Crop Science Journal of Natural Resources and Life Sciences Education
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Published in Crop Sci 16:190-192 (1976)
© 1976 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Growth and Physiological Response of Alfalfa to Diazinon and Methoxychlor Insecticides1

D. D. Wolf, G. R. Buss and R. L. Pienkowski2

An effective insecticide must control insects with minimum adverse influence on the plant. During a cultivar performance study, a commercial insecticide containing diazinon [0,0-diethyl 0-(2 isopropyl-6-methyl-4-pynimidinyl) phosphorothiorate] and methoxychlor [l,l,l-trichloro- 2,2-bis= (p-methoxyphenyl)] was applied to alfalfa (Medicago sativa L.) as a control for alfalfa weevil (Hypera postica Gyllenhal). A phytotoxic response was noted and several experiments were conducted to elucidate the nature of the toxicity. Weevil control by the insecticide was satisfactory, but total nonstrnctural carbohydrate concentration (TNC) in tap roots decreased and growth rate of shoots was not increased when compared with check treatments during 2 weeks following insecticide application. Greenhouse studies indicated that the diazinon component was phytotoxic and caused a reduced growth rate for at least 5 days. Carbon dioxide exchange rate and TNC concentration in leaves were reduced by nearly 50% for several days following application of the insecticide. Hay yields were increased by control of the insects but yield increase might have been greater if a nonphytotoxic insecticide had been used. Diazinon should not be recommended for use on actively growing plants until it has been shown that the beneficial effects out weigh the risks of phytotoxicity.

Key Words: Total nonstructural carbohydrates • Photosynthesis • Alfalfa weevil • Phytotoxicity

1 Contribution of the Agronomy and Entomology Deps., Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061. This publication was supported in part by the National Science Foundation and the Environmental Protection Agency, through a grant (NSF GB-34718) to the Univ. of California. The findings, opinions, and recommendations expressed herein are those of the authors and not necessarily those of the Univ. of California, the NSF, or the EPA.

2 Associate and assistant professors of agronomy and professor of entomology, respectively.

Received for publication June 3, 1975.




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Environmental Quality		 The Plant Genome
Copyright © 1976 by the Crop Science Society of America.