Interdisciplinary Vegetable Pest Management Potentials of Selected Cover Crops

Field experiments in Irvine, California measured pest populations in broccoli planted after two summer cover crops, cowpea (Vigna unguiculata, cultivar UCR CC 36) and marigold (Tagetes patula cv. Single Gold). Of the weed pests, we observed that Portulaca oleracea (common purslane) was the dominant species and that its population density peaked just before the first hand weeding at 184 plants per m² in broccoli following fallow. This weed also measured the highest dry biomass for broccoli planted after the fallow summer field. The fallow treatment had 5x as many purslane just before the first hand-weeding as when broccoli followed cowpea and 10x more than the marigold treatment. The population density of all weeds ranged from 216 plants in fallow plots to 54 and 42 plants in broccoli planted after cowpea and marigold, respectively, just before the first weeding. By harvest, the population of all weed spp. was 4x higher in broccoli planted on previously fallow plots relative to those in the cowpea and marigold plots. The predominant broccoli insects were Trichoplusia ni (Cabbage loopers), Pieris rapae (cabbage worm) and Plutella xylostella (diamondback moth), with loopers the most damaging. Insect pest populations were inconsistent between the cropping systems. However, the total number of insects was relatively higher on broccoli planted after the cover crops during the initial growth period than those planted into the fallow beds. Insects appeared to prefer more rapidly growing broccoli and relatively younger leaves, as leaf damage was greatest depending on the growth stages of crops. Broccoli height and canopy spread were greater following either cowpea, a nitrogen fixing legume, or marigold. While broccoli following cowpea and marigold had the greatest leaf damage, particularly at younger stages, its yields were also highest in terms of both weight and number of heads. Marketable broccoli heads were 115, 90 and 81 from cowpea, marigold and fallow plots, respectively. Similar trends were observed on the fresh weights of the marketable yields. Broccoli plants grown after cover crops matured faster than those that were planted following a summer fallow. In general, there are indications that  summer cover crops reduce multiple crop pests for a subsequent commercial crop. Although a cover crop may not provide complete pest control on its own, it may play a valuable role as a tool in any integrated pest management system, particularly for organic vegetables.

Key words: cover crops, pest management, organic farming