Reduced Colorado potato beetle densities on manure-amended plots of the long-term Potato Ecosystem Project.

One of the foundations of organic farming is an assumption that the organic production systems create a generally unfavorable environment for pest populations. Since 1999, we investigated effects of soil amendment practices on Colorado potato beetle populations in experimental potato plots and their interactions with crop rotation and two pest management approaches. Beetle densities were almost universally lower in plots receiving manure soil amendments in combination with reduced amounts of synthetic fertilizers compared to plots receiving full rates of synthetic fertilizers, but no manure. Crop rotation and pest management approaches had little or no effect. Unlike beetle abundance, plant height and canopy cover were comparable between plots receiving manure and synthetic fertilizer. Furthermore, tuber yields were higher in manure-amended plots. There was a dramatic dissimilarity in mineral composition of potato leaves collected from manure-amended and synthetic fertilizer-treated plots. We also conducted a series of no-choice assays comparing Colorado potato beetle reproduction and development on plants grown in manure-amended and synthetically fertilized soils. Female fecundity was lower in field cages set up on manure-amended plots early in the season, although it later became comparable between the treatments. Fewer larvae survived past the first instar, and development of the immature stages was slowed down on manure-amended plots. In the laboratory, first instars consumed less foliage from plants grown in manure-amended soils. Our findings indicate that manure amendment creates an unfavorable environment for the Colorado potato beetle reproduction and development. Of course, manure cannot by itself be considered as an alternative to insecticides in commercial potato production. However, it is definitely worth considering when designing fully integrated crop management systems.