Date of Award


Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)


Ecology and Environmental Sciences


Andrei Alyokhin

Second Committee Member

Stellos Tavantzis

Third Committee Member

Frank Drummond


Phosphite is a general term used to describe the salts of phosphorous acid H3PO3. It is effective in suppressing a number of plant diseases caused by oomycetes and has been shown to reduce populations of several insect species. We investigated effects of phosphite on Colorado potato beetles in the field and laboratory. Beetle numbers and defoliation on phosphite-treated plots were lower compared to the control plots during one out of two years of the study. However, larval mortality was significantly higher that year in the laboratory when larvae were fed on potato foliage excised from the potato plants treated with phosphite in the field, still suggesting the presence of a toxic effect.

Laboratory tests with excised leaves and potted plants dipped in solution of phosphite revealed lower beetle survivorship and prolonged development on the treated foliage. However, we also observed hormesis, when larvae developed more quickly on treated foliage, but suffered heavier mortality.

There was some evidence of an induced systemic defense response in the treated plants. Larval development was as expedited on the untreated portions of partially treated parts as on the treated portions. Also, larval mortality following leaf treatment followed the pattern typical of induced defense response in plants: an increase in resistance to a peak after the initial induction, followed by decay over time in the absence of additional induction.

Because of its dual properties as a fungicide and an insecticide, as well as its low toxicity to vertebrates, phosphite is a potentially good fit for integrated pest management programs. Furthermore, negative cross-resistance with commercial insecticides may provide additional benefits to using this compound.