THE CATERPILLARS ARE COMING
And there’s nothing we can do about their menacing march. But they will be stopped, for a time, by the enemies they carry within them
MAYBE it doesn't matter so much whether this particular tent caterpillar story is true or false. Its importance lies in the fact that scientists, who are notoriously quick to debunk any yarn that sounds like blarney, have been telling it for ten years and they’re still wondering if it could be true.
According to Mark Sauerbrei, an Ontario forestry official at Port Arthur, it happened during a tent caterpillar epidemic at Atikokan, the Steep Rock iron-mine town in northwestern Ontario. Millions of the ugly little caterpillars had become beautiful yellow moths, swarming to the lights. The CNR roundhouse turned off its lights and opened the steam plant’s firebox doors. The fire gave the only light and the moths streamed into it. Their oily bodies burned fiercely, making the fire so hot the foreman cut down on the coal. He finally was able to turn off the stoker entirely. Then, for several hours, it is claimed, the fire continued to blaze fiercely, fed only by the clouds of moths pouring through the firebox doors.
“Frankly, I’m a bit sceptical of yarns like that,” says Dr. Carl Atwood, head of the forest insect department of the University of Toronto, “although this one is not as impossible as it might sound, because, when the forest tent caterpillars hit one of their cyclic peaks of abundance in the north, the Bible’s locust plagues are a meagre dribble in comparison.”
After the tent caterpillar epidemic which has been ravaging the forests of eastern Canada since 1948 thousands of Canadians from the Maritimes to Manitoba believe the teeming insects could keep the fires of Hades itself burning. They transform parts of the northwoods into a hell on earth.
During their mercifully short lifetime in May and June billions of them cover large areas with a rippling crawling carpet. Poplar, oak and hard maple forests for miles look as if a hurricane has removed every leaf. They stop trains and tie up road traffic with the slime of their crushed bodies, and defy, by sheer numbers, all human attempts at control. DDT sprays wipe out millions, but billions survive.
Each caterpillar has an appetite so ravenous that it eats about eighty-six thousand times its weight in its first forty-eight, hours. The tent caterpillar infestation sweeping the Maritimes, Quebec, Ontario and forested sections of Manitoba since 1948 is probably the most spectacular forest insect outbreak in Canada’s history.
Yet foresters are not at all sure that the tent caterpillar does permanent damage, for defoliated trees produce new leaves by late July and rarely die from the attacks. However, they lose all growth that year. But the Sudbury woman who in one sweeping last spring took three tubfuls from her screen doors and windows says: “Maybe they don’t kill trees, but if this keeps on they’re going to kill me!”
Unfortunately the epidemic will return this spring at least. Eggs laid by the adult moths last summer, from which caterpillars are now hatching, form clusters on twigs and are easily counted. According to counts made last fall and winter by federal government insect rangers, some trees in Port Arthur and Lake Timagami districts have a hundred thousand eggs per tree. Unless a late spring frost hits them after they have hatched, egg concentrations such as that will leave a surplus of ninety-eight thousand caterpillars per tree, because one average seven-inch-diameter tree will feed only two thousand to maturity.
Last June a party of anglers driving north from Thessalon had to stop at a forest ranger’s post to obtain travel permits. It was a sunny day but the ranger had his lamp burning inside his cabin to read by, because door and window screens were blackened by caterpillars. “I sweep them off,” he said, “and in fifteen minutes they’re back on the screens an inch thick.”
The caterpillars had stripped the surrounding poplars and were migrating in search of new food. When an army of them reached the cabin they crawled up one wall, across the roof and down the other side. The cabin looked as if it wore a black fur coat.
anglers reached their fishing grounds they ïound the trout so gorged with caterpillars that nothing would tempt them. One angler declared: “Those fish have had so much to eat they won’t be biting for a year.”
About the same time, Nestorville, forty miles east of Sault Ste. Marie, found itself without hydro power. For hours linemen could find nothing wrong. Then they spotted a mat of caterpillars on a crossbar atop a hydro pole which had shortcircuited wires a foot apart.
When the hordes start crawling, nothing is caterpillar-proof. A woman in a Sudbury store opened her purse to pay for purchases and started to scream. A dozen or so caterpillars had found a refuge beside her change and lipstick.
If a cabin has no cracks that will admit them caterpillars get in anyway on clothing. They drop from ceilings into the cooking, spin cocoons in the bed clothing. In the bush they drop from trees like rain and get down the necks of pulp cutters where their hairy bodies set up painful skin irritations. In several areas last June pulp cutters quit work for a week or more until the caterpillars had spun cocoons.
While golfing, Alex Milne, of Port Carling, found caterpillars so thick on one fairway that they covered his ball and it took him fifteen minutes to find it. After a long putt he was baffled to see his ball roll across the cup without dropping in. The cup was filled to overflowing with a wriggling mass of caterpillars.
On concrete pavements and railway rails their crushed bodies form a coating as slippery as ice. Where caterpillars are numerous it is frequently difficult to keep trains moving. North of Sudbury freights have been uncoupled and hauled up caterpillar-greased grades in sections. At Wabigoon, near Kenora, locomotives have sat motionless for two hours with wheels spinning on a mass of caterpillars. Near Perth, in eastern Ontario, a brakeman once walked ahead of a passenger train for two hours sweeping caterpillars from the rails with a broom.
