Cabbage Integrated Pest Management : An Ecological Guide.

Major Cabbage Insect PestsThe adults can overwinter in sheltered sites where leaf litter and plant debris is present. They becomeactive with temperatures rising above 20 o C. When the temperature is always high, like in tropical lowlands,the flea beetles will not have a resting phase but just continue reproducing.Adults disperse, frequently in large numbers, on prevailing winds. They can travel long distances. Whenthey locate a suitable host crop, they settle and start to feed, often on seedling tissues below ground.After mating, the beetles lay their eggs in the soil near host plants. The eggs are pale-yellow in color,about 0.3 mm long and 0.15 mm wide. The larvae of all species are generally white or pale-yellow in colorand have very short legs. The larval head is dark in color. The larvae of most species feed on the plantroots. After living in the soil for about 4-5 weeks, the fully grown larvae are about 5-6 mm long. Theypupate in the soil. The pupa is about 2.5 mm long, white to yellow in color at first but later turns darker.The duration of the pupal stage depends on climatic factors like the temperature, it may take up to 4weeks.Plant damage and plant compensationDamage by flea beetles is most evident on seedlings. Severe damage can be caused by beetles feedingon the seedlings below soil surface, before the seedlings emerge above the soil. With emerging seedlings,the beetles chew holes, particularly in the cotyledons (seed leaves), giving them a characteristic shotholeappearance. Occasionally, seedlings may be completely destroyed. Loss can be greatest in a dryperiod when the seedlings grow very slowly after germination.Flea beetles are especially damaging to Chinese cabbage, which is sometimes used as a trap crop tokeep flea beetles off other crucifers.Apart from the direct damage they cause, flea beetles can also transmit turnip yellow and turnip mosaicvirus. Crops are at risk mainly when the new generations of flea beetle emerge. Although the plants areby then large, damage may be severe if large numbers of beetles enter the crop.Plant compensation study example for flea beetle damageHow much leaf damage can cabbage seedlings sustain? When does crop injury result in yield orquality loss? How much damage can be tolerated before something has to be done to control fleabeetles? Find out in a compensation study!Select a number of seedlings (e.g. 100) in a separate area of the nursery. Cut different percentages ofthe seedleaves (cotyledons) and different percentages of the first true leaves at different times. Forexample: cut 10, 25, 50 and 75% of the cotyledons one week after emerging, another 10, 25, 50 and75% of all leaves per plant at 2 weeks after emerging, etc. Cutting the leaves will simulate the damagedone by flea beetle (and some other leaf-eating insects). Label the different “treatments” carefully toavoid mixing up. Transplant the seedlings in a separate area of the field and include plants that havenot been cut as the control. Compare plant vigor, yield and quality during growing and at harvest time.Natural enemiesThe rainy season is not favorable for flea beetles. When the soil is more or less permanently moist,seedlings establish quick and are soon large enough to tolerate most flea beetle infestations. The sensitivegrowth stage is shortened and damage is reduced. Rain can also have a direct impact on flea beetles,washing them off the leaves, disturbing emergence from eggs in the soil, or destroying the larvae in thesoil by washing them away.Except for general predators like spiders, there are no specific predators or parasites that control fleaCabbage Ecological Guide - 200086