The Nature of Garden Pests

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Just as a weed is any plant growing where we don’t want it, a garden pest is a pest only by virtue of our opinion of it. If aphids preyed on parasitic wasps instead of vice versa, we would rush to order them from slick garden catalogs at an astronomical price per pound and give them the run of our greenhouses. If slugs suddenly became ravenous for dandelions in the lawn instead of lettuce in the garden, we would happily set up little beer busts for them every Friday afternoon instead of trying to drown them in the brew in the dark of night.

Grasshopper (order orthoptera, family acrididae)

Of course, nature being what it is, it’s difficult to separate our feelings about other creatures from our own self-interest. If you’ve watched grass­hoppers consume your entire crop of basil with the avidity of yuppies eating pesto, you might find it hard to care that their hopping ability is roughly equivalent to 6-foot-tall humans with very large thighs leaping 90 feet from a standing position.

On the other hand, if you expect to have to coexist with an extremely large number of grasshoppers this summer, understanding them better might make the relationship more tolerable. You might appreciate knowing that they have a brain in their head, albeit a wee one. It’s little more than a tiny tangle of nerves, actually, with short optic nerves going to the two large, keen compound eyes and the three or so small simple eyes (which are for sensing very short-range images or simple light/ dark shapes). A couple more nerves go to the antennae, with which they might smell as well as feel, and that’s about it. Like the proverbial chicken, a grasshopper can continue to breathe, walk, and fly for quite some time ­without its head— because it has other “brains” in its thorax and ab- domen to control those functions.

If you’ve ever tried to catch grasshoppers, for fish bait, vengeance, or just fun, you might find it useful to know that some of the common garden varieties make themselves scarce by clamping their wings together and dropping straight to the ground in midflight while you, their clever predator, are watching the trajectory traced by their brightly colored underwings and positing a landing place several feet beyond where they have actually come to rest.

Then, if you’ve actually succeeded in catching one of the above grasshoppers, you might be relieved to know that its leg comes off in your hand on purpose; the creature has special muscles for popping off endangered extremeties, and small diaphragms at the joints which close the wound to prevent blood loss or infection. And the disagreeable substance it might have disgorged onto your hand while shedding its leg is not tobacco juice, but the entire contents of its foregut or in some cases a foul secretion from a special gland for offending enemies.

Grasshoppers have a reputation for being improvident, for fiddling away the summer (rasping hind legs on tough forewings to attract a mate) while diligent ants store up grain and such for the winter. We should be grateful. What if they not only ate all the basil they could, but also carried the rest down a hole?

As for those tiny, perfectly formed, rather adorable hoppers that you begin to see in late spring, they don’t eat a thing. Just wait for a couple of months and four or five molts.

Whiteflies (order homoptera, family aleyrodidae)

If you have house plants or a greenhouse infested with whitefly, the good news is that you probably have only one kind, the greenhouse whitefly, instead of all the 1200 or so possible ones that belong to the genus. These dainty creatures, no more than 1/16 inch long, are harmless enough at the point in their lives that you’re likely to notice them flying up in little clouds around potted plants when you water or otherwise disturb them. In fact, they’re quite lovely—waxy, translucent, ethereal, with oversized sleek wings and microscopic antennae of seven segments each.

The bad news is that these minuscule creatures will, with or without the help of a mate, lay semicircular clusters of oval eggs on the underside of tender leaves, and these eggs will hatch into disgusting, hoary, blobby little nymphs, with none of the grace or beauty of their parent, that can suck the vigor right out of the host plant before you even notice that they’re there. In a warm environment, the cycle from egg to nymph to non-feeding pupa to productive adult can be completed in as little as two or three weeks.

“Warm environment” is the operative term with whiteflies. They are native to tropical climates, and come to temperate zones as stowaways on imported plants, and are perpetuated on houseplants and in greenhouses. They really can’t survive even a light freeze, but then maybe their host plant can’t either.

