Contrasting effects of sewage sludge and commercial fertilizer on egg to adult development of two herbivorous insect species

Abstract

Municipal sewage sludge containing heavy metals had a toxic effect on the development of the cabbage looper, Trichoplusia ni (Lepidoptera: Noctuidae), one of two herbivorous insects commonly found in an Ohio old-field which had been subjected to long-term sewage sludge application. Soils were removed in 1992 from an old-field following 11 years of heavy nutrient enrichment (1978 to 1988) with applications of either sewage sludge (Milorganite(®)) containing heavy metal contaminants or urea-phosphate fertilizer. Egg to adult development rate and survival of the blackfaced leafhopper, Graminella nigrifrons (Homoptera: Cicadellidae), and the cabbage looper was determined on maize (leafhopper) and broccoli (looper) seedlings grown in soils from sludge-treated, fertilizer-treated, or untreated control plots of the old-field. Fertilizer and sludgetreated soils had higher levels of N. P and organic matter, and a lower pH than the untreated control soils, while sludge-treated soils contained significantly higher concentrations of Cd, Cu, Pb, and Zn. Maize appeared to be unaffected by the three soil treatments, and survival and rate of egg to adult development of the leafhopper was not affected. Broccoli seedlings grown in both the high nutrient fertilizer and sludge soils were greener and larger than broccoli grown in control soils. However, the cabbage looper had significant larval and pupal mortality (25 to 40%) and prolonged egg to adult development on sludge-grown broccoli compared to control and fertilizer treatments. As assimilation into the food chain of heavy metals and other organic pollutants, such as PCBs, is in part a function of the interaction of soil chemistry and type of plant, the application of municipal sludges to old-fields needs to be carefully monitored, as contaminants may have significant developmental and behavioural effects on some secondary links in the old-field food chain.