Greenhouse Insect and Mite Pest Management
Insect and Mite Pests Commonly Found in Greenhouses
Leafminers
The most common leafminers are the larvae of flies, in the family Agromyzidae. These include serpentine leafminers in the genus Liriomyza, columbine leafminers in the genus Phytomyza, and the relatively new daylily leafminer, Ophiomyia kwansonis. The most common species found to infest greenhouse crops is Liriomyza trifolii, American serpentine leafminer (See Figure 27.4). They are black with a yellow spot on their backs and are about the same size and shape as fruit flies. Adults become active at sunrise.
Life Cycle
The life cycle duration depends on temperature, host plant and possibly day length. Female leafminers have a very sharp ovipositor or egglaying tube used to pierce holes in the upper surface of the leaf to extract plant sap for feeding and to lay eggs. After 2 to 4 days, eggs hatch and the larvae begin feeding (or mining) in the leaf tissue.
Types of Damage
Females feed on soft, succulent leaves by piercing the surface with their ovipositor to feed and lay eggs, which can be serious, resulting in reduced photosynthetic capacity and retarded plant growth. The leaf wounds made by the female are called “stipples” and are easily visible as distinctive raised small circular scars on the leaf surface.
Monitoring
Yellow sticky cards will capture adults. Place one trap per 10,000 square feet and monitor weekly. Research has shown that more leafminers are caught if the cards are placed horizontally, close to the ground level. Watch for mined or stippled leaves.
Cultural Management Strategies
The best initial defense against leafminers is to refuse to accept infested cuttings. Incoming plant material should be inspected for leaf stipples and active mines and held for several days to see if mines develop from leaf stipples.
Biological Management Strategies
Biological control agents include the parasitic wasps Diglyphus isaea and Dacnusa sibirica that deposits an egg in a young leafminer larva. Diglyphus is likely to be a better choice in summer, when its numbers may be supplemented by naturally occurring populations migrating into the greenhouse from outside.
Chemical Management Strategies
Contact sprays to control adults should be repeated at 3 to 4 day intervals to kill those adults that continue to emerge from puparia during the 10 to 14 days following initial treatment. Systemic insecticides can be very effective against the larvae.
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