Effects of Ecology, Life History, and Behavior Edited by Susan Mopper Department of Biology University of Southwestern Louisiana Lafayette, lA Sharon Y. Strauss Section of Evolution and Ecology University of California, Davis Davis, CA CHAPMAN & HALL I (f)p ® INTERNATIONAL THOMSON PUBLISHING Thomson Science New York • Albany • Bonn • Boston • Cincinnati • Detroit London • Madrid • Melbourne o Mexico City o Pacific Grove Cover design: Susan Mopper, Karl Hasenstein, Curtis Tow Graphics Copyright© 1998 by Chapman & Hall Printed in the United States of America Chapman & Hall Chapman & Hall 115 Fifth Avenue 2-6 Boundary Row New York, NY 10003 London SE1 8HN England Thomas Nelson Australia Chapman & Hall GmbH 102 Dodds Street Postfach 100 263 South Melbourne, 3205 D-69442 Weinheim Victoria, Australia Germany International Thomson Editores International Thomson Publishing-Japan Campos Eliseos 385, Piso 7 Hirakawacho-cho Kyowa Building, 3F Col. 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QL493.G45 1997 595.7'135--DC21 97-7322 CIP British Library Cataloguing in Publication Data available To order this or any other Chapman & Hall book, please contact International Thomson Publishing, 7625 Empire Drive, Florence, KY 41042. Phone: (606) 525-6600 or 1-800-842-3636. Fax: (606) 525-7778. e-mail: [email protected]. For a complete listing of Chapman & Hall titles, send your request to Chapman & Hall, Dept. BC, 115 Fifth Avenue, New York, NY 10003. 11 Dispersal and Adaptive Deme Formation in Sedentary Coccoid Insects Lawrence M. Hanks Department Of Entomology, University Of Illinois, Urbana, IL Robert F. Denno Department of Entomology, University of Maryland, College Park, MD 11.1 Introduction Reproductive isolation of populations may eventually lead to genetic differentia­ tion as gene pools are altered by localized selective factors and drift. The spatial structuring of genetic . differentiation depends on the spatial dimensionality of gene flow, which is a reflection of dispersal behavior (e.g., Selander 1970). Ed­ mtmds and Alstad (1978) proposed that the sedentary nature of black pineleaf scale Nuculaspis californica Coleman (Homoptera: Diaspididae) promoted the formation of demes that were adapted to the unique genotype of individual pine host plants. This specialization to the genotype of one host could reduce fitness on conspecific hosts that differ in genotype, thereby limiting the spread of scale 'populations among trees and generating a patchy distribution (Edmunds and Al­ stad 1978). Support for the demic adaptation hypothesis comes from the patchy distribution common to many species of scale insects, whereby heavily infested trees stand among others that appear free of scale (Miller and Kosztarab 1979). However, attempts to document demic adaptation in natural populations of scale insects have yielded conflicting results (Wainhouse and Howe11 1983; Unruh and Luck 1987; Cobb and Whitham 1993; Hanks and Denno 1994). In this chapter, we evaluate the potential for demic adaptation in species of the Homopteran superfamily Coccoidea, which includes armored and soft scale in­ sects, mealybugs, and their allies. Coccoids are sedentary relative to other herbiv­ orous insects and thus may be predisposed to deme formation and local adaptation to individual host trees (Edmunds and Alstad 1978; Miller and Kosztarab 1979; Hanks and Denno 1994). We begin by reviewing the natural history of the coc­ coids. Subsequently, we consider stage-related dispersal behavior to determine whether coccoid populations are especially likely to be reproductively isolated on individual hosts. Reproductive isolation and the persistence of populations on long-lived perennial hosts set the stage for adaptation to host genotype. Finally, we evaluate the evidence for demic adaptation of coccoids in published field studies to reassess the degree of support for this widely espoused hypothesis. 