ECONOMIC IMPORTANCE OF INSECTS ON REGROWTHS OF ESTABLISHED ALFALFA FIELDS IN ONTARIO

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Abstract
Damage caused by Empoasca fabae (Harris) and Philaenus spumarius (L.), caged on second and third alfalfa harvests, was additive. Numbers of E. fabae were linearly correlated negatively with plant height, leaf area, and percentage protein, and positively with percentage chlorosis. Perceptible stunting and yellowing occurred with mean infestations per stem of 0.09 E. fabae and 0.6 P. spumarius nymphs. P. spumarius adults reduced plant height only at infestations of 0.3/stem, while 0.6 nymph/stem also reduced dry weight. E. fabae reduced protein at 0.11/stem and dry weight and leaf area at higher populations of 0.17/stem. None of these parameters was affected on subsequent regrowths. Lygus lineolaris (P. de B.) could not be maintained on pre-bloom alfalfa and was not considered a pest. Adelphocoris lineolatus (Goeze) reduced protein at levels of 0.01 bug/stem and dry weight at 0.06 bug/stem but results were confounded by high mortality. Field populations of all these species were reduced by the second cutting but redeveloped on the regrowth. Although applications of dimethoate (200 g AI/ha) and dimethoate (200 g AI/ha) plus endosulphan (280 g AI/ha) were effective in reducing populations of E. fabae, Acyrthosiphon pisum (Harris), A. lineolatus, and Hypera postica (Gyllenhal), insecticides were cost-effective on only one field. This field had 1.9 E. fabae/sweep and a 3% loss in protein resulted. Plant height, density, dry weight, protein content, and overwintering survival were not significantly affected at harvest in other fields. With the exception of E. fabae, the sum of the maximum field populations, each expressed as a fraction of its threshold, did not reach 50% of an economic threshold.