Influence of postharvest alfalfa stubble on potato leafhopper, Empoasca fabae (Harris) (Homoptera, Cicadellidae). (1986)

Long stubble or lodged growth left in the field when harvesting alfalfa, Med icago sativa L., favored survival and subsequent population increase of potato leafhopper, Empoasca fabae (Harris). Nymphal and adult numbers on the regrowth were directly related to postharvest stubble height, or length of the lodged alfalfa. Predator populations were greater in plots with more green residue, but did not appreciably reduce numbers of leaf hopper nymphs or adults. Potato leafhopper, Empoasca fabae (Harris), is a major insect pest of alfalfa, Medicago sativa L., in the midwestern U.S. Initial influxes of potato leafhopper are dependent upon long distance transport from overwintering sites in the south ern Gulf states (Medler, 1957). Arrival dates and magnitude of these migrations vary greatly from year to year. Subsequent population increases and dispersal are influenced by crop management practices. Leafhoppers have been shown to dis perse from harvested alfalfa to new regrowth (Simonet and Pienkowski, 1979) and to other susceptible crops (Poston, 1975), e.g., the harvesting of alfalfa is thought to drive potato leafhopper into potatoes. Cuperus et al. (1983) showed higher numbers of potato leafhoppers in plots bordered by standing alfalfa. Radcliffe et al. (1976) showed 95% reduction in insect numbers with clean harvest. Simonet and Pienkowski (1979) showed that the postharvest survival of potato leafhopper nymphs was greatly increased when considerable green residue was left in the field. Also, leaving such residues retained adult leafhoppers and favored rapid repopulation of the harvested field (Cuperus et al., 1983; Simonet and Pienkowski, 1979). Objectives of this research were to measure effects of postharvest green residues on 1) retention of potato leafhopper adults, 2) subsequent increase in nymphal numbers, and 3) natural enemy abundance in the regrowth. Materials and Methods Experiments described here were conducted at the Rosemount Agricultural Experiment Station in east central Minnesota on cv. '520' alfalfa using third regrowth in 1980 and 1982 and second regrowth in 1983. Insect populations were sampled using a vacuum machine of our own (EBR) design. Sampling was ac complished by making 15 setdowns'Vplot of the vacuum hose (equivalent to 1 Paper no. 14,390, Scientific Journal Series, Minnesota Agric. Exp. Stn. Mention of a proprietary product does not imply endorsement by the University of Minnesota. 2 Present address, Department of Entomology, Oklahoma State University, Stillwater, Oklahoma 74078. Accepted for publication 19 August 1985. This content downloaded from 157.55.39.78 on Wed, 22 Jun 2016 06:20:17 UTC All use subject to http://about.jstor.org/terms VOLUME 59, NUMBER 2 247 Table 1. Effects of different harvest methods and postharvest green residues on densities of selected arthropods on third regrowth cv. '520' alfalfa. Samples collected 3, 7, and 14 days after harvest.ab Rosemount, Minn., 1980. Stubble Potato leafhopper Beneficials height Harvest method (cm) Adults Nymphs Hymenoptera Nabids Spiders 3 days Flail-harvested 1 7a 0a 0a 40a 10a Swathed 8 12b la 5b 49a 14a Swathed 15 14b 0a 8b 53a 11a 7 days Flail-harvested 1 0a 0a la 2a 3a Swathed 8 2a la 0a 7a 7ab Swathed 15 7b 0a 4b 32b 16b 14 days Flail-harvested 1 5a 0a 3a 3a 9a Swathed 8 6ab 2a la 6ab 10a Swathed 15 9b 2a la 8b 12a a Numbers within columns for a given date and followed by the same letter are not significantly different (P < 0.05) by Duncan's MRT. b Numbers indicate totals of 4 replications of 15 setdowns (equivalent in area to 0.48 m2) per plot of the intake hose of a vacuum machine. 0.48 m2) with a suction pressure of 500 mVsecond 9 cm from the hose mouth. Sampling was accomplished by moving the hose opening vertically into the crop, holding it near ground level for 2-3 seconds, then moving to a new site within the experimental plot. Collected material was placed in plastic bags and methyl isobutyl ketone was added to dislodge and kill the insects. Samples were frozen for later sorting and counting. 1980: Plots were 9 x 9 m (0.01 ha) arranged in a randomized complete block design replicated 4 times. Three treatments were compared: swathed leaving 8 cm stubble, swathed leaving 15 cm stubble, and flail-chopped leaving 2 cm stubble. Swathed alfalfa was removed from the plots 1 day after harvest. Insects were sampled 3, 7, and 14 days after cutting. 1982: Plots were 15 x 30 m (0.05 ha) arranged in a randomized complete block design replicated 4 times. Four treatments were compared: swathed leaving 2 cm stubble, swathed leaving 8 cm stubble, swathed leaving 15 cm stubble, and de liberately lodged by means of a seedbed packer prior to swathing at 8 cm. The alfalfa was windrowed and field-cured for 8 days before removal. Insects were sampled 8, 12, 16, 22, and 29 days after cutting. 1983: Plots were 15 x 30 m (0.05 ha) arranged in a randomized split block design. The treatments compared were: identical to those used in 1982. The alfalfa was windrowed and field-cured for 7 days before removal. One day following removal of the alfalfa, one half of the field was sprayed with carbofuran at 0.56 kg Al/ha resulting in a split plot design with four replications. The insecticide was applied in 46 1/ha water at 2.1 kg/cm2 with a tractor-drawn boom sprayer. Insects were sampled 8, 12, 15, 20, 25, and 29 days after cutting. This content downloaded from 157.55.39.78 on Wed, 22 Jun 2016 06:20:17 UTC All use subject to http://about.jstor.org/terms 248 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY H V 5 Xx .15cm o . Nv -8c -^ v n 2 cm