POPULATIONECOLOGY Labeling an Egg Parasitoid, Anagrus epos (Hymenoptera: Mymaridae), with RubidiUlll within an Overwintering Refuge ANDREW CORBETI, BROOK C. MURPHY, JAY A. ROSENHEIM, ANDPAUL BRUlNSl Department of Entomology, Universityof California, Davis, CA 95616 Environ.Entomol.25(1): 29-38 (1996) ABSTRACT French prune trees provide overwintering habitat for the egg parasitoid Ana- grns epos Girault (Hymenoptera: Mymaridae), an important natural enemy of the the grape leafhopper, Erythroneura elegantula Osborn (Homoptera: Cicadellidae). French prune trees were treated with rubidium during the fall of 1991-1993 to assess the potential for obtaining an elemental label in overwintering A. epos. Multiple applications of 5,000-ppm solutions of RbCI to French prune foliagefrom late August to early October resulted in increases in foliar rubidium content of up to 200 times the naturally occuring concentration. Rubidium content of adult prune leafhoppers, Edwanuiana prnnicola Edwards (Homoptera: Cicadellidae),col- lected from treated trees was up to 130 times the naturallyoccurring level. Naturallyoccuring rubidium content of A. epos was 0.052 ng Rb per individualbased on a sample of 498 wasps collected from 3 separate, untreated grape vineyards. Mean rubidium content was 0.205 ng Rb in A. epos collected in the spring directly from French prune trees that had been treated in the fall with RbCI. This is 3.9 times the background level and indicates successfullabeling of overwinteringA. epos. However,there was substantialoverlap in rubidium content between A. epos from treated trees and those from untreated sites. A. epos reared from rubidium- enriched host eggs and maintained in the lab on a mixture of honey and water e1iliibited minimal decay of the elemental label during the adult lifetime. Success of this labeling ap- proach has made possible the direct study of the dispersal of A. epos from treated French prune tree refuges into adjacent grape vineyards. KEY WORDS Anagn.ls epos, western grape leafhopper, grapes, elemental labeling, refugia, agricultural landscapes INSECTS OFTEN COLONIZE crops from external persal and colonization, particularly from overwin- habitats during the early period of crop growth. tering habitats, is critical to understanding regional This colonization occurs either from established population dynamics of both herbivores and nat- populations on annual crops that are in the middle ural enemies. It is especially important in the suc- of their growing season (Stern 1969, Brandenburg cessful integration of overwintering refuges within and Kennedy 1982, Fleischer et al. 1988, Corbett annual cropping systems for the enhancement of et al. 1991), or from native vegetation or perennial natural enemies (Pickett et al. 1990, Landis and crops in which the insect overwinters (Doutt and Haas 1992, Corbett in press). We present in this Nakata 1973, Thomas et al. 1991). The amount of article a method for the direct study of early-sea- colonization experienced by an agricultural field son dispersal of a parasitic wasp from an overwin- depends in complicated ways on the nature of the tering refuge. This method involves the chemical surrounding agricultural landscape (for example, labeling of overwintering wasps with the trace el- the proximity and size of potential sources of col- ement rubidium, with the goal of their subsequent onizers), on the dispersal behavior of the insect, as recapture in an adjacent crop following spring well as on climatological and physical aspects of emergence. the environment (for example, Lewis and Dibley Rubidium has been used successfully to label 1970). In most cases, the source of colonizers of a both herbivorous and entomophagous insects in particular crop is not easily determined, and with- the field (Payne and Wood 1984, Fleischer et al. out detailed study it is not necessarily clear what the contribution is by various surrounding land- 1988, Theony et al. 1992; see Akey et al. 1991 for scape components (Brandenburg and Kennedy a thorough review). Because of its location be- 1982, Bishop and Riechert 1990, Wratten and tween potassium and cesium in the periodic table, Thomas 1990). Elucidating the dynamics of dis- rubidium behaves as a chemical analog to potas- sium; therefore, it readily replaces potassium in bi- IDepartmentof Chemistry. ological tissues (Berry et al. 1972). At low concen- 0046-225X/96/0029-0038$02.00/O © 1996 EntomologicalSocietyofAmerica 30 ENVIRONMENTAL ENTOMOLOGY Vol. 25, no. 1 trations, rubidium appears to have no detectable Our research focuses on Anagrus epos Girault. effects on survivorship or behavior (VanSteenwyk an egg parasitoid of the western grape leafhopper, 1991); therefore, it can be used without disrupting Erythroneura elegantula Osborn (Homoptera: Ci- the natural dispersal behavior of an insect. The cadellidae), an important pest of grapes in Califor- amount of rubidium in biological samples can be nia. A. epos is a native species that attacks numer- determined relatively easily through the method of ous leafhopper species; parasitism of grape atomic emission spectrophotometry (that