Impact of Insecticide Applications at Various Alfalfa Growth Stages on Potato Leafhopper (Homoptera: Cicadellidae) Abundance and Crop Damage

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Abstract
The effects of four experimental treatments (insecticide applied at 1 week, 2 weeks, or 3 weeks of growth after the first cutting and an unsprayed control) on potato leafhopper (PLH), Empoasca fabae, abundance and its damage to alfalfa were measured. The 2 years of this study differed in the time of PLH population increase and in the damage to alfalfa. Four days after the first alfalfa cutting, PLH adults and nymphs numbered 29.4 per m2 in 1982, but they numbered 5.1 per m2 in 1983. Oviposition during the first week of regrowth in 1982 resulted in a large increase in nymph abundance, while in 1983 a large increase in nymph abundance was associated with oviposition during 2–3 weeks of regrowth. Dry matter and protein yields were highest when insecticide was applied 1 week after cutting in 1982, but in 1983 yields did not differ among treatments receiving insecticide. Only the yield from the 3-week treatment was significantly greater than the yield from the unsprayed treatment. Thus, PLH damage to alfalfa occurred during most of the growth of the second crop, and damaging populations occurred either shortly after the first cutting or subsequent to colonization 2 weeks after cutting. These results suggest that crop protection from PLH requires frequent assessment of PLH abundance.