Ecological Entomology (1994) 19, 399-401

SHORT COMMUNICATION

Eliminating early-season leaf-tiers of paper birch reduces abundance of mid-summer species NAOMI CAPPUCCINO and M A R C - A N D R E MARTIN Groupe de Recherche en Ecologie Forestikre, UniversitC du Quebec ?i MontrCal, MontrCal, Quebec, Canada

Key words. Acrobasis betulella, Betula papyrifera, Coleotechnites, leaf-tiers, positive interactions, Pseudotelphusa.

Introduction

Studies on the interactions among species of phytophagous insects sharing a host plant have mostly focused on com- petition (Damman, 1993). However, interactions between herbivores may also be positive. Although the importance of positive interactions relative to competition is not known, they may explain at least some of the positive correlations that are quite often seen among herbivores sharing a host-plant (e.g. Root & Cappuccino, 1992; Fritz et af. , 1987).

One mechanism of positive interaction among Lepi- doptera is the mutual use of leaf-shelters. Late-season species are often found in the shelters built by early-season feeders (Carroll & Kearby, 1978; Rose & Lindquist, 1982; Cochaux, 1969). This mechanism is present in the lepi- dopteran community of paper birch Betula papyrifera Marsh (Cappuccino, 1993), where mid-summer leaf-tiers, mostly oecophorids and gelechiids, colonize the leaf- shelters made by the birch tube-maker Acrobasis betullela Hulst. (Pyralidae) and other early feeders. Later in the season, A. betulella females oviposit in the leaf-ties made by the mid-summer species.

There have been different approaches to the study of interactions between species on the same trophic level, and these approaches often yield conflicting results. Detailed mechanistic studies may lead to the interpretation that competition between two species is important, an interpretation that is not often supported by community- level studies of co-occurrence (Feinsinger & Tiebout, 1991). A less common but more powerful approach is to manipulate species abundance to elucidate the population- level processes that translate mechanisms into community- level patterns (Feinsinger & Tiebout, 1991; Naeem, 1990). The objective of the present study was to determine whether

Correspondence: Dr Naomi Cappuccino, Groupe de Recherche en Ccologie Forestiere, UniversitC du QuCbec a Montreal, CP 8888, succ. A, Montreal Quebec H3C 3P8, Canada.

the mutual use of leaf-shelters by birch microlepidoptera is a mechanism that has important consequences for the population abundance of the species involved. By taking advantage of removals of early-season shelter-builders as part of a study on the dynamics of extinction and recolon- ization in A.betulella, we document the influence of early- season shelters on the abundance of mid-summer species.

Methods

A . betulella becomes active at budburst, tying leaves together into durable shelters. In July the shelters are colonized by a suite of species, of which Pseudotelphusa spp. and Coleotechnites spp. (Gelechiidae) are the most abundant. These species can make their own leaf-shelters by tying pairs of leaves together into ‘leaf-sandwiches’, but have a tendency to colonize pre-formed shelters, including those made by humans with string and paperclips (Cappuccino, 1993). Pseudotelphusa and Coleotechnites appear to be univoltine in northwestern Quebec (N. Cappuccino, unpublished data). Further details of the natural history of these species are provided by Cappuccino ( 1993).

The study took place on the islands and shoreline of Lake Duparquet in northwestern Quebec. Twenty-seven islands and five shoreline sites with birches between 3 and 15m tall were chosen. Between 10 and 30 June 1992, A.betulella was removed from one shoreline site and thirteen islands. Sites chosen for removal were similar to control sites except that the islands tended to be smaller (between approximately 600 and 2000 m2; controls ranged from about 1500 to 15,000m’). Removal islands had between two and ten birch trees. Some of the larger control islands had over 100 trees. Removals would not have been feasible on these larger islands.

We removed A.betulelfa by climbing the trees, sys- tematically searching the foliage for the large leaf-shelters made by this species and removing them by hand. The goal

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