Controlling Garden Pest Populations

Habitat is also important as a limiting factor for animal
populations. For example, when garden slugs are reproducing
beneath boards that are placed directly on the ground as
walkways in wet weather, by simply placing fine dry sawdust
under the boards you can reduce the slug population while
avoiding the problem of muddy footpaths at the same time.

Biological controls, including diseases and natural
enemies, are particularly important in keeping down pest
populations. While most gardeners are aware that toads,
spiders, some birds, and other predators eat insects, it is
not as widely recognized that an insect’s major natural
enemies are other insects. The natural enemies can be
broken up into two groups: predators (which usually eat a
lot of their prey at one sitting, and eat many different
species of pests), and parasites (insects which live on, or
inside, another insect, lay eggs inside, and usually cause
the host to die). Examples of predators are ladybird
beetles (Coccinellidae), lacewings (Chrysopidae and
Hemerobiidae), and syrphid (Syrphidae), or hover, flies.
The parasites are so tiny that most people are not aware of
them and they have no common names. Most are members of the
order Hymenoptera and are referred to as miniwasps.

The distinctions are important because parasitic insects
are extremely “host specific”. For example, the parasite
that attacks the cabbage aphid doesn’t attack the rose
aphid. These are the “silver bullets” of pest management,
and learning how to conserve them, import them, and release
them is a very important pest management technique. They’re
already out there; in fact, they are the reason you’ve been
raising anything at all, because without them all your
plants would be devoured by the herbivorous insects. The
cardinal principle of good pest management, therefore, is
to disrupt these natural controls as little as possible, or
you may end up making things worse instead of better.

Unfortunately, the injudicious use of synthetic chemical
insecticides may do just that: make the problems worse or
create some where none were before. These results can occur
in several ways.

Resurgence is one common effect of an insecticide
treatment: You see some bugs on your plants, you treat them
with a pesticide, and in anywhere from a few days to a
number of weeks later, the same kind of bug is back again,
only this time in higher numbers than before. What
happened? Well, you accidentally killed of the pest’s
natural enemies along with the pest. It is rarely possible
to kill off all the pests, and without any predators or
parasites to hold them in check, those that didn’t die, or
those that flew in from your neighbor’s yard, were able to
multiply without restriction.

The pests are plant feeders, or herbivores, and tend to
move around less than their predators, the carnivores. The
predators must actively seek their prey, thus covering more
ground and coming into contact with more of the poison.
Furthermore, there may be a lot fewer predators out there
than pests. Each ladybird beetle must eat a lot of aphids
to survive, so there will only be as many as its food
supply can support. Also, the predators may take longer to
reproduce themselves.

Another reason that the natural enemies do not “resurge” as
quickly as the pests, after pesticide treatments, is that
occasionally a material is selectively more toxic to the
predators and parasites. Research shows that this is the
case with carbaryl (marketed as Sevin), which is
particularly poisonous to bees and some other Hymenoptera,
the order which includes parasites of such insects as
aphids, scales, mealybugs, and caterpillars. Thus, as a
consequence of the use of carbaryl, the pest insects may
actually resurge to greater numbers than before.

Resistance is a second major effect of exclusive reliance
on synthetic chemical insecticides. Here we are, devilishly
concocting the most exquisite pesticides in our
laboratories, at enormous costs, and somewhere out there,
in every population of pest insects, there is an insect
that is resistant to it. Each time you spray an insect
population, you change the balance between susceptible and
resistant individuals, because all the resistant insects
survive to reproduce, while most of the susceptible insects
are killed. Therefore, the more you use a pesticide to
control insects, the faster you produce a population of
resistant pests.

Secondary pest outbreaks are a third problem caused by
injudicious use of insecticides. To understand this you
must remember that there are insects everywhere, on every
plant, that you are not aware of. All the time that you are
focused on and concerned with the highly visible pests,
there are small populations of other insects that could
cause problems if they were not under good control by their
own natural enemies. When you treat the area with an
all-purpose spray, there is a very good chance you will
kill off some of the parasites and predators of these other
potential pests. Then, suddenly, you’ll have a whole new
problem on your hands.

