Some Observations on the Management of Turnip Insect Pests
Sarwar, Muhammad, Economic Review
Turnip (Brassica rapa L.) is a root Brassica crop, member of the mustard (Cruciferae) family and is therefore related to cabbage and cauliflower. Turnip has been used as a vegetable for human consumption since long times. Turnip root has been a popular livestock fodder where ever the crop can be grown. For most of that time turnip roots have been managed as forage, because its roots are valuable energy sources for young ruminant animals. Livestock graze turnip tops and roots readily, and the forage is of high quality. Turnip produces high-quality forage if harvested before heading. Livestock eat the stems, leaves and roots of turnip plants. Above-ground parts normally contain 20 to 25% crude protein, 65 to 80% digestible dry matter, about 20% neutral detergent fiber and about 23% acid detergent fiber. The roots contain 10 to 14% crude protein and 80 to 85% digestible dry matter. Turnip roots usually are higher in glucosinolates than the tops and leaves. The high levels of glucosinolates (which can cause thyroid enlargement in young growing sheep and cattle) can be a problem if turnip forage is fed for long enough. Oral or subcutaneous iodine administration can alleviate thyroid problems. To minimize the potential for animal health problems from these factors, forage from turnips should be fed in combination with other forages. The temperate varieties are sweeter (Purple Top, White Globe, Snowball) where as the tropical types are more pungent (red, white or dull colored).
Turnip crops are attacked by two different flea beetles, which eat holes in the cotyledons and first leaves, chew stems and cause extensive plant loss. The cabbage flea beetle and the striped flea beetle feed exclusively on Brassicas, including related weeds such as yellow rocket. Problems with these flea beetles are much greater when Brassicas are grown under conventional tillage. Both flea beetles can be controlled with insecticides applied to the soil at planting. Red turnip beetles are an occasional pest of canula, rapeseed and mustard, the use of direct seeding, "zero till" and "chemical summer fallow" practices may contribute to this insect becoming an important crop pest.
Turnip beetles infestations can be reduced or prevented by using good cultural practices, their infestations originate in stubble fields previously seeded to canula, rapeseed or mustard or in other fields containing heavy stands of weeds of the mustard family, which have not been cultivated in the fall or early the following spring. For this reason, avoid the practice of under seeding canula, rapeseed, or mustard with forages. Cultivation is an extremely effective means of reducing red turnip beetle eggs, larvae and pupae. Cultivation after harvest buries the eggs and when they hatch, the larvae are unable to burrow out of the soil. Fall cultivation may cause 75-100 percent mortality of newly hatched larvae the following spring. Cultivation also controls the larvae by destroying host plants. When the host plants die, most larvae starve to death, for they are incapable of travelling very far food. A similar situation occurs when herbicides are applied to control volunteer canula, rapeseed, mustard, or weeds of the mustard family while the insect is still in the larval stage. Cultivation from mid-May to mid-June will subject pupae to mechanical injury, predation, and desiccation reducing the potential of economic injury by red turnip beetles. When adult beetles invadenewly seeded canula, rapeseed and mustard fields in June, it may be necessary to use chemical insecticides to control them. The affected fields should be examined carefully, to determine the density of beetles and whether the beetles are causing economic damage. Since, they usually walk into fields from the margins and are concentrated in a moving front, insecticide application can usually be confined to plants along the field margins. If invasion occurs late in June, the adults are inliely to cause economic damage, because plants are relatively large and the beetles will soon be entering the summer resting stage in the soil. …