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"Pesticides
commonly used on ornamental plants
and their impact on beneficial insects"
By Mario Lanthier and Tania Jensen,
CropHealth Advising & Research,
2001.
- Acephate
- Bacillus thuringiensis
- Diazinon
- Dimethoate
- Dormant oil
- Insecticidal soap
- Insect growth regulators
- Pirimicarb
- Other insecticides
- Fungicides
- Miticides
- Notes on terminology
When
using pesticides, always read the label for registered plants and pests, the
rates of application, and safety instructions
Uses and mode of action
Acephate is very similar to dimethoate with both systemic and contact
action. It is of low toxicity to
humans and is thus a safer option when pesticide use is necessary.
Impact on beneficials
Acephate is highly toxic on contact to all beneficials and has a moderate
residual action.
Uses and mode of action
Various strains of Bacillus thuringiensis are available.
Common products are made of B.t. kurstaki (B.t.k.), applied for the control
of leaf-eating caterpillars, and B.t. israelensis, applied for the
control of fungus gnat larvae. With
B.t.k., thorough coverage of the plant with the spray solution is
critical, as the caterpillars must ingest the product to be affected.
Impact on beneficials
Products containing B. thuringiensis have no contact toxicity to
most beneficials and have short residual action.
Uses and mode of action
Diazinon is inexpensive and has an extensive label for ornamental plants
and insect pests. Many
landscape companies and nurseries use it routinely.
It works by disrupting the nervous system of insects and is of moderate
toxicity to humans.
Impact on beneficials
Diazinon is highly toxic on contact to most beneficials and has a
moderate residual action.
Uses and mode of action
Dimethoate is used for both contact and systemic control of many insect
pests. Dimethoate works by
disrupting the nervous system of insects. It
is of high toxicity to humans and should be used with caution. When dimethoate is applied to the bark of trunks or large branches, the
sap carries the product to the leaves where aphids are feeding. Hence,
Dimethoate is very useful for large trees in urban areas
where a treatment of the full canopy is not practical.
Impact on beneficials
Dimethoate is highly toxic on contact to all beneficials and has a long
residual action.
Uses and mode of action
Dormant oil is applied for the control of insects and mites overwintering
in the egg stage and works by suffocating the eggs close to hatching.
It is of low toxicity and is valuable for early season control of aphids,
scales, spider mites and caterpillars.
Impact on beneficials
Oils are toxic on contact to most beneficial but have short residual
activity.
Uses and mode of action
Insecticidal soap is applied for the control of soft-bodied insects and
mites such as aphids, leafhopper nymphs, scale crawlers, psyllid nymphs and
spider mite motiles. It works by
breaking down the outer tissues of the insect and mite and by interfering with
internal membrane functions. It is
a product of very low toxicity.
Some factors limit the efficacy of this product.
Soap works only by direct contact and excellent coverage is mandatory to
achieve good results. It works
poorly in hard water, under low relative humidity, or on plants with
hairy leaves.
Impact on beneficials
Soap is toxic on contact to most beneficials but has no residual
activity.
Uses and mode of action
Insect growth regulators mimic insect hormones involved in egg
development, larval moulting, or chitin formation.
Many of the more recently registered pesticides are insect growth regulators
with a narrow-range of action and low toxicity to humans.
Impact on beneficials
Insect growth regulators vary in their contact toxicity to beneficials,
but in most cases they have low contact toxicity and short to moderate residual
action.
Uses and mode of action
Pirimicarb is effective only on aphids and will not control other insects
such as caterpillars, sawflies or thrips.
This product gives very rapid control of aphids and is of moderate toxicity to
humans.
Impact on beneficials
Pirimicarb is of low toxicity to lady beetles, but exhibits intermediate
toxicity on contact to predatory mites, aphid midges and minute pirate bugs.
Other insecticides occasionally used by landscape companies and nurseries
include carbaryl, deltamethin, endosulfan, malathion, permethrin, and phosmet.
The products range from low to high toxicity and should be used with
caution.
Impact on beneficials
All these insecticides are highly toxic on contact to beneficials and
have moderate to long residual activity.
Different fungicides are applied to prevent or control diseases on
ornamental plants. Most are of low
toxicity to humans.
Impact on beneficials
Most fungicides are not toxic on contact to beneficials. Exceptions include sulphur, thiram and zineb, which can be
toxic on contact to predatory mites and aphid midges, and benomyl, which is
highly toxic on contact to predatory mites and minute pirate bugs.
Products commonly used on ornamental plants include dicofol, dienochlor,
fenbutatin oxide, sulphur and lime sulphur.
All are of low toxicity to humans.
Impact on beneficials
Dicofol is highly toxic on contact to most predatory mites and aphid
midges. Dienochlor is highly toxic
on contact to most predatory mites and has long residual activity. Fenbutatin oxide and sulphur have intermediate toxicity on
contact to predatory mites. Lime
sulphur is applied during the dormant season when beneficial insects are not
active.
- “Toxicity to humans” refers to the
acute toxicity as measured with the LD50 (lethal dose to 50%).
- “Toxicity on contact” refers to
beneficials being killed during the application of the product.
- “Residual activity” refers to
beneficials being killed after they migrate to the sprayed plant. The residual activity can be short (hours), moderate (days)
or long (weeks).
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