The influence of corn plant phenology on the dynamics of adult western corn rootworm, Diabrotica virgifera virgifera, populations was studied during 1988 and 1989 in com fields artificially infested with eggs. Fifty percent of adult emergence from the soil occurred by day 194 in 1988 and day 203 in 1989. In both years, adult emergence was synchronized with corn flowering, eggs were recovered in soil samples approximately four days after reproductive females were first observed in the population, and oviposition was essentially complete about 25 days after it began. The number of reproductive female beetle-days accumulating per m 2 was similar in both years. Approximately two times as many eggs were laid in 1988 (1239 eggs 1m2) as in 1989 (590 eggs 1m2). The difference in egg density may have been caused by differences among years in the temporal synchrony of reproductive bee tles with flowering corn. Daily survival rates of adults were high while corn was flowering; exhibited a gradual decline during grain filling; and decreased rapidly during the grain drying stage. The western corn rootworm, Diabrotica virgifera virgifera LeConte, is among the most destructive insect pests of corn, Zea mays, grown in the midwestern United States. The species is univoltine and overwinters in the egg s age. Eggs, larvae, and pupae are subterranean. Eggs hatch in the spring, and larvae feed on the roots of corn plants. Larval feeding reduces the ability of corn plants to absorb nutrients and moisture from the soil and makes them susceptible to lodging. Adults emerge from the soil in the summer and feed primarily on the leaves, silks, and pollen of corn. The potential for large, damaging, larval populations in fields is generally deter mined by the number of eggs laid there the previous summer; however highly variable and unpredictable mortality during the immature life stages makes predic tion of damage difficult. The number of eggs laid per unit area in a field is related to the density of female beetles, their fecundity, and residence time in the field. Beetles of both sexes feed on a variety of corn tissues, but survival and fecundity rates vary with the tissues upon which they feed, and consequently, on the growth stage of corn plants in the field (Elliott et at. 1990a). Dispersal rates of adults are also influenced by corn plant phenology. Beetles are attracted to fields of silking and pollinating 'USDA, ARS Northern Grain Insects Research Laboratory, Rural Route #3, Brookings, SD 57006. 2Current AddressUSDA, ARS Plant Science Research Laboratory, BOl N. Western Rd., Stillwater, OK 74075. 1 Elliott et al.: A Descriptive Study of the Population Dynamics of Adult Diabro Published by ValpoScholar, 1991 160 THE GREAT LAKES ENTOMOLOGIST Vol. 24, No.3 corn, but tend to leave fields in which corn has advanced to more mature stages of growth (Hill and Mayo 1974, Godfrey and Turpin 1983). Recent advances in laboratory rearing methods for D. virgifera that permit pro duction of large numbers of D. virgifera eggs (Jackson 1986) have made it possible to infest relatively large field plots with egg at densities similar to those encountered in naturally infested fields. Field tests conducted over several years have failed to demonstrate differences in the biology f beetles developing from e gs laid by colony beetles and those developing from eggs laid by field collected beetles (J. R. Fisher unpublished data). Thus, artificial infestation may be a useful method for initiating D. virgifera field populations with dynamics similar to those of naturally occurring populations. Previous studies of the seasonal population dynamics of adult D. virgifera related patterns of adult emergence from the soil, oviposition, and population change to calendar date and heat units (Hein and Tollefson 1985, Hein et al. 1988). There have been no studies of adult D. virgifera field populations in which the processes of emergence from the soil, reproductive development, oviposition, and mortality were studied in relation to corn plant phenology. Our objectives were to measure beetle emergence from the soil, sex ratio, reproductive development, mortality, and ovipo sition in relation to time during the growing season and corn plant phenology. Because extreme annual variation in the climate of eastern South Dakota (particu larly during winter) makes natural infestations of D. virgifera sporadic and unpre dictable, we chose to conduct our studies in artificially infested fields. MATERIALS AND METHODS Field studies were conducted in eastern South Dakota during 1988 and 1989. Each year, a single 0.4 ha field on a research farm located adjacent to the USDA, ARS Northern Grain Insects Research Laboratory, Brookings, South Dakota, that had been planted to wheat the previous year (1988) or fallowed (1989) was infested with D. virgifera eggs obtained from a laboratory colony maintained by the methods of Jackson (1986). Corn (,Pioneer 3978') was planted and fields were infested with eggs