To enjoy the presence of these tropical visitors, examine an egg cluster with a magnifying glass. Enjoy the neat patterns in which they’re positioned. Marvel at the tiny, slender stalk each one sits atop. Wonder how they do that.

Slugs (order stylommatophora, family limacidae)

Nobody loves a slug. These slimy, boneless gastropods, which consist of little more than a big “foot” with a mouth and stomach, range in size from pinheads to foot-long bad dreams, and come in tens of thousands of varieties. Unfortunately for the ones that encounter humans with salt shakers, their skins are permeable and give them little protection. That’s one reason they produce such prodigious amounts of mucus from glands all over their bodies—to protect themselves from drying out. The other reason is navigational; they need a slippery trail to squish along on their soft bottom surfaces.

What is it about eyes that causes such a strong positive response in humans? Stick a couple of limpid, expressive eyes on any old thing and we immediately sympathize. Its eyes, even though they’re beady little items poking oddly up on slender stalks and operating independently, are what prevent slugs from being completely odious in appearance to many of us. The smaller tentacles located just next to the eye stalks are for smelling and feeling.

Eyes aside, the most interesting part of a slug is the part you’re not likely to see: the sole of the foot. It’s covered with fine, short cilia that help the creature move along, and at the forward end is a mouth that is surprisingly intricate. This small, round orifice has strong jaws and a long tonguelike structure called a radula which has thousands of tiny, raspy teeth. The radula not only reduces food—your whole lettuce crop, perhaps—to manageable particles, but also carries it into the avid and waiting mouth.

While most of us find little redeeming value in slugs, they do serve at least one useful purpose in the world of herbs: they pollinate Canadian wild ginger (Asarum canadense). Other than that, perhaps the best that can be said is that they prompt us to get down close and intimate in our ­gardens. And in the process of waging slug battle, we see sights we otherwise might overlook.

Slugs generally spend all day in dark, moist places. They hibernate all winter, and if the summer is excessively hot and dry, they might also go into summer hibernation (aestivation), too. So aside from eating, what do their lives consist of? Sex, of course. Go out at night with a flashlight, and you might find a pair of slugs engaged in courtship and mating—a surp­risingly lengthy and complex affair. Slugs are hermaphroditic, each capable of producing both eggs and sperm. Depending on their species, they might dance and circle around each other, caress, and press the soles of their big feet together before piercing each others’ bodies with sharp spicules that stimulate sperm production and then inserting their respective penises into the others’ appropriate openings. All this is accompanied by massive amounts of slime, and might go on for hours.

That’s just the way it is with slugs.

Aphids (order homoptera, family aphididae)

Aphids are simple-looking little creatures with lives full of drama and plot. On the face of things, they simply suck enormous quantities of plant juice through their slender, two- or four-tubed proboscises and excrete correspondingly enormous amounts of sticky, sweet “honeydew”. Some species actually have waxy coatings to keep from drowning in their own excrement, and many are attended by ants, wasps, and other insects that are attracted by the saccharine effluent. On closer examination, though, aphids can be seen to have a life cycle of great mystery and wonder.

Aphids hatch in early spring from eggs that have typically wintered over on woody stems and tree bark. Their appearance coincides with the emergence of tender new green growth, which they probe their little sapsuckers into straightaway. They are all females, and all wingless. Within a week or two, these first females, or “stem mothers”, begin to produce young at the rate of several a day. Not eggs: they produce tiny living clones of themselves, also female, also wingless. These offspring mature quickly, often beginning to reproduce in a week or less. And so on. It’s no wonder our flat-leaved parsley or fennel seems to develop huge aphid populations almost overnight.

Now, all the while these mother aphids have been replicating themselves indiscriminately, they and their progeny have also been sucking the life out of their host plant. Just when they’ve almost eaten themselves out of house and home, a generation appears that is also female, but winged. This crop of babies flies away to a new host plant, which it proceeds to populate with wingless females. And so on.

For at least some species, another curious pattern of behavior comes into play: rotating food sources. The hop-aphid, for instance, has been observed to live for three generations on plum trees, then for eight generations on hopvines, after which it migrates back to plum trees for three or so generations more.