Last June in northern Michigan highway traffic on one hill was stalled a couple of hours when one curious driver stopped to look at the mat of caterpillars crossing the pavement. When he tried to start again his wheels spun as though the car was in wet snow.
Fortunately the caterpillars stay around for only about two weeks in late May and June when mosquitoes and black flies are sabotaging the tourist trade anyway.
Few other insects illustrate more forcefully the capacity for explosive increase possessed by the insect kingdom. The tent caterpillar population rises and falls in fairly regular cycles. For years it will survive tenaciously with a population as low as one caterpillar per acre of forest. Then suddenly, about every ten years, the population bursts into an epidemic. Nature’s controls, disease and parasites, attack the caterpillars with an efficiency that man’s chemicals cannot match. Then, as suddenly and mysteriously as they first appeared, the caterpillars are gone again, a gruesome memory until the cycle returns.
There are half a dozen or more species of tent caterpillars in North America, named for their habit of building tentlike webs in trees to which the colonies retreat for protection during daylight. One, the orchard or eastern tent caterpillar, pitches ifs tent in wild cherry or apple trees and is a serious orchard pest, but it never goes wriggling across the landscape in a furry tidal wave like its northern cousin. The one that stops trains and denudes whole forests is the forest tent caterpillar. Oddly enough it’s so busy eating, or crawling around looking for something to eat, that it never takes time to build a tent. A tent caterpillar without a tent may be the family eccentric but, in the world of nature where success is measured only in how many of a species survive to produce another generation, the forest tent caterpillar is the insect kingdom’s wonderboy.
When it’s fat, well fed and finicky the forest tent caterpillar prefers poplar leaves, but after all the poplar greenery has Continued on page 48
Caterpillars Are Coming
Continued from page 23
been munched down to the bare twigs it roams around and eats practically everything except poison ivy. A diet of poplar seems to do extraordinary things to its sex life. If poplar-fed the caterpillar which becomes a female moth lays twice as many eggs as one reared on other fare, such as some unfortunate gardener’s spinach. This is why infestations of forest tent caterpillars have become more devastating. Poplar springs up in dense secondgrowth stands after forest fires. Thousands of square miles of forest once pine and hardwood are now predominantly poplar. The result has been a super-race of tent caterpillars.
Northerners who think they have their troubles with caterpillars can experience a revengeful satisfaction in the knowledge that the insect has its troubles too. Probably fewer than one out of every hundred of last summer’s eggs will successfully complete their life history and become adult moths this summer.
The yellowish-brown moths with a wingspread of about one and a quarter inches appear in late July and each female lays from one hundred to three; hundred and fifty eggs. Birds eat the; eggs by millions, but the main foe both to eggs and the caterpillars later are diseases and other insect parasites. No matter how small, every creature has a smaller parasite which lives upon or in it and the tent caterpillar has more than its share.
The eggs remain on the trees all winter and if they survive birds and parasites each egg hatches into a tiny caterpillar in May. The entomologists call this the insect’s larval stage. By late June each caterpillar is one-and-ahalf inches long, a couple of thousand times heavier than when born six weeks before. When they strip the tree on which they were born they drop to the ground and move like an army in search of another tree not already stripped by others which have preceded them. Frequently they die of starvation in the search.
In the larval stage the battle with parasites becomes fierce. At least twenty-five parasitic species of wasps and flies seek out tent caterpillars on which to deposit their own eggs. The caterpillar is defenseless against them and may have a dozen minute parasite eggs deposited on it, or just beneath the skin. Only one is required to kill it.
When the parasite egg hatches, the grub begins eating into the doomed tent caterpillar’s tissue. With ghoulish precision it avoids the caterpillar’s vital organs because the parasite can’t afford to have its host die until the parasite itself is mature.
If the tent caterpillar doesn’t die of starvation, if it isn’t eaten by birds, if it doesn’t drown in a thunderstorm, if it doesn’t die of disease (it has a host of virus and bacterial parasites as well) and if it isn’t killed too soon by an overzealous internal parasite that ate too much too fast, it is ready when full grown for the next stage of its life history. Appetite is now replaced by wanderlust. It drops from the tree and roams in search of a cranny in which to spin a cocoon. The cocoon is an oval-shaped jacket of yellow silk and within it the transformation from a caterpillar to a moth takes place. In this silken sleeping bag the caterpillar becomes a brown shell-encased pupa and the pupa, in turn, develops the sex organs and delicate wings of the adult moth.
But if the caterpillar harbored a
parasite or two before it started to pupate the parasite grubs are still eating. The tent caterpillar pupa dies, eaten alive from within.
If the tent caterpillar has been one of the lucky few and remained unparasitized, the moth emerges in ten days or two weeks. For the first time the insect now has a sex. Female moths stay close to their cocoons until a mate finds them. Their love life is not very clearly understood, but it appears that the males find them by scent.
After the eggs are laid, ensuring a new generation, the moths live four or five days, swarming to lights, eating nothing, and then die.
