Effects of manure and chemical fertilizers on insect pest populations on collards

Advocates of organic agriculture often assert that plants supplied exclusively with nutrient from biological materials are more resistant to insects than those grown using chemical fertilizer. This proposition was examined using collards grown in five treatments: two with fresh manure at two levels (12 and 220 ha⁻¹); one with ‘sheet composted’ manure at 220 ha⁻¹; a chemical fertilizer treatment (785 kg ha⁻¹ of 13:13:13 NPK); and a control treatment with no added nutrients. Population densities of two flea beetles and two lepidopteran pests were monitored weekly during a growing season. Plant size and foliar levels of reduced nitrogen (N), nitrate nitrogen (NO₃⁻-N), water and total glucosinolates were also measured four times during the season. During population peaks, flea beetle densities were significantly higher on plants receiving chemical fertilizers than on plants receiving similar amounts of macronutrients from manure. Flea beetle, imported cabbage worm and diamondback moth densities were significantly higher on untreated plants than on plants receiving any added nutrients. Flea beetle densities were also significantly higher on plants grown with ‘sheet composted’ manure than on those grown with fres manure. The nutrient treatments affected plant size, reduced N, NO₃⁻-N, water and glucosinolate content. These differences were not clearly associated with differences in insect population sizes but some patterns in the data are discussed. Although the potential exists for manipulating soil amendment regimes to affect pest insect populations in collards, an understanding of the mechanisms involved will require further investigation.