Chapter 15. Harmful Soil Insects and Other Pests

Any natural, uncultivated field or forest soil without insects living in it would probably be poor. However, a carefully tended Garden­er's Loam without insects and other harmful pests (or at least a minimum population of them) is not only possible but desirable.

Discovery of chlorinated hydrocarbons of high potency (be­ginning with D.D.T. during World War II) gave us weapons of amazing efficiency to use against harmful soil-inhabiting pests. Properly used (they can be abused) these are not only safe to handle but endure for years in soil. No gardener can really appreciate their value until he has seen a lawn completely freed of grubs for four or five years with a single application of a chemical such as chlordane.

Such potency against insects naturally raises fears of the effects of these chemicals on human beings and animals. Oddly enough, most insecticide materials of this type are actually less dangerous to higher animals than are older chemicals they replaced. A case in point is chlordane which took the place of lead arsenate as a turf treatment against grubs. Chlordane is so safe that it is approved for use (with only normal caution) as a household insecticide, for spray­ing living quarters, basements, kitchens and other areas where the old lead arsenate would be highly dangerous. Arsenate of lead has been responsible for many deaths: I have yet to hear of a single person being killed by chlordane.

Other equally effective chemicals are available for use on insect species not too easily killed with chlordane. Most of these materials are rather specific in their action and not as useful as chlordane for a general insect-proofing treatment.

Insecticide Residues

Buildup of harmful insecticide residues in soil is possible, even though the material used is quite safe to apply. Such accumulations are almost always the result of misuse and failure to follow di­rections.

Insecticide residues harmful to plants are not new; injurious ef­fects of copper following regular use of Bordeaux mixture on grape­vines in France were reported in the nineteenth century. One of the earliest such reports in America goes back to 1908—it dealt with arsenate of lead accumulation in an apple orchard.

Soil type has a great deal to do with harmful effects of such residues. For example, the home owner who has established a lawn after years of work on a sandy loam, only to find it infested with grubs, would do well to think twice before using arsenate of lead or calcium arsenate as a grub control.

Several years ago, hi connection with experimental work on cal­cium arsenate as a long-term crabgrass control, I went back over the literature to discover what harmful effects calcium arsenate might have had on grasses when used in heavy applications. I found that grass was amazingly tolerant, except on sandy soils. On clays and clay loams, cereal-type grasses and others were not injured at rates that ran into thousands of pounds of calcium arsenate per acre, ap­plied over a two- or three-year period. Soy beans withstood doses of as high as 30 pounds of actual D.D.T. per thousand square feet on clay, but were injured on sandy soils by doses as low as 8 pounds per thousand square feet. If available iron was added to the soil, injury on sandy loam did not show up at calcium arsenate rates lower than 15 pounds per thousand square feet. In New Jersey, lima bean, snap bean and turnip seedlings were killed by applications of 30 pounds per thousand square feet. The few seedlings that did survive, how­ever, were able to grow into normal plants once their roots pen­etrated past the soil layer on which the calcium arsenate had become fixed.

Potent Pair

Today, of older materials used for soil treatment, only calcium arsenate and lead arsenate are of much importance. Both are being used as pre-emergence crabgrass controls at rates of about 10 pounds to a thousand square feet of lawn area. When applied to lawns, these two chemicals form a layer of treated soil in which germinating crabgrass seedlings cannot survive. They work by sub­stituting for phosphorus, starving the seedling for that element.

The correction of arsenate toxicity is obvious: heavy applications of superphosphate will undo practically any case of toxic soil due to either of these chemicals.

Each year sees a decrease in reports of lead and copper toxicity as these older insecticides are dropped from modern spray schedules. Today, our concern is with residues of chlorinated hydrocarbons such as D.D.T. and chlordane. A further problem is that of residues from chemical weed killers, which is covered separately in Chapter Sixteen.

The Big One

The most serious residual problem likely to be experienced by home gardeners is from D.D.T. used for soil treatment. Grasses are highly tolerant: doses of as high as 30 pounds to a thousand square feet have shown no injury. Theoretically, since D.D.T. is used largely to grub-proof lawns, there should be no human hazards from overdoses. But, of course, grass plots may not always remain as grass plots; if they are converted vegetable gardens, the D.D.T. overdose hazard becomes real.

In my own case, I converted my vegetable garden into a lawn area when we acquired a summer home. However, I can see a time when, with children grown and married, we may give up summer jaunts and resume vegetable gardening. If you, too, foresee the possibility that food crops may someday be grown on present lawn areas, the use of large doses of D.D.T. for grub-proofing and so forth would seem unwise. On lawn areas which are not so likely to be plowed up and converted, this chemical (which costs much less than chlordane) is the preferred treatment for many soil insects.