on 12 May each year. The entire length of each row of corn in a field was infested with eggs at population densities of 2612 and 2275 eggs per m 2 in 1988 and 1989, respectively, using methods described by Sutter and Branson (1986). Corn was planted at densities of 5.22 and 4.77 plants per m 2 in 1988 and 1989, respectively. To facilitate sampling each year, the field was partitioned into 12 rectangular sub plots of equal size. Two emergence traps similar to those described by Fisher (1984) were positioned at random locations within each sub-plot (total 24 traps). The emergence traps used were 0.91 m wide in 1988 and 1.0 m wide in 1989 (one row width) and were 0.61 m in length (three times the plant spacing within rows). Each trap was centered over three plants within a row. Emerged beetles were collected from traps three times weekly on alternate days. The number and sex of beetles in each collection were recorded. The popUlation density of adult beetles in the field was determined three times each week by counting all beetles on an entire corn plant and the soil surface and weeds surrounding the plant. Beetles were counted on each of four plants selected in a haphazard fashion from within each sub-plot (total 48 plants). Hanway's (1966) 0-10 numeric rating system was used to estimate the growth stage of each sampled corn plant. We combined plant phenology data into three groups. We considered corn plants in the field to be in the flowering stage from the date at which 10070 of plants had advanced at least to stage 4 (tassel visible) to the date at which 90070 of plants had advanced beyond stage 5.5 (pollination complete, silks beginning to turn brown), plants to be in the grain filling stage from the date at which more than 90070 of plants had progressed beyond stage 5.5 to the date at which 90070 of plants had 2 The Great Lakes Entomologist, Vol. 24, No. 3 [1991], Art. 5 https://scholar.valpo.edu/tgle/vol24/iss3/5 1991 THE GREAT LAKES ENTOMOLOGIST 161 advanced beyond stage 8 (a few kernels with dents), and plants to be in the grain drying stage from the date at which more than 90% had advanced to stage 9 (all kernels with dents). Soil samples were taken once each week beginning approximately one week after beetles began to emerge from the soil and continuing until the study was terminated. Each soil sample consisted of 12 subsamples (one from each sub-plot). A subsample consisted of 10 cylindrical cores taken with a 5.4 cm diameter bulb-setter to a depth of 15 cm. Each core was taken directly within a row at the base of a corn plant selected in a haphazard fashion from within the sub-ploL The 10 cores were sifted through a I cm screen, thoroughly mixed, and 0.47 I of soil was removed for processing. Eggs were washed from each subsample using the method of Shaw et al. (1976) and floatation in magnesium sulphate. All D. virgijera eggs were counted and identified to species by chorion sculpturing (At yeo et al. 1964). On the date the study was terminated each year, an estimate of the absolute population density of eggs was obtained using the frame method (Foster et al. 1979). The frame was one-half the row width long and one-half the plant spacing within rows wide. A sample consisted of 24 subsamples (two subsamples from each sub plot); each subsample consisted of the soil dug to a depth of 20 cm within a single frame placed at the base of a plant chosen at random from within the sub-plot. Soil from each subsample was sifted and mixed as described above, and 0.471 of soil was removed for processing. were washed from the soil, counted, and identified to species as described above. Twice weekly, 50-75 female beetles were collected from within the field, taken to the laboratory, and dissected to determine their reproductive status. A 1 to 5 scale was used to rate ovarian stage of development (Cinereski and Chiang 1968). Beetles with ovaries rating 1-2 were considered reproductively immature, beetles rating 3-4 were reproductively mature, and beetles rating 5 were post-reproductive (Short and Hill 1972). Survival rates of beetles were estimated once each week. One-hundred female and 50 male beetles were collected from within the field and placed in screen cages described by Elliott et al. (1990a). Each cage enclosed a single plant, and 5 male and 10 female beetles were placed in a cage. Growth stages of the 10 caged plants were representative of those in the field. Cages were left in place for 48 hours after which the number of surviving beetles of each sex was determined. A proportional daily survival rate was calculated from survival data for each 48 hour period. In making calculations we assumed that the survival rate of beetles was constant each day during the 48 hour period. Total beetle-days per m 2 over the season were calculated for components of the adult population by fitting a cubic spline function to observed time series of popula tion density data and calculating the
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