Insects of the Greenhouse
by Jennifer Bartz
Greenhouses are built in many different shapes and sizes, but they
all serve a similar purpose - growing plants. No matter how small a greenhouse is
it will at some point contain insects and weeds. With the abundance of food and
a warm, humid atmosphere, this makes for an ideal environment for insects.
There are a handful of damaging insects in which greenhouses
will encounter, some worse than others. Whiteflies, scales, thrips, aphids,
mealybugs, and spider mites are of the most common. Some others that may be
present are fungus gnats, shore flies, cutworms, cabbage loopers, leaf rollers,
and leafminers.
Trialeurodes vaporariorum, the greenhouse whitefly, is a powdery white 
insect, that which is not a true fly. These insects are very
common amongst greenhouse crops. Whiteflies may cause stippling of leaves by
feeding on the plants 
with their piercing-sucking mouthparts. These "flies"
are usually found on the underside of leaves and will leave the leaf if
disturbed. Not only do they damage the plant by feeding on it, they also
excrete a sweet, sticky substance known as honeydew that acts like a growth
medium for black sooty mold, a greenhouse fungus. Due to the fact that
whiteflies are resistant to most insecticides, cultural and mechanical control
practices are very important in controlling these pests.
Scale, another greenhouse pest, feeds by using its
piercing-sucking mouthparts in which they inject toxic saliva into the plants,
the cause of the plant's deaths. They are also known for excreting
honeydew on the surface of the leaves. Scales may be controlled by physical or
mechanical removal. Insecticidal soaps may also be used for many of these species.
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The Western flower thrips, Frankliniella occidentalis, is
one of the most difficult to 
control due to its relatively small body size. Regardless of
their size, thrips cause major damage by rasping the plant's epidermis and
sucking up the exuding sap. They also damage blossoms, causing them to become
distorted and discolored. A buildup of thrips is favored by hot, dry
conditions. It is not uncommon for these insects to be resistant to
insecticides therefore requiring the use of biological controls.
Aphids, also called plant lice, are one of the most easiest to
identify because of the resemblance of their 
cornicles to "tailpipes." Aphids feed by sucking plant
sap from leaves or stems resulting in the curling or puckering of the leaves.
These pests also excrete honeydew and are prone to viral transmission. Many
methods are used to control aphids such as the use of high-pressure water
sprays, the introduction of predators and parasites, and insecticidal soaps.
An occurrence of a white powdery substance on a leaf is an 
indication of a mealybug problem. Pseudococcus longispinus feed
on the entire plant by ingesting the sap. They also excrete large amounts of
honeydew which gives rise to black sooty mold. Insecticides are not very
effective in depleting a mealybug population. Biological controls and
insecticidal soaps are a good source of control.
Of the five different types of greenhouse mites, the two-spotted
spider mite, Tetranychus urticae, and the cyclamen mite are the two that are of greatest concern
among greenhouse crops. T. urticae may be recognized by the fine webbing
that can occur on the entire plant. In general, mites feed on the underside of
leaves by using their piercing-sucking mouthparts. Damage is not only done to
leaves but to flower buds as well leaving them deformed or discolored. Mites
thrive in high temperatures and low humidity, exactly what most greenhouse
environments are like. Miticides are used in the control of mites; however,
repeated applications are needed since some stages of the life cycle are
resistant.
Those described above are some of the most common pests. The
latter create problems, but are not as prominent.
One such pest is the fungus gnat. They are mostly likely found in
potting soils or mixes where the larva causes damage by feeding on organic
matter or attacking bulbs, roots, and other 
fleshy organs. Yellowing of plants 
and rapid wilting are signs that this pest is present. To keep this
insect under control, sanitation is of major importance. This may be
accomplished by avoiding excess watering and keeping both the benches and
floors free from organic matter, weeds, and debris. The use of pasteurized soil
as helps cut back the insect population.
Shore flies are similar to fungus gnats in looks but differ by
feeding habits. They feed on algae that grow on media, floors, benches, or
pots. Sanitation is the key in controlling the insect.
The partial remains of leaves, stems, and fruits may recognize the
presence of cutworms. However, trying to catch them in the act may be hard
since they nestle themselves in the soil or mulch during the day and feed only
at night. Spraying with Bt in July and August serves as a good control of these
pests.
Another caterpillar, similar to the cutworm, is the cabbage
looper. Both being destructive during their larval stages, loopers feed on
leaves and are also characteristic of cutting the entire plant off at the base.
Webbing or rolled leaves is an indication that leaf rollers are
present. They spin webs at branch tips, pulling the leaves together such that
they might feed on the enclosed buds, leaves and developing fruit. Bt or
Rotenone sprays are a few means of control.
Leafminers do exactly what their name sounds like they do; they
mine the leaf for food. The larvae infest the leaf by burrowing between the
upper and lower epidermis leaving narrow, winding paths where they have fed.
These areas are left slightly discolored.
Insects must be monitored and controlled at all times since the
environment remains constant and the food supply is continuous. This may be
done using various methods:
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monitoring/cultural controls
(yellow sticky cards, inspections, sanitation)
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biological controls
(beneficial insects)
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chemical controls (insecticidal
soaps, miticides, oils, fungi, bacteria)
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mechanical controls (pruning
plants, vacuuming or crushing insects)
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insect screening (mesh
screens placed over vents)