Urban and peri-urban vegetable production (UPVP) systems offer farmers enhanced market opportunities with better economic returns. UPVP systems significantly contribute to urban food supplies, especially of vegetables. The impact of UPVP systems is limited by a number of factors including insect pests and disease problems. Insects are developing resistance to many of the most widely used pesticides in UPVP, prompting many farmers to grow vegetables under protective structures which block the entry of pest and prevent crop infestations.
There are two types of protective structures: permanent and temporary. AVRDC – The World Vegetable Center has designed suitable nethouse and plastic house structures and appropriate maintenance procedures that emphasize preventing entry of all damaging insects, thereby reducing the need to use pesticides.
Permanent nethouses, if carefully constructed and properly maintained, can last for at least five years. The most widely used structure is 2 m high made of galvanized iron tubing covered with nylon net. Coarser net (16- to 32-mesh size) is used for larger sized insects, such as lepidopteran moths, whereas finer net (50- or 60-mesh size) is used to exclude smaller insects, such as thrips, whiteflies, and aphids. Coarser nets allow a free airflow with minimum build-up of temperature inside. The nethouse is usually constructed to cover an area of 500 m2.
It has been demonstrated that 15 cycles of various leafy vegetables could be produced free of any pesticide use without losing yield or quality over a two-year period. In addition to leafy vegetables, tomato, eggplant, cabbage, cauliflower, broccoli, yard-long bean, and bitter gourd can also be grown successfully in nethouses. In northern parts of India, such as Punjab, AVRDC introduced adaptations to the original design, especially in the shape of the nethouse. A net-room with a double door entry point was provided at the front side, using poly-grip assembly to fix the net, and with the net stitching parallel to the hoops, which makes the nethouses more durable under the climatic conditions of the region.
These are permanent structures lasting for about four to five years, which utilize transparent but sturdy rainproof plastics used mainly for longer duration fruit vegetables such as tomato, sweet pepper, and cucumber. These plastic houses not only protect the crop from rain, but also keep the insect pests away, thus allowing profitable tomato production during the hot-wet season
?The usual dimension of the plastic houses is 25m × 6.5m. Each house has a semicircular top covered with clear plastic treated with an ultraviolet (UV) light protectant. In the center of the semicircular top, a 30 cm portion along the entire length is left free of the plastic cover, but closed with nylon net. This facilitates ventilation and reduces the buildup of heat inside. The ventilation facility is made rainproof by erecting a transparent plastic shade over the top. The side walls are covered with nylon netting. At the front side, a 60-mesh netting double door is installed.
There are two types of a temporary plastic house, which is used for a single growing season: a single-bed type with an arched roof and a double-bed type with an A-shaped roof covered with UV-protectant polyethylene film. The width is 2.4m for the single-bed shelter and 4.8m for the double-bed shelter. The height at the center of both types is 2.4 m. The sides are covered with nylon netting to exclude insect pests and prevent rain from blowing in. The growth and yield of cucumber, sweet pepper, and small-headed cabbage were significantly improved when grown under rain shelters during the hot-wet season in Vietnam. In addition, these structures significantly reduced the incidence of fungal and viral diseases.
Low Net Tunnels
Growers who cannot afford the high initial construction costs of nethouses or plastic houses can grow vegetables under temporary net tunnels. The net tunnels are constructed with U-shaped iron or aluminum bars, which are covered with nylon netting over each bed. Leafy vegetables such as Brassica rapa var chinensis, B. oleracea var. alboglabra, B. rapa var. parachinensis, and B. juncea grown under tunnels were less damaged by heavy rain and insect pests. This resulted in a reduced number of insecticide applications, better produce quality, and higher marketable yield. Amaranthus yield has tripled under net tunnels in Vietnam.
Insect populations on cauliflower under net tunnels were greatly reduced, by 80% in Cambodia; marketable yields were 1.5 to 2.0 times greater under net tunnels than in the open field. Growing head cabbage under net tunnels in the Solomon Islands reduced insect incidence by 38-72%, and resulted in significantly higher economic returns.
Vegetable production under protective structures reduces yield losses from insect pests, diseases, and heavy rains, and results in higher productivity and returns per unit area. Protective structures enable growers to produce vegetables successfully during the off-season, which enhances the availability of fresh produce at times when vegetables are usually in short supply. Because vegetable prices are higher during the off-season, growers also enjoy higher returns per unit area.