Leaf Ties As Colonization Sites for Forest Arthropods: an Experimental Study

Ecological Entomology (2004) 29,300–308 Leaf ties as colonization sites for forest arthropods: an experimental study JOHN T. LILL andROBERT J. MARQUIS Department of Biology,University of Missouri-St Louis,St Louis,Missouri,U.S.A. Abstract. 1. Secondary colonization of leaf shelters constructed by caterpillars has been reported from a number of systems. Both the mechanism (larval or adult movement vs. oviposition) and the cues used by arthropods in locating leaf shelters,however,have received little attention. 2. Artificial leaf shelters (i.e. leaf ties or pairs of leaves clipped together to form sandwiches) were constructed on understorey white oak (Quercus alba L.) trees and the abundance and species composition of arthropods colonizing and ovipositing on leaf pairs was examined in three treatments: occupied leaf ties (containing a leaf-tying caterpillar),unoccupied leaf ties,and non-tied control leaves. 3. The density of arthropods present in the occupied and unoccupied leaf ties after two weeks was seven and four times greater respectively,than non-tied controls. The guild composition of these early colonists differed among treatments,with the highest densities of leaf chewers,scavengers,and predators in occupied ties and the lowest densities in non-tied controls. 4. The densities of all arthropods ovipositing on leaf pairs in the occupied and unoccupied leaf tie treatments were four and three times greater than non-tied controls. Leaf-chewing insects (including leaf-tiers and non-tying inquiline species) and scavengers showed strong oviposition preferences for tied leaves. However,most species of leaf-tying caterpillars and the psocids (Psocoptera) did not distinguish between occupied and unoccupied ties,suggesting that these groups do not use occupancy-related cues in selecting oviposition sites. Key words. Community structure,ecosystem engineering,indirect interactions, leaf shelter,oviposition, Quercus alba. Introduction factors,including plant morphology,the density of conspe- cifics,and the presence of natural enemies,also influences To explain variation in abundance of insect herbivores on host plant choice (reviewed in Thompson & Pellmyr,1991). individual plants,ecologists have traditionally focused on For predaceous arthropods,such as ladybird beetles,the variation in nutrient status and/or defensive chemistry of presence of herbivorous prey (e.g. aphids) has been shown edible plant parts,since these can influence insect perform- to have a large influence on oviposition site selection (Evans ance measures linked to fitness (e.g. preference– & Dixon,1986). As a consequence,many models of plant– performance linkages; Price et al.,1995). While nutrition herbivore,predator–prey,and host–parasitoid population and secondary chemistry play an important role in host dynamics either implicitly or explicitly include information selection for some herbivorous insects (especially butter- (or assumptions) about arthropod movement and ovipos- flies; Renwick & Chew,1994),a variety of other ecological ition site selection (Myers,1976; Ives,1992; Dwyer,1995). For most arthropods,however,the understanding of the cues used to locate feeding and oviposition sites is poor. Correspondence: John T. Lill,Department of Biological Implicit in many studies of herbivore oviposition behav- Sciences,George Washington University,340 Lisner Hall,2023 iour is the idea that during some phase of its life cycle,an G Street,NW,Washington,DC 20052,U.S.A. E-mail: lillj@ insect has both the opportunity and ability to distinguish gwu.edu differences in quality among potential plants or plant parts 300 # 2004 The Royal Entomological Society Leaf tie colonization 301 as a food source for itself or its offspring. Some species do and E. brizo (Bdv. & Leconte),appear to use the ties only as indeed make oviposition choices and feeding site choices a shelter,feeding outside of the shelter at night. Most that positively influence their fitness or that of their off- species feed primarily by skeletonizing the leaf epidermis spring (Singer et al.,1988; Thompson,1988; Brody & inside the shelter,where they typically spend their entire Waser,1995). Many insects,however,are highly cons- larval development,occasionally adding leaves as food trained in their ability to select among host plants owing becomes scarce. Occupied ties usually contain frass and to their mode of dispersal. In particular,species that dis- insect remains (exuvia,head capsules,etc.) amidst the silk perse as juveniles (recently hatched or post-diapause) often webbing,which probably provides food for scavenging have limited mobility,offering few avenues for host plant arthropods. Upon reaching the pre-pupal stage,the cater- assessment (Dethier,1959; Cain et al.,1985; Hunter & pillars usually exit the leaf ties and drop to the litter where Elkinton,2000). Many generalist caterpillar species may they pupate. Secondary colonization of these leaf ties by also hatch in the canopy and passively disperse by balloon- other leaf-tying species and by non-tying inquilines (herb- ing,letting wind and gravity determine their eventual ivores,scavengers,predators,and parasitoids) is extremely destination (Holmes & Schultz,1988). Even for individuals common,resulting in sequential occupation by an array of that arrive at a suitable plant via oviposition,encounters arthropods over the course of a season (Lill,2004). with natural enemies or physical disturbance may often cause Previous work in this system showed that the presence of them to fly,drop off,or spin down on a silk thread to a new leaf ties on white oak trees significantly increased both the plant (Gross,1993). Thus,understanding the factors that abundance of total arthropods (R. J. Marquis & J. T. Lill, structure communities of arthropods found on different unpubl. data) and the species richness of insect herbivores plants would be enhanced by consideration of the various (Lill & Marquis,2003) at the whole plant level. It was not colonization routes employed by the resident species. clear,however,whether these increases were due to One ecological factor that has been shown to positively increased oviposition in/on the ties or to some other effect, influence arthropod colonization is the presence of leaf such as increased survival or decreased emigration of insects shelters. Many different kinds of arthropods (e.g. spiders, on trees with shelters. Furthermore,for those species that weaver ants,caterpillars,sawflies,and beetles) create leaf were observed recruiting by oviposition into existing leaf shelters by binding,tying,rolling,or webbing leaves ties,the cues used to locate and/or select among oviposition together with silk (Wagner & Raffa,1993; Berenbaum, sites are unknown. The approach used in this study was to 1999; Anderson & McShea,2001; Fukui,2001). These compare the colonization of pairs of leaves in three experi- shelters often are secondarily colonized by a variety of mental treatments: (1) leaf ties that initially contained a arthropods that use them as a refuge and/or a feeding site shelter-building caterpillar; (2) leaf ties that were initially (Carroll & Kearby,1978; Damman,1987; Cappuccino, unoccupied; and (3) non-tied leaves. It was predicted that 1993; Cappuccino & Martin,1994; Martinsen et al.,2000; both colonization and oviposition would be higher on tied Lill & Marquis,2003; Lill,2004). As such,the arthropods leaves than on non-tied leaves,and that moths (and other that build these leaf shelters are acting as physical ecosys- arthropods) would either show a preference for,or would tem engineers,organisms that modify the biotic or abiotic have an easier time locating,occupied ties,due to occupancy- environment which in turn,influences resource availability related cues (leaf damage,frass,silk,etc.). It is believed that for other organisms (Jones et al.,1994,1997). In her study this study is one of the first to characterize colonization of the birch tube-maker, Acrobasis betulella (Hulst), patterns for an entire assemblage of arthropods. Cappuccino (1993) demonstrated that adult oviposition is an important means of secondary colonization of existing Methods leaf rolls. With the exception of this study,however,the mode of colonization by the secondary occupants has not Study site been explicitly examined. An understanding of the mode(s) of colonization of leaf shelters is important because it pro- This experiment was conducted at Cuivre River State vides an insight into the potential mechanisms underlying Park,located near Troy,Missouri,U.S.A. The park is a observed increases in local arthropod abundance on plants northern extension of the Ozark plateau and is a second containing shelters (Lill & Marquis,2003). growth mixed oak-hickory forest with an understorey of In the present study,the effects of leaf shelters on colon- flowering dogwood (Cornus florida L.),sassafras ( Sassafras ization of white oak (Quercus alba L.) leaves by a diverse albidum Nees),sugar maple ( Acer saccharum Marsh), community of leaf-dwelling arthropods were examined for a redbud (Cercis canadensis L.),and various oak saplings,and site in eastern Missouri (U.S.A.). Once leaves have fully an overstory consisting mostly of white oak (Quercus alba L.), expanded and hardened (late spring/early summer),white black oak (Q. velutina Lamarck),and hickory ( Carya spp.). oak is colonized by a rich caterpillar fauna (22 species in six families; Lill & Marquis,2003) that constructs leaf ties by binding overlapping leaves into leaf sandwiches using silk Oviposition

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