Aphids are major greenhouse pests because of their large host plant ranges, ability to produce high populations rapidly, ability to transmit plant viruses, their resistance to many commonly used insecticides and finally, difficulty in detection.
There are more than 4,000 species of aphids, but only a few of them are pests in the greenhouse. The most common aphids in the greenhouse are: GREEN PEACH APHID (Myzus persicae) and MELON/COTTON APHID (Aphis gossypii).
Aphids are small (I – 3 mm), fragile, soft-bodied and pear-shaped insects generally found in colonies. Their antennae are approximately one-half the lengths of their body. The green peach aphid has long, laid-back antennae and bumps on the inside of the base, while the melon/cotton aphid has shorter, darker antennae without bumps. Color can vary (with temperature, season and population density) from green to rose pink (green peach aphid) and yellow, light to very dark green or nearly black (melon/cotton aphid). Aphids are distinguishable from other insects by the presence of a pair of cornicles or “tail pipes” on the rear of the abdomen. Green peach aphids’ cornicles are the same color as their body and darker at the tip while melon/cotton aphids have entirely dark cornicles and they are always darker than the body. In the greenhouse aphids are usually wingless, but when the population density becomes great, young aphids develop wings and migrate to another area to begin a new colony.
Aphids in greenhouses are all female and give birth to more live females without mating. The fact that under greenhouse conditions adults give birth to live young, means that there are no dormant stages, such as eggs or pupae, which are less susceptible to insecticides. One adult can produce 50 to 250 young during her lifetime, depending on the host plant and its nutritional status. The nymphs can mature and begin reproducing in 7 to 10 days. The life expectancy of the adult can be from 7 to 21 days, making possible more that 30 generations a year in the greenhouse.
Aphids feed by inserting their piercing-sucking mouthparts into the vascular system (phloem) of the plant and sucking out the sap. This causes discoloration, curling, crinkling and wilting of leaves, malformation and distortion of buds and shoots, leading to plant stunting and deformities, reducing the vigor of the plant. They also have the potential to transmit virus, which can cause further problems.
When feeding, aphids excrete honeydew, a sugar-rich, sticky substance, that coats leaves and flowers and which is a good medium for black sooty mold to grow. Molted skins also reduce the aesthetic value of ornamental plants. Aphids feed on all parts of the plant, but prefer tender young tissue. Green peach aphids often congregate near the growing tip of the plant, while melon/cotton aphids are more evenly distributed throughout the entire plant, making it harder to detect an infestation.
Excellent control is achievable using all control methods together. INTEGRATED PEST MANAGEMENT (PM) which incorporates cultural, biological and chemical techniques will maximize control and minimize waste of effort and resources.
Aphid control is easier when they are detected and controlled in the early stages of an infestation, and at the beginning of a crop production cycles, therefore a regular monitoring program is a very important part of aphid control, especially considering their small size and similar color to plants. Screening vents may reduce migration into the greenhouse from surrounding fields. Yellow sticky cards should be placed to detect presence of winged aphids as they move into or around the greenhouse. Inspection and quarantine of incoming plant material and inspection of plant foliage (undersides and growing tips) weekly will help in early detection of this pest. Plant debris should be disposed of promptly. Weed control inside and outside of the greenhouse is very important because weeks can be sources of infestation and refuge – between crops.