Host Discrimination by Seed Parasites

Host Discrimination by Seed 4

Parasites

Frank J. Messina

Department of Biology, Utah State University, 5305 Old Main Hill,

Logan, UT 84322–5305, USA

Introduction

Angiosperm seeds are typically well provisioned with nutrients.

Resources derived from the endosperm are needed for the successful

germination, growth and establishment of seedlings (Parrish and Bazzaz,

1985; Härdling and Nilsson, 1999). Not surprisingly, many herbivores

include nutrient-rich seeds as a major component of their diets (Mattson,

1980). We can define as seed parasites those herbivores that feed and

develop wholly or mainly within host seeds (Price, 1997). Depending on

the site and density of infestation, a seed parasite may kill the seed

embryo, decrease the survival of the emerging seedling or have little

effect on its host (Cipollini and Stiles, 1991; Jaegeret al., 2001). In some

parasites, juvenile stages cannot move between seeds and must complete

development within a single ‘natal’ seed. These species thus exploit hosts

in a way that makes them similar to many fruit parasites and insect

parasitoids (see Vetet al., Chapter 3, and Strand, Chapter 7, this volume).

Insect taxa that commonly act as seed parasites include seed beetles

(family Bruchidae), weevils (Curculionidae), grain borers (Bostrichidae),

seed flies (Anthomyiidae and Tephritidae) and chalcid seed wasps

(Torymidae and Eurytomidae).

Unlike free-ranging granivores, such as rodents and ants, seed

parasites tend to have specialized diets, and many infest seeds before they

disperse from the parent plant. Janzen (1980) found that 75% of the

seed-beetle species in a Costa Rican forest were associated with a single

plant species. Narrow host ranges of seed parasites may reflect fine-scale

adaptation to the timing of the host reproduction, as well as to the specific

chemical and physical defences of seeds. Plant investment in defence

traits should depend in part on the fitness value of various tissues,

CABInternational2002.The Behavioural Ecology of Parasites
(eds E.E. Lewis, J.F. Campbell and M.V.K. Sukhdeo) 65