Except where cost is a factor, as when a large lawn must be treated, chlordane is the most satisfactory soil pest control for amateur use.

Benzine hexachloride, called BHC for short, is widely used on farm crops. It has no place in the home garden. Less than a pound to 1,000 square feet will seriously injure many plants. It contam­inates all root crops so strongly that they cannot be eaten.

SOIL-INHABITING INSECTS

Many insects spend all or part of their life cycle underground but do not properly come within the scope of this chapter. For example, the tomato hornworm, a thick, ugly green insect familiar to anyone who grows tomatoes, does go underground to pupate but since it does not feed there, and cannot be controlled by soil treatment, it is not included in my discussion of soil-inhabiting insects. Proper to this chapter is the May beetle or June bug; it spends three years underground, feeding and injuring plants, during which period it can be controlled with soil insecticides.

The best way to attack insects that spend part of their life cycles underground and cannot be reached by chemicals is to dig or plow all areas not in use as soon as possible in fall, leaving clods and lumps rather than raking the areas to a level surface. If you delay this job until killing frosts have occurred, it will not be too effective. Most insects have laid their eggs by mid-September in the North. I find that control is much more complete if the soil is turned twice, or, better, three times before the first crusting by frost occurs. This exposes insect pests, as well as weed seeds, to migrating fall birds, many of which leave the North before frost comes. Fall rains wash out buried eggs, exposing them to frost action and to birds that re­main over winter.

Pests Controlled in the Soil

Ants: Treatment of soil with chlordane disposes of many ant species annoying in the garden. Soil under peonies especially should receive this attention to kill ants that take honeydew from the buds. If ant nests can be located, dusting or spraying with chlordane will mean much better control, as this destroys young ants which might otherwise not be reached. When ants march in from surrounding woods or from other properties, use sweetened ant poisons or pre­pared baits.

White Grubs: Many species invade gardens and lawns. Larvae or grubs of the May beetle or June bug are particularly destructive.

The grubs spend parts of three years underground. Eggs are de­posited in June and hatch a month later into larvae that feed on decaying vegetable matter and on roots of plants. They continue feeding through a second year, during which they do severe damage to lawns. Early in spring of their third year, they stop feeding and turn into adult beetles, emerging to begin the life cycle over again.

Application of chlordane or calcium arsenate will destroy the first and second broods of grubs, but may not show much effect on the brood that is about to emerge as adult beetles in May and June. This misleads those who have used chlordane; they think it is of no value. However, for at least four years following date of treatment, no living grubs will be found, once the spring brood has emerged.

In lawns, grub injury may be limited (a paler green color in patches may indicate loss of roots and poor nutrition due to feeding of second-year brood), or may be severe. When severe, the roots often are cut off just below the surface so sod can be rolled back like a carpet, exposing feeding grubs underneath. In less severe cases, removing a square foot or two of light, weak sod and digging into the soil should be enough to reveal whether the dirty-white curled larvae with blackish heads are present.

Japanese Beetle: These differ from June bugs in their life cycle, but, like them, do severe damage by eating at plant roots when underground. Japanese beetle grubs spend 10 months in larvae form, surviving one winter. In late May or early June the grubs stop feed­ing and pupate, emerging as adult beetles. They live for little over a month, feeding on leaves and flowers and laying eggs, usually select­ing lawns or other grass areas for the latter purpose. Late-hatched adults may survive as late as early October.

Although these beetles are voracious eaters of prized flowers and foliage, we are concerned with the underground, root-eating period of their life cycle, which begins when the young hatch from eggs during late summer. By fall the grubs are half grown. They feed on roots until checked by cold, resuming again in spring. D.D.T. treat­ment will control grubs in soils for up to six years, while chlordane will do so for at least four years. Even longer effectiveness is provided by a biological control measure—the introduction into the lawn of the so-called milky disease. Prepared spore cultures are on the market.

Termites: Although these pests are usually considered destructive only to foundations of houses, they can become serious pests in soils containing any amount of partially decayed wood or other woody matter. They work beneath the surface and burrow inside stems and are not detected until dying plants lead the gardener to investigate. They are much more troublesome where organic fertilizers are used.

To control, remove any wooden stakes, edgings or other sources of cellulose, then dose soil with chlordane. Where termites are active, be sure to protect fences, stakes and other wood used in gardens with a wood preservative such as copper naphthanate or pentachlorophe-nol. Avoid using organic fertilizers.

