Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00001733
11/12/99
Natural control
Insects reproduce rapidly. They are capable of building
up tremendous populations in a few years or even a few
months. Fortunately, natural enemies of insects, such as
pathogens, parasites and predators, usually prevent
populations of insects from reaching outbreak
proportions. The regulatory role of natural enemies is
referred to as biological or natural control. Biological
control may also include the release of predators or
parasites, and the application of insect pathogens, such
as microbial insecticides.
An understanding of the natural enemies that regulate
insect populations is critical to the success of
landscapers and nursery managers because destruction of
natural enemies will most certainly result in outbreaks of
insects that feed on ornamental landscape plants. Each
insect species has a complex of viruses, bacteria, fungi
and protozoans capable of infecting individuals and
producing disease. Some of the more spectacular diseases
are the viral infections of caterpillars that leave
thousands of flaccid, black caterpillars hanging dead from
tree branches, and the fungal diseases of grasshoppers and
flies that cause the insects to climb up the plants, where
they die clinging to the very top. Some pesticides have
adverse effects on these pathogens of insects. In
particular, many fungicides suppress important fungal
pathogens of insects.
Parasites of insects live in or on the bodies of their
hosts, where they feed during at least part of their life
cycle. Many insect parasites are referred to as
parasitoids because they consume a large portion of their
host, eventually killing it. Most insect parasites are
flies, bees and wasps. There are at least six important
families of fly parasites and 10 families of important
wasplike parasites. In just one family of parasitic
wasps, the ichneumons, more than 3,100 species have been
identified in North America. Most ichneumon species are
specialized to parasitize only one or a few host insect
species. Insect parasites are very susceptible to
insecticides, and some may also be killed by other
pesticides, such as fungicides and herbicides.
The final groups of natural enemies are the predators.
Predators usually feed on insects smaller than
themselves, ingesting one or more for a single meal. In
general, they are very active insects that seek out other
insects to feed on. Some of the more important families
of predators are ground beetles, tiger beetles, ladybird
beetles, social wasps, lacewings, some stinkbugs, assassin
bugs, nabid bugs, robber flies, syrphid flies and some
midge flies. A tremendous number of species of predaceous
insects exist. For example, more than 2,500 species of
ground beetles live in our area. Like parasites,
predaceous insects are susceptible to insecticides. In
some cases, they are far more sensitive to insecticides
than the plant feeding insects they prey on.
Homeowners and landscapers should avoid unnecessary
destruction of natural enemies. Avoid using insecticides
unless you are certain they will give you the necessary
reduction of insect pest numbers. Applying insecticides
to resistant life stages of insects, such as the eggs of
many insects, adults of armored scales or gall insects
inside plant tissue, may kill off natural enemies and so
result in increased numbers of plant feeding insects.
Also, insecticide drift may destroy natural enemies on
other plants in the area. In general, the use of
pesticides destroys natural enemies and interferes with
natural control. Use cultural and biological controls
whenever possible to prevent the need for insecticides.
Host plant resistance to insects
Some plants are more susceptible to insect attack than
others. Plants have many mechanisms of defense against
insects. Plant leaves may contain natural chemicals that
adversely affect insects feeding on them. Some plants
have extremely hairy leaves or thick, waxy cuticles that
deter insect feeding. Other plants have developed a
tolerance to insect feeding that allows them to sustain
substantial defoliation without suffering serious injury.
These are all considered host plant resistance.
In some cases, certain varieties of a plant are more
susceptible than other varieties. For example, many
varieties of tatarian honeysuckle are extremely
susceptible to attack by the honeysuckle witches'-broom
aphid, but at least two varieties, 'Clavey's Dwarf' and
'Arnold Red', appear to be highly resistant. Using
resistant cultivars in the landscape can significantly
decrease the amount of maintenance necessary for those
plants.
Some species of plants are more prone to insect problems
than others. European white birch trees are very
susceptible to birch leaf miner and bronze birch borer
attack. No resistant varieties are available. Avoid
problem plants such as European white birch, and select
more insect-tolerant plants when possible.
Microbial insecticides
Some naturally occurring pathogens of insects have been
produced and formulated as microbial insecticides. By far
the most widely used microbial insecticide is Bacillus
thuringiensis, sold as Dipel, Thuricide and Sok-Bt. This
bacterium produces spores containing a toxin that is
deadly to many lepidoptera larvae (butterfly and moth
caterpillars). The toxin crystal dissolves only under the
high gut pH conditions found in caterpillar digestive
systems. B. t. is not toxic to mammals and has little
effect on most natural enemies, so it is strongly
promoted for use in IPM programs designed to preserve
natural enemies.
Two other microbial insecticides are commercially
available but are not used on woody ornamentals: milky
spore disease for Japanese beetle larvae, and B.t.i. for
mosquito larvae.
This information is for educational purposes only. References
to commercial products or trade names does not imply
endorsement by MSU Extension or bias against those not
mentioned. This information becomes public property upon
publication and may be printed verbatim with credit to MSU
Extension. Reprinting cannot be used to endorse or advertise
a commercial product or company.
This file was generated from data base ZZ on 01/19/00.
Data base ZZ was last revised on 11/12/99.
For more information about this data base or its contents please contact
heatley@msue.msu.edu . Please read our
disclaimer for important
information about using our site.