Effects of Potato Leafhoppers on Soybean Plant Growth and Yield

Various densities of adult potato leafhoppers were established on 'Essex' soybean at plant growth stage V2 in caged field plots. Cages were removed two weeks later, and plots remained uncaged for the rest of the growing season. Direct counts of nymphal leafhoppers on plants shortly after cage removal confirmed that a gradient of infestation levels was attained. In both years of the study, higher levels of infestation resulted in significant reductions in each of the following: plant height, number of nodes per plant, and average internodal length in the lower portion of the plant. Yield was significantly reduced at higher infestation levels in the first year of the study, but not in the second. Rainfall patterns were markedly different during the two growing seasons, and it is suggested that early season moisture stress may exacerbate the effect of potato leafhopper infestation on soybean yield. The potato leafhopper, Empoasca fabae (Harris), is a migratory insect which typically is first collected each year in Kentucky alfalfa fields during May (Parr and Pass, 1989). In addition to alfalfa, the potato leafhopper is found on many other host plants, including nearly all leguminous crops. During earlyand mid summer, it is consistently one of the most abundant insects found on soybean in the midwestern United States (Helm et al., 1980, and references therein). Population growth of the potato leafhopper on soybean and the damage it causes to this host plant are influenced by plant pubescence. Experimental soybean lines with glabrous or appressed pubescence characters are highly susceptible to damage from this insect (Johnson and Hollowell, 1935; Broersma et al., 1972). Com mercial soybean varieties, however, typically are covered with normal, erect tri chomes (Bernard and Singh, 1969) and usually appear to suffer little damage from natural infestations of potato leafhoppers. In experiments using caged 'Amsoy' soybean (normal pubescence), Ogunlana and Pedigo (1974a) determined economic injury levels for potato leafhoppers at various soybean growth stages. In a companion study, in which they sampled potato leafhoppers in soybean fields throughout two growing seasons at four locations in central Iowa, leafhopper densities never reached the economic injury level (Ogunlana and Pedigo, 1974b). Poston and Pedigo (1975), however, found that removal of alfalfa at first harvest caused migration of potato leafhoppers from alfalfa plots into adjacent soybean plots, sometimes leading to damaging population levels on the young soybean plants. Others have suggested there is little evidence that typical natural populations of potato leafhopper on soybean cause significant damage or apparent effects on yield (e.g., Turnipseed and Kogan, 1976; Helm et al., 1980). Several factors led us to investigate the effects of potato leafhopper on soybean plant growth and yield. First, and most importantly, soybean growers periodically 1 Present address: Department of Entomology, University of Georgia, Georgia Experiment Station, Griffin, Georgia 30223. Accepted for publication 4 September 1993. 30 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY inquire about this insect (and the possible need for control) when they notice potato leafhoppers associated with "hopperburn" and stunted plants during the early summer. In our studies of other soybean insects we, too, had observed severe early season symptoms of leafhopper damage in some years, but we did not know if such injury during vegetative growth stages of soybean resulted in yield reduc tion. Also, we were uncertain about the applicability of results determined pre viously for 'Amsoy' soybean, an indeterminate, early maturing variety (Ogunlana and Pedigo, 1974a), to varieties currently grown in Kentucky, especially later maturing, determinate varieties. Finally, studies in Kentucky alfalfa (Parr and Pass, 1989) and our observations in soybean indicated that numbers of colonizing leafhoppers varied tremendously from year to year. During years of highest pop ulations, it seemed at least feasible that potato leafhoppers might reach infestation levels on soybean that could reduce yield. Even if hopperburn and plant stunting did not result in yield reductions, such information would be important in order to make appropriate recommendations to growers. Our objectives were to determine the effects of the potato leafhopper on plant growth and whether symptomatic injury (e.g., plant stunting) led to yield reduc tions. The variety 'Essex' was used because it is a late-maturing, determinate variety widely grown in Kentucky. Effects of early season infestations were studied because the potato leafhopper typically is most abundant in Kentucky soybean fields during the early part of the growing season (Raney and Yeargan, 1977). Materials and Methods general: All studies were done near Lexington, Kentucky, using 'Essex' soy bean planted in rows with 0.9 m spacing between rows. Soybean seedlings were thinned at growth stage VI (Fehr and Caviness, 1977) to a density of 26 plants per m of row. Soybeans were planted into tilled soil; the herbicides alachlor and linuron were applied at planting (2.2 and 1.1 kg active ingredient per ha, respec tively). Cages made of wooden frames covered with nylon screen (mesh size = 0.8 mm) were set in place one to three days prior to artificial infestation of selected plots with additional potato leafhopper adults. Each cage measured 0.9 x 1.8 x 0.9 m (1 x w x h) and enclosed a total of 1.8 m of soybean row (i.e., 0.9 m in each of two adjacent rows). In both 1988 and 1989, cages remained in place for two weeks following artificial infestation of soybean plants. At the end of that period, cages were removed and the plots were not caged for the remainder of the season. Following cage removal, direct visual counts of leafhopper nymphs were made to determine if the treatments had produced the desired differences in infestation levels (sampling details for each year are provided below). Nymphal densities, rather than adult densities, were chosen as an index of success in establishing differential infestations because it is virtually impossible to accurately determine adult densities without disturbing the plants. Potato leafhoppers used in these studies were collected by sweep net from nearby alfalfa fields several hours prior to their introduction into the field cages. No attempt was made to determine the sex of leafhoppers introduced into cages, but there is no reason to suspect sex ratios differed among the groups added to different