Slugs and Snails: Baits containing metaldehyde will lure these pests out of their holes to feed and die. Remove flat stones or shingles under which they hide. Dusting paths and walks with hydrated lime or dry wood ashes often traps them and dehydrates them. Sometimes dabs of either material placed on the soil and covered with a shingle will trap them by the dozens.

Millipedes, Sowbugs and Pillbugs: Chiefly these are signs of poor housekeeping in greenhouses: remove all organic matter and dust under benches with chlordane. (Dusting with chlordane is also the outdoor remedy.)

Sweet Potato Weevil: Grubs burrow through sweet potato roots and stems. Apply one pound of 10 per cent D.D.T. dust to 1,500 square feet of garden or use 20 ounces of SO per cent wettable D.D.T. to a gallon and a half of water, applying this to 1,000 square feet.

Wireworms: Hard, buff or ecru-colored segmented worms are very destructive to root crops. Same treatment used for sweet potato weevil will control wire worms.

Relative Importance

Many of the more destructive insects are sheltered and fed by partially-decayed organic matter. This is one reason why compost­ing plant wastes in a compost pile is better practice than applying them to soil and plowing under. In the compost pile, insect control measures can be more easily taken. The use of calcium cyanamid in the last turning of the pile will check breakdown temporarily, but will get rid of grubs.

Earthworms (see Chapter Ten) will be killed by most of the previously mentioned treatments.

Many Moles

Moles are of minor importance except where they are abundant, when they can be a real nuisance. You may be surprised to learn that moles do not eat roots, tulip bulbs, and so on, although often blamed for disappearance of these plant parts. Moles do approach or touch underground parts of plants as they burrow through soil looking for insects on which they feed. In doing so, they often open a passageway for mice or ground squirrels which do eat bulbs.

Because they feed solely on insects and other soil animal life, moles can best be driven out of a lawn or garden by treating the soil with insecticides such as chlordane, dieldrin, calcium arsenate, ar-senate of lead, or DDT. With their natural foods eliminated, moles will quit burrowing through the treated soil.

NEMATODES

The tiny (sometimes microscopic) thread-like creatures called eelworms or nematodes (nemas to the scientist) are the most mys­terious and least understood of all creatures inhabiting the soil. Zoologists regard them as a race apart from other worm-like soil dwellers. They were known to early microscope workers, but con­sidered of no economic importance and generally ignored until about 30 years ago.

Some animal-parasitic nematodes are beneficial, but practically all plant-parasitic species (both leaf and root nematodes) are in­jurious. And contrary to common opinion, most nematodes are not killed by freezing. There are nearly 10,000 known species of nema­todes, several hundred of which are known to attack plants. (Much work is yet to be done in classification.) Because of the ease with which these creatures mutate and produce new forms, the nematode threat is enlarging.

The first recognized soil-pest menace to plant life in America (or at least the first considered important by Federal authorities) was the importation of nematode-infested narcissus bulbs from Europe in 1926. In that same year a disease called spreading decline was classified as a serious threat to citrus grooves in Florida, but was not associated with nematodes at the time.

All during the 1930's the science of nematology was relatively neglected. Then in 1941 the discovery of a full-scale infestation of a species called golden nematode on Long Island set off a furious "crash program" to bring our knowledge of nematology into line with the threat presented by these creatures. The golden nematode, known to be a dangerous pest of potatoes, is one of the cyst-forming species, which possesses a peculiar type of female immortality. Older females do not die but form an egg-like cyst which can remain dormant for years. This cyst then regenerates when conditions are favorable. Once in a soil, cysts can survive for decades, without any outside source of food or water. Years after the soil has been fumi­gated and treated with nematocides, cysts may still regenerate, so a constant watch and recheck of the area must go on.

In 1953, the serious spreading decline of Florida citrus was traced to the burrowing nematode as the carrier. Since that time, nearly $3,000,000 has been spent in an effort to eradicate this pest from citrus groves, but the end is not yet in sight.

In the Garden

We now know that every commonly-grown fruit and nut crop is attacked by one or more nematode species. Vegetable crops are parsitized regularly, even in the North, and damage done to these food crops is estimated at over $100,000,000. It is on these crops that the home gardener is most likely to encounter nematodes. They form knots or lumps on the roots of many plants, most conspicuously on carrots. Other crops are attacked internally without visible symp­toms other than a decline in vigor and productivity. Tomatoes, beans, okra and many vine crops can drop as much as 50 per cent in productivity without any sure sign of infestation. Foliar symp­toms on many plants, such as premature dropping of leaves, stunting and yellowing are not clear-cut enough to mark them as different from the symptoms caused by certain nutritional or insect disorders.

