Horticulture Principles and Practices

SUMMARY
Herbicides are chemicals used to control weeds. They may be classified according to
selectivity (as selective herbicides, which kill only certain plants, or nonselective herbicides),
timing of application (as preplant, preemergence, or postemergence herbicides),
mode of action (as either contact or translocated), and chemistry (as either organic or
inorganic herbicides). Herbicides may be formulated as liquids or granules. They are
applied by either broadcast, or band application. Sometimes they are applied as a spot
application to control highly localized weed infestation. The effectiveness of herbicide
application depends on several factors, including plant species, type of herbicide, time
of application, weather factors, age of weeds, and soil factors.
MODULE 5
GREENHOUSE PEST CONTROL
Greenhouses are enclosed structures and hence have environmental conditions that are
different from the general open environment. Further, the greenhouse environment can
be controlled, and thus the pest incidence can be controlled to some extent. However,
even with best efforts, certain pests occur in greenhouses.
8.19 COMMON GREENHOUSE INSECT PESTS
Some of the most common and economically important greenhouse pests are described
in the following list.
1. Aphids. The most common aphid found in greenhouses is the green peach aphid
(Myzus persicae). They may be winged or wingless. Whereas the winged green
peach aphids are brown in color, the wingless ones are yellowish-green or pink.
Aphids attack a wide variety of greenhouse plants. Younger leaves that are
attacked become distorted, and older leaves show chlorotic patches.
2. Fungus gnats. Important species of the fungus gnat are Bradysia spp. and Seiara
spp. Gnats are gray-colored, long-legged flies. They live on the soil. The stage in
their life cycle that constitutes a pest problem is the larva stage. Gnat larvae are
white worms with black heads. They prefer soils that are rich in organic matter,
feeding on soil fungi and decaying organic matter. However, they can also feed
on underground storage organs and the roots of young seedlings.
3. Leaf miners. Leaf miners are insects that in the larval stage tunnel between the
outer layers of leaves. Their activity blemishes leaves severely. Chrysanthemums
are particularly susceptible to leaf miner attack; the chrysanthemum leaf miner is
called Phytomyza atriconis. Other species exist.
4. Mealybugs. Mealybugs (Pseudococcus spp.) are oval-shaped piercing insects that
secrete a waxy covering over their bodies, causing them to appear white. This
waxy layer forms a protective covering that makes pesticidal control of bugs
difficult. To be most effective, mealybugs are controlled by spraying the nymphs.
The economic damage they cause to plants is similar to aphid attack. Mealybugs
also secrete honeydew, which attracts black mold to grow on the plant.
5. Mites. Mites belong to the spider or scorpion family (Arachnida). They are very tiny
in size and develop well under conditions of high humidity and low temperatures of
about 16°C (60°F). A common species is the Steneotarsonemus pallidus (Cyclamen
8.19 Common Greenhouse Insect Pests 275