is, mea- leafhopper regularly approaches levels of '=:90%. suring the emission of element-specific wave- A. epos overwinters within the egg stage of its host. lengths by excited atoms) (Akey and Burns 1991). Because grape leafhopper overwinters as an adult, Because rubidium occurs naturally in plant and in- A. epos cannot overwinter withi.n vineyards but sect tissues at fairly low and uniform levels (across must instead colonize from external overwintering geographic areas), relatively small increases in ru- sites, particularly the overwintering eggs of leaf- bidium result in a reliable elemental label that is hoppers using Rubus spp. within riparian habitats detectable above the natural background concen- (Doutt and Nakata 1973). Kido et al. (1984) dem- tration. onstrated that A. epos also overninters in eggs of Rubidium labeling has been achieved either the prune leafhopper, Edwardsiarw prunicola (Ed- through direct sprays of RbCl solution onto foliage wards) (Homoptera: Cicadellidae), on French of herbaceous plants (Graham et al. 1978, Wolfen- prune trees. As a result, research has proceeded barger et al. 198.2,Fleischer et al. 1988) or through on the potential for use of French prune trees as injection of RbCl solutions into the xylem of woody overwintering refuges for A. epo:, within vineyard plants (Fleischer. et al. 1991, Theony et al. 1992). agroecosystems (Pickett et al. 1990, Murphy et al. Herbivorous insects obtain a label through feeding in press). The research presented in this article was on rubidium-enriched plant tissues. Predatory in- undertaken with the goal of using rubidium label- sects (for example, Geocoris spp. [Jackson 1991], ing to quantify the contribution of French prune Onus spp., and miscellaneous spiders [Graham et tree refuges to early-season A. epos populations in al. 1978]) obtain detectable labels through feeding adjacent vineyards and to study A epos dispersal on labeled herbivorous insects. Parasitic Hyme- behavior following emergence. In this article, we noptera also have been shown to obtain a rubidium focus on an evaluation of the feasibility of labeling label through development within a labeled host A. epos overwintering in French prune trees. An (Payne and Wood 1984, Jackson et al. 1988, Hop- analysis of recaptures of labeled individuals will be per and Woolson 1991). presented in subsequent articles. Obtaining a detectable rubidium label in an in- Successful labeling of A. epos overwintering in sect emerging in spring requires the retention of French prune tree refuges for the study of dis- persal and colonization requires the successful the label through an overwintering stage, typically lasting many months. The longevity of a rubidium completion of a series of steps. Specifically,the fol- lowing 4 questions need to be addressed: (1) Is it label (the time following removal from the Rb possible to enrich the rubidium content of French source during which a label is detectable above the prune foliage late in the season, during the period background level) is largely dependent on the when prune leafhopper females that will lay over- amount of time spent feeding on the source and wintering eggs are developing? (2) Will feeding by the amount of feeding done after removal from (or immature prune leafhoppers on rubidium-en- dispersal from) the source (VanSteenwyk 1991). In riched foliage result in labeling of adults and their principle, an insect in a quiescent, overwintering eggs? (3) Do A. epos that have overwintered in phase should retain the rubidium label because prune leafhoppers eggs in treated French prune there will be no feeding, and loss of the Rb trees contain levels of rubidium that are measur- through some other route is unlikely. Results by ably greater than untreated A. epos? (4) Will A. Wolfenbarger et al. (1982) with the boll weevil, epos with increased rubidium content retain the Anthonomus grandis grandis Boheman (Coleop- label over a significant portion of their life? In this tera: Curculionidae), support this proposal. They article we present the results of an evaluation of found detectable levels of rubidium in boll weevils these steps. in March, 6 mo following application of RbCl so- lution to cotton fields. They hypothesized that these individuals may have overwintered near the Materials and Methods study field. Such long-term labeling should also be Study Sites. We applied rubidium to French possible for a developing parasitic wasp using an prune trees during the fall of 1991, 1992, and 1993 overwintering herbivorous host. Evaluation of ru- to obtain labeled A. epos emerging: the subsequent bidium labeling of parasitic wasps has so far been springs (1992, 1993, and 1994, respectively). In done either in the laboratory or in the field within 1991, rubidium was applied at 2 sites at E. & J. the same growing season as treatment with RbCI. Gallo Ranch near Livingston, CA (Merced Coun- To date there are no published accounts of at- ty). The Hayes' site consisted of sixty, 2-yr-old tempts to label a parasitoid of an overwintering French prune trees =:05 m in height in 2 staggered herbivorous host in the field.