ALFALFA HARVEST STRATEGY EFFECT ON LYGUS BUG (HEMIPTERA, MIRIDAE) ANDINSECT PREDATOR POPULATION-DENSITY - IMPLICATIONS FOR USE AS TRAP CROP IN COTTON

Lygus bug, Lygus hesperus Knight, and beneficial insect populations (O rius sp., Geocoris spp., and Nabis spp.) were quantified in alfalfa st rips, which were interplanted within cotton fields. The propensity of alfalfa strips to retain lygus bugs, thereby keeping them out of cotto n, and the utility of alfalfa strips as a reservoir for beneficial ins ects were evaluated from May to August in 1990 and 1991. The following three alfalfa cutting strategies were examined: (1) uncut alfalfa, (2 ) a 28-d strip-cut treatment, and (3) a 35-d strip-cut treatment. Inse ct populations were quantified in both the newer alfalfa growth and th e older alfalfa growth of the strip-cut treatments. Lygus bug densitie s were significantly higher in the uncut treatment than in the other t reatments, peaking at 520 per 1.9 m2 in 1990 and 350 per 1.9 m2 in 199 1. Lygus bug levels in the strip-cut treatments were low in the newer alfalfa growth (averaging 41.8 per 1.9 m2) and only slightly higher in the older alfalfa growth of the 28-d strip-cut treatment (averaging 6 5.3 per 1.9 m2). Populations were relatively constant within these thr ee treatments throughout the season. However, in the older growth of t he 35-d cutting regime, Lygus bug populations averaged 103 per 1.9 m2. Densities of beneficial insects were also higher in the uncut than th e strip-cut treatments, but the higher predator densities apparently d id not offset the high Lygus bug population increase. The uncut treatm ent also lacked Lygus bug egg and nymphal mortality from the alfalfa c utting as compared with the strip-cut treatments. The less frequent cu tting, in the 35-d compared with the 28-d treatment, probably also all owed for a buildup of the Lygus bug population. Lygus bug instar distr ibution, sex, and predator species were also examined in these treatme nts. It appears that to optimize Lygus bug management, while still pro ducing high densities of predacious insects, strip-cut alfalfa should be used and cut on a 28-d schedule.