Technique for Rearing Mite-Free Southern Pine Beetle, Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae), Adults1

Southern pine beetles can be reared free of phoretic mites from naturally infested bark if the bark is removed from the tree and air dried. Bark removal does not reduce the number of beetles that emerge. On the average fewer than 1 % of the beetles emerging from removed bark carried one or fewer mites, and 85% of the beetles emerging from attacked bark carried one or more mites. Seven species of mites were found on beetles emerging from attached bark. About 15 species of mites are commonly transported by the southern pine beetle (SPB), Dendroctonus .frontalis Zimmermann 1868 (Moser and Roton 197 1, Kinn 19761, with the proportion of phoretized flying adults conservatively estimated at 40% (Moser 1976). The trophic habits of these phoretic mites range from predaceous, mycophagous, nematophagous, or saprophagous to various combinations of feeding preferences. Some are detrimental to the SPB, others are neutral or beneficial (Kinn 1967, 1980, Moser, 1975, Wilson 1980). Many numerous and complex relationships exist among these mites and the SPB; hence mites should be excluded from studies to determine the capacity of SPB to attack trees and produce broods in the absence of selected associates. At present there are no efficient methods for obtaining large numbers of SPB free of phoretic mites. Mechanically removing mites invariably injures the beetles because the elytra have to be lifted. Rearing beetle adults from eggs on artificial media is tedious and time consuming; very few adults are produced (Bridges 1979). Kinn (1979) first documented that desiccation of the phloem killed some mite species. Subsequent studies by the authors showed that desiccation might severely reduce or eliminate all mite populations, resulting in virtually no phoretic mites on emerging beetles. In this study we decided to see whether mite-free populations of SPB could be reared from desiccated host material. Materials and Methods Four loblolly pines (Pinus tneda L.) infested with SPB were cut during February, May, August, and December 1980. The trees were from a natural stand about 35 years old in Catahoula Ranger District of the Kisatchie National Forest. Trees were felled when the SPB broods were pupae in the outer bark. Pupae do not carry phoretic mites (Roton 1978). Samples were taken from the bole area where the pupal stage was most concentrated and where competition from other bark beetle species was least. Typically, the sample area extended 5 m through 11 m from ground level. No attempt was made to normalize infested bole heights within trees (McClelland et a1 1979), because it was not our objective to sample tree populations systematically. The infested bole area of each tree was divided into five sections. Each section was subdivided into three sample bolts trimmed so each contained 2,880 cm2 of bark area. The uppermost bolt of each section was a survey bolt. A sixth survey bolt was taken from just below the first section. The other two bolts of each section were randomly assigned to one of two treatments. For one treattnent, the bark was removed and exposed to the air inside the laboratory for 36 h. For the other treatment (control) the bark was not removed. Material from both treatments was placed in individual rearing cans inside the laboratory and kept at about 20°C and 50% relative humidity. The can interiors were dark with little or no air movement. The survey bolt of the middle section was used to determine how long mites survived on the inner surface of drying bark. The mite surveys were made <24 h after the bolts were cut, then daily until mites were no longer observed. In a preliminary survey at the start of the experiment, all the bark of the other five survey bolts was examined to establish which species of mites were present and to estimate their relative abundance. Inner bark moisture was measured with a model RC1 Delrnhorst moisture detector. Readings were taken from the mid-section of the five survey and five control bolts of each tree <30 min after they arrived from the field. Moisture readings were also recorded from the inner bark of the five stripped bolts per tree. Data were analyzed using the Statistical Analysis SysTo determine the effect of bark removal on beetle emergence an analysis of variance was run using a splitplot design. The whole units were locations along the infested bole, and the subunits were treatments. The response variable was the total number of beetles that emerged. To determine which species of mites the beetles carried most often, a split-plot design was used with 'Trade names and company names are included to identify equiplocations on the infested bolt as the whole units and mite ment used, and do not constitute endorsement by USDA. Received for ~ublication 29 October 1982: acce~ted 1 1 Julv 1983. species as the subunits. Duncan's multiple range test ' 2 S ~ S Institute, Inc., SAS ciicle, Box 8600, Cary, NC 77511. was used to compare means. November 1983 MOSER AND BRIDGES: TECHNIQUE FOR REARING MITE-FREE SPB 943 FIG. 1 . Beetle emergence from removed bark.