MANAGING INSECT PESTS: AN ECOSYSTEM VIEW

All of the above problems are really a result of taking too
narrow an approach to managing insect pests. What is needed
is an overall view that takes into account the entire
ecosystem of which the plant and the insect that feeds upon
it are a part.

In general, a garden is an ecosystem of living (biotic) and
nonliving (abiotic) parts. Light, air, water, and minerals
from the soil are converted into living matter by the green
plants, and the plants are fed upon by herbivores, or
plant-eaters. These, in turn, are fed upon by carnivores,
which are fed upon by secondary carnivores, which may fall
prey to tertiary carnivores. The predators and parasites of
insects described above are all part of such a chain. All,
in turn, are fed upon by the detrivores, or decomposers,
the fungi and bacteria which break down biotic materials
and return them to the soil where they become available to
plants once again.

INTEGRATED PEST MANAGEMENT

Integrated pest management is a decision-making process
which combines all possible strategies with the aim of
suppressing the population of a pest below the level where
it causes economic or aesthetic damage. The strategies may
be cultural, physical, biological, or chemical. An
essential aspect of this approach is monitoring your garden
to determine what level of pest population is a problem.
This means going out and making counts. You must figure out
how many insects there are per plant, or per leaf or stem,
and then watch to see if their number increases. When the
population is a certain size, and you determine that the
plant damage is intolerable, you should then do something
about the situation. Your goal is really to suppress your
pest population to below that level. We’re not talking
about eradication, but pest suppression, which will keep
the ecosystem from oscillating wildly, and will allow it to
cycle at a considerably more moderate level.

Examples of some cultural controls are the selection of
varieties resistant to the pest, and management of watering
and fertilizing techniques. Aphids, for example, are very
sensitive to nitrogen levels in the plant. By
overfertilizing you can actually increase their
populations. On the other hand, thrips (order,
Thysanoptera), tiny juice-feeding insects that often look
like straight dark lines approximately 1/16 of an inch
long, may appear in response to dryness. By increasing the
water to the plant, you may reduce the damage that they
cause. We have used this method in managing thrips found on
tomatoes.

A common physical means of control is pruning, either to
remove the insects physically, or to remove the favored
habitat of an insect, such as the inner canopy of sucker
growth on a tree, which is favored by certain aphids.

Biological control is the manipulation of the biological
components in the ecosystem. This may mean enhancing
natural controls already present, or introducing missing
components that would be desirable. For example, there is a
naturally occurring disease of certain caterpillars called
Bacillusthuringiensis , marketed under
the trade names of Dipel, Thuricide, and Biotrol. If
sprayed on a plant when the caterpillars are large enough
to feed on both sides of the leaf, it will give good
control of the pest without harming any of its natural
enemies.

Many people buy carnivores such as ladybird beetles or
praying mantids to release in their gardens as general
predators. However, if not previously defatted, the
ladybird beetles must fly to burn off stored body fat
before feeding, so they will shortly leave the area in
which they are released. Mantids are very unselective in
what they eat and are probably better bought for purposes
of observing and learning about insects than in hopes that
they will satisfactorily control pest populations.

Green lacewings, excellent general predators, can also be
bought commercially. Because they are shipped and released
in the egg stage and hatch into voracious larvae that
cannot fly, lacewings are useful biological control tools.
Unfortunately, they are not yet produced commercially in
many sections of the nation. The only large-scale insectary
we know that offers them for sale by mail is Rincon-Vitova
Insectaries, Inc., Oakview, California.
Wheast, a food spray that is a by-product of the cheese
industry, can be used to feed and enhance the numbers of
lacewings and some other common predators found in the
garden. It can be obtained from the same company.

Chemical tools are also part of an integrated pest
management system, but they are used only as a last resort,
when all other methods have failed and the damage is
intolerable. They are used only where needed and carefully
timed so as to cause the least disturbance of the natural
controls. The most selective chemicals are chosen, to
affect the pest but as few other organisms as possible. And
they are applied very infrequently, so they will work when
you need them and will not cause new problems. Used in
reduced quantities, they will be less likely to pose
hazards to the environment and human health.