The odds against any one completing the life cycle and emerging as a moth are high. But nature stacked the cards so that the tent caterpillar as a species can hardly lose. One pair of moths may leave three hundred and fifty eggs. If only four of these, or about one percent, survive to maturity the tent caterpillar population will be doubled the next year.
Man is virtually helpless against the power of increase possessed by the tent caterpillar. Individual owners can protect their own properties with DDT sprays, but effective widespread aerial spraying is out of the question, for at six hundred dollars a square mile this would cost millions.
Private property owners can sometimes remove the egg clusters with a pruning pole and destroy the caterpillars before they hatch, or a DDT spray from a hand pump or tank sprayer will destroy practically all the caterpillars if applied in May just after they have hatched. A spray of water and “wettable” DDT should be used. Oil solutions in drenching sprays from ground-operated equipment burn and injure trees.
Nature’s Great Destroyer
If there are unsprayed poplars within a quarter mile, however, caterpillars from these will move in. They may be kept out in .sandy soil by surrounding the protected area with a steepsided, six-inch-deep trench. The caterpillars fall in and can’t climb out. In hard soil a trench won’t stop them. In fact, one Sault Ste. Marie tourist proprietor’s sand trench didn’t stop them last year. It filled up with caterpillars, then those behind crawled across without knowing the trench was there. A surer method is to spray a twenty-foot strip surrounding the property with DDT.
Caterpillars can be stopped on a wide scale only by parasites and disease. Occasionally epidemics are wiped out when severe late spring frosts destroy the larvae. Starvation is frequently an important agent in reducing the populations. But the enemy that finally routs the tent caterpillar hordes with a defeat so decisive that it takes the caterpillars ten years or more to recover is either disease or the parasites.
Last year a virus disease caused practically a hundred percent mortality among caterpillars in some areas around Thessalon. But throughout the north parasites generally appear to be waging the main battle.
Parasites, which have a tough time to survive when caterpillars are scarce, may be so rare in the first year of a new outbreak that only one percent or less of the caterpillars are attacked. But as the caterpillars increase so do the parasites. Usually, nature’s great destroyer — starvation — weights the balance suddenly in the parasites’ favor.
Here is what a census of a typical segment of tent caterpillar population might show. The first year’s population
might be ten thousand caterpillars and one hundred parasite larvae—a parasitism incidence of one percent. By the second yéar these would multiply to one hundred thousand caterpillars and five thousand parasite grubs—a parasitism of five percent. The third yeai might start with one million caterpillars and enough parasites to wipe out one hundred thousand, a potential parasitism of ten percent. But by this time the caterpillars have multiplied far beyond the point where there is enough food to support them and nine hundred thousand of those one million caterpillars may starve.
The parasites attack only mature larvae or pupae; there is no wastage of parasite eggs on half-grown caterpillars which are going to starve anyway. So, by the time the parasitic flies and wasps attack, starvation has already reduced that million caterpillars to one hundred thousand. This is just about what the parasites can handle, so that potential parasitism of ten percent has suddenly become one hundred percent, or close to it, and the tent caterpillar has lost another campaign.
Where are we now in that bugs vs. bugs battle? In many areas parasites appear ready for the knockout punch this summer. In some sections the parasites appear to have yet another year or two to go before they gain supremacy.
Every June scientists put a finger on the pulse of the battle by collecting sample lots of tent caterpillars and pupae and examining them for parasitism. As a cross-check, egg counts on sample trees are made each fall. Forest insect rangers like wiry Jim McDonald, of the Soo, do this work under the direction of Dr. M. L. Prebble, the young red-headed director of the federal government’s forest insect laboratory at Sault Ste. Marie.
Prebble’s expert on parasitism, Lloyd Sippell, expects that the parasitism incidence may hit ninety-five percent in many areas this year.
But even as the battle surges toward the inevitable tent caterpillar defeat, nature is already paving the way for a future tent caterpillar comeback.
The most numerous parasite, the one which is building up to strike the tent caterpillars’ coup de grâce, is a big blue fly, resembling an overgrown housefly with red eyes, black stripes, and a droning flight that makes him sound like a miniature jet plane. Scientists call him Sarcophaga aldrichi. Northerners know him as the Mexican horsefly-
The female “horsefly” is parasitism’s perfectionist. She doesn’t waste time with eggs, she deposits on the cocoon a living maggot that starts eating at once. As a parasite this maggot is a hundred-percenter from whom nothing escapes. Other parasite species can live side by side in the same ten! caterpillar pupa. But not this one. If there are other parasites ahead of it they too are eaten.
In a grim sort of way this is a boon for the tent caterpillar. The slaughter is so complete that, ironically, it assures that the tent caterpillar as a species will survive. For, in wiping out the tent caterpillars, the fly also wipes out all other parasites. A small number of tent caterpillars escape and produce the next generation and now Sarcophaga is their only foe. But the caterpillar population is so low that now even the killer fly can’t find them, and its population also slumps to near-zero.
The tent caterpillar, in defeat, has actually won. Its armies are reduced a millionfold, but the comeback ahead is practically unopposed and in another decade the caterpillars return in their billions. ★