You may have been wondering whether make aphids exist or whether sex is superfluous among this group of matriarchs. Not to worry. Just as the season is ending, a generation is born that includes both females and males, both somewhat smaller than their predecessors, with no mouth parts. They don’t need any; their job is to mate and produce eggs before winter.

What I’ve described is a typical life cycle of the sort of aphid you’re likely to find among your herbs. Other sorts exist that have truly remarkable relationships with ants. The eggs of corn aphids, rather than wintering over in exposed fields, are carried underground by ants and tended along with ant eggs and larvae until spring. Because they hatch before there is young corn for them to live on, the ants tunnel in among the roots of smartweed and move the hatchlings there to feed. When the young corn has germinated, the ants once again move the sluggish aphids to where the best meal is and take their reward in the copious honeydew which the aphids produce.

Aphid infestations may have pushed you beyond enjoying the oddity of their nature; well, it could be worse. In 1943, clouds of winged aphids, all from one migrating generation, descended on lower Manhattan, plastering shop windows and cars and blanketing pedestrians. Now that’s aphids.

Spider Mites (order acarina, family tetranychidae)

While it’s a bit silly and sentimental to project human attributes onto lower orders, it’s also hard not to think of spider mites as tiny thugs. Though less than 1/64 inch long, their mouthparts can pierce the toughest leaf surfaces, the males fiercely defend egg-laying females, and in general they act a lot like the aggressive spiders they’re related to, albeit on a microscopic scale. Even the bristles on their abdomens—which function as sensors—make them look tough. Not that you’re likely to see this kind of detail without a microscope. Spider mites are rarely spotted on a plant before their damage becomes apparent. And it’s often hard to be sure they’re there without shaking the plant over a piece of white paper and observing whether any of the bits and specks that fall off try to crawl away.

Somewhat more readily observed are the delicate mats of very fine ­webbing. Rather than thinking of these as disfigurements, try to imagine the female spider mites extruding the gossamer threads from spinnarets near their mouths and weaving them into shelters for their eggs and developing young. Given time, they can completely ­encase a leaf or whole plant in webbing—a prodigious output of silk from such minute creatures.

The common two-spotted mite is often called red spider mite, though it’s a rusty red only at less active times during its life cycle; more usually it’s pale ­yellow or greenish.

Consistent with the belligerent nature of their kind, spider mites can be controlled by being pitted against other predatory mites that aren’t herbi­vorous. The resulting Armageddon would be sight to behold, if one could.

Underwings and positing a landing place several feet beyond where they have actually come to rest.

Then, if you’ve actually succeeded in catching one of the above grasshoppers, you might be relieved to know that its leg comes off in your hand on purpose; the creature has special muscles for popping off ­endangered extremeties, and small ­diaphragms at the joints which close the wound to prevent blood loss or ­infection. And the disagreeable substance it might have disgorged onto your hand while shedding its leg is not tobacco juice, but the entire contents of its foregut or in some cases a foul ­secretion from a spe­cial gland for offending enemies.

Grasshoppers have a reputation for being im­provident, for fiddling away the summer (rasping hind legs on tough forewings to attract a mate) while diligent ants store up grain and such for the winter. We should be grateful. What if they not only ate all the basil they could, but also carried the rest down a hole?

As for those tiny, perfectly formed, rather adorable hoppers that you begin to see in late spring, they don’t eat a thing. Just wait for a couple of months and four or five molts.

Selected Reading

Coldrey, Jennifer. Discovering Snails and Slugs. New York: The Bookwright Press, 1987.
Evans, Howard Ensign. Life on a Little Known Planet. New York: E.P. Dutton, 1966.
Farb, Peter. The Insects. New York: Time-Life Books, 1962.
O’Toole, Christopher, editor. The Encyclopedia of Insects. London: Equinox (Oxford), 1986.
Tinbergen, Niko. Curious Naturalists. New York: Basic Books, 1958.
Wootton, Anthony. Insects of the World. London: Blandford Press, 1984.