Preliminary study of the biology of the white grub Phyllophaga fimbripes (LeConte) (Coleoptera: Scarabaeidae).

A taxonomic description of the larva of the white grub Phyllophaga fimbripes (Cole?ptera: Scarabaeidae) is provid ed. Rangeland at the Pawnee National Grasslands in northeast Col orado was surveyed for P. fimbripes damage to blue grama grass, Bouteloua gracilis (H.B.K.), and damaged areas mapped. Soil be neath brush plants, and soil in transition zones between beetle killed and non-killed areas contained the largest populations of first-instar larvae. Soil is less dense in brush areas, and it was hypothesized that beetles find it easier to burrow in these areas. The adults were observed on several nights, and appeared to be weak fliers, flying only for a brief period each night. The flight season in 1978 and 1979 lasted from mid-July to early August. The population declined sharply between fall, 1977 and spring, 1978. White grubs may be serious pests of rangelands. Larvae feeding under ground on the roots of grasses have caused extensive damage to rangelands in the United States (Davis 1918). The white grub Phyllophaga fimbripes has been implicated in the destruction of considerable areas of perennial forage grasses on some parts of the Pawnee National Grasslands in north eastern Colorado. This area is important to the cattle industry of Colorado. The purpose of this study was to determine the biology of this insect, and its impact on the grasses of the region. Studies were undertaken in the field to determine the extent of grass mortality, beetle site preferences, popula tion trends, and habitat distribution. In addition, observations of adults dur ing the flight season were made and the larva was formally described. The adult of P. fimbripes was first described by LeConte (1856). The first taxonomic study of Phyllophaga larvae was performed by Hayes (1929), who used the mouthparts and stridulating organ as key characteristics. This system has proven generally satisfactory. It was revised by Boving (1942) in a monograph which includes descriptions of, and a key to, the known Phyllophaga species. Received for publication 28 November 1979. This content downloaded from 207.46.13.112 on Mon, 03 Oct 2016 05:01:35 UTC All use subject to http://about.jstor.org/terms 702 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY Fig. 1. Blue grama sod killed by Phyllopohaga fimbripes feeding. Note dead blue grama crowns. Photograph taken in April 1979. Materials and Methods experimental area: Most of this work was done at the Central Plains Experimental Range of the Science and Education Administration, USDA, 60 km northeast of Fort Collins, Colorado. The area is flat to rolling and is dominated by blue grama grass, Bouteloua gracilis (H.B.K.) Lag ex Steud. A co-dominant is plains prickleypear, Opuntia polyacantha Haw. The field in which the population and site preference studies were conducted also contained four-wing saltbush, Atiplex canascens (Pursh) Nutt. Areas of grass which have been destroyed by beetles are dominated by the annuals Russian thistle, Salsola kali tunnuifolia Tausch, and the goosefoots, Che nopodium spp., and the low perennial scarlet globemallow, Sphaeralcea cocci ne a (Pursh) Rydb. survey: The western portion of the Pawnee National Grasslands in northeastern Colorado was surveyed visually for typical grub damage during July and August of 1978. population and site preference: Sampling for first-instar beetle lar vae was accomplished by taking core samples (10 cm diameter x 24 cm depth), with a hand operated metal sampler. The soil beneath three habitats was samples: four-wing saltbush, plains prickleypear, and brush-free. Each This content downloaded from 207.46.13.112 on Mon, 03 Oct 2016 05:01:35 UTC All use subject to http://about.jstor.org/terms VOLUME 53, NUMBER 4 703 Fig. 2. Healthy blue grama sod. Photograph taken April 1979. of these habitats was sampled in kill, non-kill, and transition zones, for a total of 9 treatments. A kill area (Fig. 1) was defined as an area with less than 10% blue grama cover, a non-kill area (Fig. 2) as one with greater than 45% blue grama cover, and a transition zone (Fig. 3) as the intersect of these two zones. Core samples were divided into 3 portions in the field to determine vertical distribution of grubs in the soil. There were 10 replica tions per treatment for each of two sampling periods. The first sampling period was from October 4 to November 15, 1977, and the second from September 9 to November 3, 1978. The samples were transported back to the laboratory, sifted through 16 mesh screen, and the number of first-instar grubs per sample determined. Temperature and precipitation data from the Experimental Range was obtained from the Natural Resources Ecology Laboratory at Colorado State University, and an attempt was made to correlate weather with beetle pop ulation trends. Tests of relative soil compaction and surface temperatures for 6 treat ments were conducted in an effort to isolate factors that might influence the population distribution. For this study transition zone treatments were omitted. Twenty surface temperatures were taken at dusk for each treatment This content downloaded from 207.46.13.112 on Mon, 03 Oct 2016 05:01:35 UTC All use subject to http://about.jstor.org/terms 704 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY Fig. 3. Transition zone between beetle-killed blue grama sod and healthy sod. Photograph taken in April 1979. with an infrared thermometer (Mikon Instruments Inc., Ridgewood, NJ). Fifteen replications of relative soil compaction were determined with a pen etrometer (Humboldt Manufacturing Co., Northridge, IL) for each treat ment. A replication was defined as the average of five penetrometer readings taken in a particular area, e.g., the base of a brush plant. Data were analyzed with analysis of variance at the 5% level, and means were separated with Tukey's HSD. A Vx + 0.5 transformation was used for data from the first-instar sampling due to the large number of zeroes in the data. adult feeding and activity: Numerous plant species were examined in the field during the period of beetle flight activity for evidence of beetle feeding. A blacklight trap was erected in a beetle damaged field 6.6 km west of Briggsdale, Colorado to monitor the length of the adult flight season. Flight behavior was observed on two evenings during late July, 1978 and five evenings during late July, 1979. larval descriptions: First and third-instar larvae of Phyllophaga fim This content downloaded from 207.46.13.112 on Mon, 03 Oct 2016 05:01:35 UTC All use subject to http://about.jstor.org/terms VOLUME 53, NUMBER 4 705 WYO. WWMU NATIONAL GRASSLANDS