240 I Lawrence M. Hanks and Robert F. Denno 11.2 Natural History of Coccoid Insects The family Coccoidea comprises about 6,000 described species that have been al­ located to as many as 20 families (Miller and Kosztarab 1979). Coccoid families are distinguished by their production of wax fibers that clothe the body or form specialized structures (Table 11.1). Like their closest living relatives the aphids, coccoids are small-bodied insects that feed on plant tissues through needle-like stylets. Eggs are deposited in a single mass under the waxy cover of the female or in a waxy ovisac. The tiny ( < 0.5 mm long) mobile first instars, called crawlers, disperse and soon settle to feed on the host. First ins tars usually remain immobile once they have settled. Female coccoids are neotenic, developing through two to three additional instars before maturing into apterous, reproductively mature nymphs that range in size from 0.5 to 35 mm (Miller and Kosztarab 1979). The morphology of adult females has been used in determining the phylogeny of the Coccoidea and suggests a general decline in mobility from primitive to advanced families (Table 11.1). Male coccoids are paurometabolous, completing two nymphal instars, a "pre­ pupal" stage, and a "pupal" stage before emerging as adults (Miller and Kosz­ tarab 1979; Koteja 1990). Adult males are morphologically similar across coccoid families, usually being < 1 mm long, winged, and lacking functional mouthparts (Giliomee 1990). Males rely on pheromones to locate females (Beardsley and Gonzalez 1975; Miller and Kosztarab 1979). In some species, or variants within species, males are produced in very small numbers or not at all, and females are reproduced parthenogenetically (Nur 1971; Beardsley and Gonzalez 1975; Miller and Kosztarab 1979). Host plants of coccoids are diverse, including bryophytes, gymnosperms, and angiosperms, both monocots and dicots (Davidson and Miller 1990). Woody perennials are their most common hosts, and on these hosts coccoid populations may persist for many decades (Davidson and Miller 1990; McClure 1990a). Polyphagy is common (Beardsley and Gonzalez 1975), and host ranges can be extensive, such as the 236 plant genera in 88 families that are hosts of Aspidiotus nerii Bouche (Davidson and Miller 1990). Coccoids feed on nearly every plant part, from roots, stems, branches, and leaves to fruit (Beardsley and Gonzalez 1975; Miller and Kosztarab 1979). Feeding stylets may be many times longer than the body and tap into a variety of plant tissues, including parenchyma, me­ sophyll, cambium, and phloem (e.g., Glass 1944; Agarwal 1960; Hoy 1961; Williams 1970; Walstad et al. 1973; Blackmore 1981; Sadof and Neal 1993; Ca­ latayud et al. 1994). 11.3 Dispersal, Behavior, and Capability The supposition that coccoid insects are predisposed to demic adaptation pre­ sumes a sedentary nature (Hanks and Denno 1994). Although some of their life Table 11.1 Characteristics of Adult Females for the Common Families of Coccoidea That Feed on Perennial Plantsa Group Family Common name Characteristics of females / Margarodoidea Margarodidae Giant coccids, Ground Covered with loose wax and most with pearls legs well developed, or nymphs legless in waxy cyst with adult having legs present and reduced (ground pearls). Margarodoidea Ortheziidae Ensign coccids Covered with hard, waxy plates, legs well developed. Lecanoidea Pseudococcidae Mealybugs Covered with mealy wax secretions, legs usually well developed. Lecanoidea Eriococcidae Felt scales Similar to pseudococcids in form, without waxy secretions, but may be covered with wax during oviposition. Lecanoidea Dactylopiidae Cochineal insects Covered with wooly wax or waxy plates, legs reduced. Lecanoidea Coccidae Soft scales Exoskeleton usually without wax, or wax ornate or amorphous; legs present or vestigial. Lecanoidea Cryptococcidae Cryptococcids Covered with waxy filaments, legs vestigial. Lecanoidea Asterolecaniidae Pit scales Covered with tough waxy film or embedded in waxy mass, legs vestigial or absent. Diaspidoidea Diaspididae Armored scales Females legless, under a hard cover of wax and tanned protein. a From Howell and Williams 1976; Miller and Kosztarab 1979. Groups are listed in phylogenetic order from primitive to advanced. 242 I Lawrence M. Hanks and Robert F. Denno stages (nymphs of both sexes and adult females) have little or no mobility, coc­ coids nevertheless can move very effectively between hosts, as evidenced by the rapid spread of introduced pest species (e.g., Bean and Godwin 1955; Anderson et al. 1976; Hennessey et al. 1990). In this section, we discuss the dispersal behav­ ior of coccoids and the potential for reproductive isolation of populations on indi­ vidual