doubles again). But to ignore biological control is also a mistake. Many species-specific insects and diseases have been used very successfully to control insect pests. The real answers lie in a careful look at all of the available choices of weapons, with serious debate about each of the consequences, and honest looks at the impacts to the environment, however hard this may be. As conservation biologists we need to explore both sides of the thin green line.
Some of the people who study chaos theory have mused that the flap of a butterfly’s wing could change the weather miles away; but if conservation biologists don’t make a “flap” about “whether” or not some of the pest control agents should be used, it may very well impact butterflies from miles away.
1. Luna moth ©Virge Kask, Uconn-Storrs
2. H. cecropia moth ©David Wagner, Uconn-Storrs
3. H. cecropia larva ©David Wagner, Uconn-Storrs
4. H. cecropia larva ©David Wagner, Uconn-Storrs
5. Male botfly (Cuterebra austeni) on a lek site ©Jeff Boettner, UMass Amherst
6. Botfly (Cuterebra fontinella) ©Jon Atwood, Antioch College, NH
Arnaud, P. H., Jr. 1978. A host-parasite catalog of North American Tachinidae (Diptera). Publication 1319. U.S. Science and Education Administration. Washington, DC.
Boettner, G. H., J. S. Elkinton, and C. J. Boettner. 2000. Effects of a Biological Control Introduction on three Nontarget Native Species of Saturniid Moths. Conservation Biology 14(6): 1798-1806.
Brower, L. P., G. Castilleja, A. Peralta, J. Lopez-Garcia, L. Bojorquez-Tapia, S. Diaz, D. Melgarejo, and M. Missrie. 2002. Quantitative Changes in Forest Quality in a Principal Overwintering Area of the Monarch Butterfly in Mexico, 1971-1999. Conservation Biology 16(2): 346-359.
Hochberg, M. E. 2000. “What, Conserve Parasitoids?” In Chapter 17, M. E. Hochberg and A. R. Ives, eds., Parasitoid Population Biology. Princeton University Press, New Jersey. 366 pgs.
Howarth, F. G. 1991. Environmental Impacts of Classical Biological Control. Annual Review of Entomology 36:485-509.
Miller, M., and G. Aplet. 1993. Biological Control: a little knowledge is a dangerous thing. Rutgers Law Review 45:285-334.
Piegler, R. S. 1996. Catalog of parasitoids of Saturniidae of the world. Journal of Research on the Lepidoptera 33: 1-121.
Simberloff, D. and P. Stiling. 1996. How Risky is Biocontrol? Ecology 77 (7): 1965-1974.
Stork, N. E. and C. H. C. Lyal. 1993. Extinction or ‘co-extinction’ rates? Nature 366: 307.
Windsor, D. A. 1995a. Equal Rights for Parasites. Conservation Biology 9(1): 1-2.
Windsor, D. A. 1995b. Endangered Interrelationships: The Ecological Cost of Parasites Lost. Wild Earth 5 (4): 78-83.
Zimmer, Carl. 2000. Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures. Free Press, New York. 298 pages.
The views and opinions expressed in all articles that appear in "Conservation Perspectives" are those of the authors and do not necessarily reflect those of NESCB.