Overlooking tiny spider mites on a plant will be difficult once they've covered the leaves with their fine webbing.
By Tom Quirk
Controlling these pests is the key to success in the garden — for you and your plants.
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.
Nevertheless, this isn’t the case with many pesky garden bugs. Here’s some insight into the detailed nature of these organisms and some suggestions for getting what you want without giving them what they want!
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, 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 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 abdomen 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 mid-flight 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 extremities, 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 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.
If you have houseplants or a greenhouse infested with whiteflies, the good news is that you probably have only one kind — the greenhouse whitefly — instead of the entire 1,200 or so that belong to the genus. These dainty creatures, no more than 1/16 inch long, are harmless enough at the point in their lives when 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 and 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 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, 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.
And when you’ve had enough of the egg clusters, you’ll likely want to remove them before you’re graced with many more whiteflies. Completely remove any and all affected foliage. Wash remaining stems and leaves with insecticidal soap. The sticky, yellow traps available at hardware or garden stores will attract and put an end to airborne whiteflies.
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 eyestalk 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. To catch and capture slugs, take a flashlight to the garden an hour or so after sundown. You can remove the slugs and dispose of them in a bucket of soapy water, or if you can stand the messy aftermath, sprinkle salt on them. Another option is the use of natural slug and snail control products or traps that lure slugs out. These products are likely available at your local garden store but check the labels to make sure they’re not harmful to pets or wildlife.
Another plant protector against slug feasting is the material on the soil’s surface surrounding your plants. Remember the smooth “foot”? Creating a rough area of sharp edges will cause slugs enough discomfort to steer clear of your plant. Try sprinkling sand, eggshells, broken snail or sea shells, diatomaceous earth or hardwood shavings around plants to serve as a barrier.
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, 2- or 4-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 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 per day. But 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, laying eggs in winter to hatch the following spring and continue the tremendous bug-reproductive and plant-counterproductive process.
For at least some species, another curious pattern of behavior comes in to 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 hop vines, after which it migrates back to plum trees for three or so generations more.
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 that 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.
To rid your plant of these pesky pests, begin by removing all infected leaves. Using your fingers and warm, soapy water, dislodge any remaining pests or sticky residue from stems. For a large plant, spraying with a strong jet of water several days in a row with particular attention to the undersides of leaves may also help remedy the situation.
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 detail without a microscope. Spider mites rarely are 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 spinnerets near their mouths and weaving them into shelters for their eggs and developing young. Given the 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; 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 predators that aren’t herbivorous. The resulting Armageddon would be a sight to behold, if one could. However, if you’re not interested in adding more bugs to the mix, spraying insecticidal soap or citrus oils on them dries out the outer layer of these pests. A strong blast of water can help control infestations as well. Some garden hose adapters are especially suited to spraying the underside of the leaves. Whether you choose insecticidal soap or water, repeat the application for several days.
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