Ornamentals are also attacked. Over 600 different species of trees, shrubs, flowers and lawn grasses are known to attract nematodes. The root-knot nematode alone attacks more than 500 different spe­cies, causing severe loss of plant vigor. One of the worst in its effect is the foliar nematode that attacks chrysanthemums in the green­house bench. It is able to move up stems and across leaves on the film of water that forms on the leaf by condensation. When it reaches a stomatal opening, it penetrates into the heart of the leaf where it is all but immune to any form of control.

During the past two years many members of the American Rose Society have become increasingly aware of the damage done by nematodes, and numerous articles on these pests have appeared in the Society's journal.

WHAT CAN BE DONE?

The nematologist in one southern state told me recently that he had never checked a garden soil without finding one or more nema­tode species parasitic on plants. He was reasonably sure that no such thing as a nematode-clean soil existed in his state. Most scientists feel that once a soil has become infested, complete eradication is almost an impossibility, even if only non-cyst-forming types are involved.

Since eradication is impractical the one course open to us is to reduce the population to a point where plants can survive and grow reasonably well in spite of some infestation. The first step in such a program should be exclusion. Since nematodes are so widely dis­tributed throughout the United States, this might seem like locking the barn door after the horse is gone. However, the introduction of new and more virulent species is always a possibility. I feel certain that the use of southern-grown tomato and cabbage plants in the North has been a means of annually reinfesting many growing fields with nematode species which do not ordinarily survive northern winters.

A case in point regarding infestation of soil by introducing plants is that of Radopholus similis, the burrowing nematode which causes the spreading decline of citrus. This nema does not survive in the North, but is known to infest more than 100 ornamentals, many of them house plants which are shipped to northern markets from Florida. This pest is showing up in greenhouses in the North, ap­parently brought in on Florida foliage plant shipments.

Chemical Treatment

Formerly we were limited to controls which could be used only when the soil was not occupied by living plants. These included applications of chloropicrin, dichloropropene, ethylene dibromide and similar volatile chemicals. Newer chemicals of this type include Dow Telone, Shell D-D, Vapam and Mylone. These are employed on greenhouse and coldframe soil used for propagating purposes (where the chemicals can kill 100 per cent of all nematodes) and for treating rows in the nursery (where complete control is not achieved, but nematodes are set back sufficiently so plants can make near-normal growth).

There is another chemical, scientifically designated as 1,2-di-bromo-3-chloropropane, and sold under the trade names Nemagon and Fumazone, which can be used around living plants without injuring them. Also, certain phosphate insecticides are available that can be applied to the soil for absorption by the plant to kill internal nematodes. To date, however, chemical control is not 100 per cent effective.

Natural Controls

Several years ago, I was at the U.S.D.A. station at Beltsville, calling on Dr. Charles Wechsler, a root-rot specialist. He had been studying nematodes as carriers of disease organisms, and at the moment happened to have a culture which contained both nematodes and a ring fungus. He asked me to look in the microscope and ob­serve the natural death of a nematode. As the nema poked its way into the odd ring-shaped organ on the fungus, the ring closed, trapping its victim so it could not escape. This was my first introduc­tion to the fungus enemies of nematodes, which I discovered were fairly numerous. Some, like the specimen I saw, actually close on the body of the nematode. Others work like an eel-pot; these have a ring that allows the victim to enter with its head, and then trap it so it cannot move either forward or backward.

Other fungus species have been aptly named Lethal Lollipops. These have a ball-like structure covered with a sticky substance on which the nematode is caught like a fly on sticky flypaper.

These various fungi exist in all soils high in organic matter. They tend to die out, however, if moisture content of soil fluctuates up and down; they prefer a uniformly moist home. Their action against nematodes comes under the heading of biological control.

I have discussed with several nematologists the possibility of con­trolling nematodes by encouraging the ring and lollipop fungi. The consensus is that while these fungi are useful in keeping down the population, they tend to be self-limiting, since when the population is reduced only partially, the fungi tend to die out. Apparently, they depend upon nematodes' bungling into their traps, and if not enough victims are around, they starve.

There are predatory nematodes too; they feed on their plant-eating nematode relatives, but cannot multiply to a point where they will effect full control. In other words, with nature's help, science is fighting a holding battle while working toward a total solution.

Chapter Digest

Modern chemicals have given the gardener some highly potent weapons for his endless campaign against injurious soil-inhabiting insects and various other creatures. However, there is another side to the coin: these chemicals have harmful effects if misused. Where possible, natural (bio­logical) control measures should be employed in partnership with chemical warfare. The discussion of controls for specific enemies gives greatest emphasis to nematodes, for they pose a grave and too-little-understood threat to almost all kinds of plants in all parts of the country.