WHAT YOU CAN DO TO UNDERSTAND INSECTS IN YOUR GARDEN

[1] Start by overcoming some of your fear of insects. They
are part of the natural world (indeed there may be more
insect species than any other form of life), and few are
any problem to man. Forget the concepts of “good” bug and
“bad” bug used to sell you products you don’t need. Even
the plantfeeding insects are necessary. Those that eat the
weeds help to keep them in check, and the ones that feed on
cultivated plants are important too. You will never
eliminate (and do not want to eliminate) the pest insects,
because you have to have the pest insects around in order
to keep around the natural enemies that control them.
That’s why there’s no such thing as a “bad” bug–there
should even be some of the “worst” pests around to make
sure you have the predators and parasites around so that
the next “bad” bug will have enough enemies there to feed
upon it.

[2] When you see plant damage, ask yourself, is it
tolerable? There is no way to have plants without the
insects that feed on them, and there is, therefore, no way
to have no insects. We have to readjust our values to
tolerate some plant damage.

[3] When the damage involved is truly intolerable, then you
need to determine the true cause. It is easy to blame an
insect or other animal that is highly visible during the
day (such as an earwig or cutworm curled up inside a hole
in a tomato) while the real culprit is something else that
does its damage by night. In fact, going out after dark
with a flashlight may be the only way to discover what is
going on. (On the tomatoes in our garden, for example,
slugs were eating holes at night that provided refuge for
the insects during the day.)

[4] When you have correctly identified the animal causing
the problem, learn something about its habits and life
cycle. You can do this through direct observation, reading,
or asking your local university or county agricultural
extension advisor. He or she also may be the best person to
help you identify the animal that you have found.

[5] Knowing something about the pest, you can start to
consider how you can modify the environment to affect it.
For example, in our back yard we were severely bothered by
an invading garden snail ( Helix aspersa ). By
looking around, we discovered we had actually created some
choice breeding areas for the snails in a large bed of
succulents and under boards used for pathways, where we
discovered a great many baby snails and snail eggs. So we
reduced the large bed to a few specimen plants, separated
so that air could circulate around them and dry them of
after rains or watering, and we put dry sawdust under the
boards. This helped to reduce the snail population almost
immediately.

[6] Use handpicking. To continue with our snail example, we
made a point of checking the garden for snails in the early
evening with a flashlight every week or so. Those we found
in the vegetable beds were collected by hand and fed to our
chickens. (We could also have stepped on them and put them
in the compost pile.)

[7] Try barriers. Isolating beds with dry sawdust can
reduce snail and slug traffic between beds. Tin cans,
opened at both ends and shoved down into the mulch a few
inches to provide a collar around freshly transplanted
seedlings, have proved effective against the same pests.
Nets and chicken wire are protection against birds in early
spring. Cages made of aluminum flyscreen will keep out the
cabbage maggot and other root burrowing larvae of flies. Be
inventive!

[8] Concoct some ingenious traps. We turned over clay
flowerpots set among the plants to collect snails that
wander in during the warm, sunny day and emerge to feed at
night. Rolled up newspapers give earwigs places to hide.
Flytraps can be simply constructed and baited with dog
manure or garbage.

[9] Provide habitat and food for beneficial organisms that
are the natural enemies of insect pests. Compost mulches
favor the shiny black carabid beetles (Carabidae) that eat
cutworms, for example. By increasing the diversity of the
soil surface, you increase the habitat for beneficial
insects, particularly some of the predacious ground
beetles.

Plant flowers in the vegetable garden, particularly
shallow-throated ones that provide easy access for the
little mini-wasp parasites mentioned above. Many of the
beneficial insects need some pollen and nectar during the
season to provide protein for egg laying. So having some
plants in flower all during the season may actually help to
suppress the potential pest populations. This is an example
of companion planting that works!

