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Methods for Evaluating Insecticide Effects on Alfalfa Growth¹

ABSTRACT

It has been shown that certain insecticides commonly used on alfalfa (Medicago sativa L.) can be phytotoxic. In this study, stem length and stem elongation rate were compared as screening techniques for measuring responses of potted alfalfa plants to insecticides. The measurements were made in two environments (low and medium light) and with several rates of four insecticides. The insecticides were (i) diazinon (O,O-dimethyl O-(2-isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate, (ii) heptachlor (1,4,5,6,7,8,8-heptachloro-3a,4,7,7a-tetrahydro-4,7-methanoindene), (iii) carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate), and (iv) methoxychlor (1,1,1-trichloro-1,2-bis (p-methoxyphenyl) ethane) plus malathion (S-(1,2-bis(ethoxycarbonyl)ethyl)O,O-dimethyl phosphorodithioate) in a 1:1 mixture.

Stem elongation (change in stem length) between two points in time was more effective in detecting an insecticide effect than was absolute stem length. Daily elongation rates as estimated by length changes over 2- or 3-day intervals for the 2 weeks following treatment gave the highest levels of significance. When variability was reduced by selecting stems for uniform initial lengths, the coefficient of variation (CV) for elongation rate was about 20%. At each sampling time, it would take about 50 measurements/treatment to detect a 10% difference in means in 80% of the trials with that CV. Number of measurements required is, thus, a limitation in comparing numerous treatment combinations.

Measurements of stem elongation did show phytotoxic effects of diazinon and methoxychlor plus malathion. Previous work had shown that these treatments could be phytotoxic. No other significant growth responses were observed. The diazinon effect showed up only in the low-light environment, indicating that light level influenced the response to the insecticide.

Key Words: Medicago sativa L. • diazinon • heptachlor • carbofuran • methoxychlor • malathion • phytotoxicity • screening method • stem elongation

¹ Contribution from the Dep. of Agronomy, N. Y. State College of Agric. and Life Sci., Cornell Univ., Ithaca, NY 14853, as Agronomy Series Pap. 1209.

² Associate Professor of Agronomy.

Received for publication March 14, 1977.