Summer World_ A Season of Bounty - Bernd Heinrich [56]
About 150 species of insects alone have been released deliberately for biological control. To test for the presence of parasitoids, Boettner regularly puts thousands of hand-reared silk moth caterpillars into the wild. Sometime later he retrieves them to study their parasite load. He has found that in some localities 100 percent of the silk moth caterpillars are infected (killed) by the one introduced fly alone, in less than one week. That’s a disastrous first inning for the caterpillar in its game with just one of its many enemies, as it tries to reach adulthood. The caterpillars are normally exposed to these flies not for one week but for a month, and in order to turn into adults they have to survive not only this exposure but the pupal stage as well. Add to this mortality caused by flies, also that of caterpillar-foraging birds, and by the fungal, bacterial, and viral diseases that also affect the larvae, and it is a wonder that any ever remain to spin a cocoon.
I had, perhaps by chance, observed one very prominent moth’s precipitous decline on my little plot of land on the hill in Maine. However, there are thousands of species, all invisible links in the fabric of any ecosystem. It is hard for anyone to care about any of them, because so little is known about them. And probably not much ever will be known—because there is no apparent proximal reason to know about them except curiosity for its own sake. “Every country boy” was familiar with one very conspicuous moth—the Calosamia—in the past, as Holland wrote in 1903. Is anyone familiar with it now? There are thousands of other more or less invisible species. Yet we don’t have people able to identify even most of the big brightly colored “canaries,” never mind important players like Gambrus wasps that are “biocontrol agents” and play a vital role. There is even debate about how many and what kinds of Gambrus wasps there are, and we can’t afford to lose any of them.
I think the lesson of the moths is the value and power of even the rarest players in the maintenance of a natural ecosystem, and the danger that even a few members of an alien species can disrupt it. High summer temperatures allow for rapid growth, and the long growing season then permits several generations in small animals. Generation times that are speeded up to a year or less, and mortality due to predators and parasites that is then ramped up to at least 99 percent, may appear alien to us. But in such a system any small difference in an inherited trait has an extremely high chance of being “noticed” in terms of evolution. The parasitoids (and bacteria and viruses) should therefore always be “ahead” of the slow reproducers. Of course the parasite’s raison d’être is not to kill. It is to live. But if its host’s population is dense, then virulence is adaptive because the parasite can grow without restraint within its host, eating every bit to proximally put out the maximum number of offspring. Killing the hosts in the process does little harm to the parasite, which simply jumps onto the next host. However, once the population of hosts becomes very rare, then any parasites that kill the host quickly become extinct, and the benign parasites are selected instead. It’s a case of “overexploitation of the resource.” It is wildly successful—for a while. But once the resource—in this case, live hosts—become rare, then the parasite, or the bacterium, or any other infective agent will be committing suicide if it kills, because it will then die with its host. At low population the tables turn, and then only the benign parasites will live, a situation analogous to what happens when a society produces an invention that allows it to tap new resources or invade virgin territory. The exploitive strategy is favored when resources are plentiful; but when they run out or there is no place else to go, the frugal strategies then persist preferentially.
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New England Longhorns
23 July 2005. TODAY I FOUND SEVERAL FOOT-LONG LIVE oak twigs on the