[10] Modify your cultural methods. Make efforts to
determine whether your watering, fertilizing, and pruning
or clipping methods are affecting the pest populations. One
strategy for dealing with pests is overhead sprinkling,
which we recommend, or direct water sprays, taking the hose
and washing off a plant that has an insect on it. Not only
do you have direct effects (the physical effects of the
water hitting the plant, knocking off the insect, or
breaking its tissues or mouth parts), but also you will
increase the humidity levels of the plant or plant tissues,
and this could produce fungal outbreaks in the insect
populations.

HOW TO CHOOSE AN INSECT PESTICIDE

When you’ve tried everything and nothing seems to work
adequately, you need to stop and ask yourself if the plant
in question is important enough to you to warrant
supporting it with a chemical crutch. It may simply be the
wrong variety or species for that spot. Your overall
climate, or the mini-climate, may be unsuitable for that
plant.

If you decide that you wish to keep the plant, then you may
wish to resort to the use of a chemical tool. Here are some
guidelines for choosing and using them.

[I] Pick a pesticide that is least toxic to mammals. Of
course, water and water-and-soap sprays are relatively
harmless from that point of view. Some detergents even have
phosphorus in them, which is a plant nutrient. However,
when using a commercial detergent compound, always test it
on a small portion of the plant first, since you don’t know
what other chemicals might be in the material. Such an
approach is also useful with other pesticides.

Toxicity of pesticides is measured by LD 50 , or the lethal
dose required to kill 50% of the test animals, usually mice
or rats. The LD 50 is expressed in milligrams per kilogram
of body weight of the test animal, either dermal (when
contact is through the skin) or oral (through eating). The
higher the number, the less toxic the dose. Thus pyrethrin
with an LD 50 of 3,000 is assumed to be less toxic than
rotenone with an LD 50 of 400, or diazinon with an LD 50 of
108. In some commercial greenhouses where constant use of
pesticides against whiteflies has produced populations
resistant to everything else, some growers now use a
material called Temik with an LD 50 of 0.93. This is so
toxic that a drop accidentally spilled on a sweaty hand
could kill a person.

If you wish to use a product and are curious about its
toxicity, you can usually find the LD 50 of any
commercially available compound by looking it up in a
reference work such as “Commercial and Experimental Organic
Insecticides” from the Bulletin of the Entomological
Society of America, 15 (2): 85-148, often available in
libraries or agricultural extension offices.

However, it is important to remember that the LD50 doesn’t
tell you everything. Materials are not tested in
combination, so it will tell you nothing about synergistic,
or combined, effects with the other contaminants in the
environment to which you may be exposing yourself. Also,
the tests are ended and summarized after a short period of
time, and the effects may take longer to show up. This was
the problem with Kepone, which was thought to be safer than
it turned out to be. This is also the problem with
materials such as chlordane and DDT, which are now
suspected of causing cancer many years (usually 20 to 25)
after exposure, but which were originally regarded as
relatively nontoxic to humans.

[2] Pick the most species-specific material available. This
is the advantage of Bacillus thuringiensis ,
mentioned earlier. It is very selective, affecting only
certain caterpillar pests and not their natural enemies.

[3] Choose pesticides that break down quickly to harmless
materials. All of the botanicals, as they are
called–pyrethrin, rotenone, and sabadilla are
examples–break down rapidly under sunlight and
bacterial action, and their breakdown products are
nontoxic.

[4] Spot treat. Use only as much as you really need where
you need it, confined to the problem area. Use a bait, if
one is available, before resorting to a broadcast
treatment.

[5] Time your treatments. Don’t spray just as the pest
population is beginning to decline because large numbers of
their natural enemies are present. Give the parasites and
predators time to catch up; they always lag somewhat behind
their prey. Learn to recognize the common insect predators
and parasites of your area so you’ll be able to see how
well they are doing.

This whole approach to insect management is hardly simple.
But it is worth it to keep yourself healthy, your garden
thriving, and the insect populations, both plant and insect
feeders, in balance with one another. This balance may
ultimately be essential if we want to be able to continue
growing plants at all.

EDITOR’S NOTE: In order to help you identify, and thus
decide how to control, your garden’s pests, we’ve included
a guide to some of the more common vegetable-eating insects
on pages 94 and 95 of this issue.