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Watch for insect problems in float systems

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By Rick Greenwell

Frequent close inspections of float trays can help to catch insect pest problems early and to minimize seedling losses. Variegated cutworms are the greatest concern, and some damage has been reported. Cutworms found during the week of April 25 were in the 0.75 inch range, just big enough to cause visible damage. Rounded holes in leaves lying on or near the media surface and cut stems are signs of cutworm feeding. They can totally consume very small seedlings.
Variegated cutworms generally hide in the media when not feeding but may be caught up on plants during periods of low light. Small cutworms are light brown to black with a single row of light spots along the center of the back. As they grow, the caterpillars turn light gray and the spots become less distinct.
Variegated cutworms feed for about three weeks, reaching a length of just over 1-1/2 inches when full-grown. Most of the feeding is done during the last few days of their development and many plants can be destroyed in infested trays.
Variegated cutworm moths begin to fly in March and lay clusters of about 60 eggs on small plants. Consequently, cutworm damage usually occurs in distinct “hotspots” and is often near side curtains where moths can get enter. Orthene, or other acephate products, can be used for cutworm control in float systems. Spray areas where damage is being seen and buffers around them because the cutworms will spread outward as they feed and grow.
Wet media and surface algal growth are major factors that contribute to problems with fungus gnats and shore flies in greenhouses and float systems. Adults are the noticeable stage but the larvae can damage small plants by chewing on leaves or damaging developing roots.
It’s easy to overlook a few gnats that appear first but with a short generation time, populations can build quickly. Yellow sticky cards, available through greenhouse suppliers, provide an early detection system.
Orthene (acephate) sprays will kill adults and reduce their numbers but it is difficult to eliminate the flies because the larvae are down in the algae or under the protective cover of the leaves. Preventive control is based on keeping media from getting too wet and eliminating as many standing ground puddles as possible.
Fungus gnats are small (1/8”) black flies with comparatively long legs and antennae, tiny heads, and one pair of clear wings. Females lay tiny ribbons of tiny yellowish white eggs in the growing media that hatch within 4 days. The clear larvae are legless and have black heads. Larvae feed for about 14 days and pupate near the surface of the medium. Adults live only about a week. Under greenhouse conditions, about 20-25 days are required to complete a generation.
Shore flies also are small gnats with short antennae, red eyes, and heavier, darker bodies. A pair of smoky wings with several clear spots can be seen when looking closely at the insect. They are good fliers and can be seen resting on most any surface in the greenhouse.