11.2.5. Seed predation

Plant seeds usually contain higher levels of nutrients than other tissues, providing for the growth of the seedling. Specialist seed-eating insects use this resource. Notable seed-eating insects are many beetles (below), harvester ants (especially species of Messor, Monomorium, and Pheidole), which store seeds in underground granaries, bugs (many Coreidae, Lygaeidae, Pentatomidae, Pyrrhocoridae, and Scutelleridae) that suck out the contents of developing or mature seeds, and a few moths (such as some Gelechiidae and Oecophoridae).

Harvester ants are ecologically significant seed predators. These are the dominant ants in terms of biomass and/or colony numbers in deserts and dry grasslands in many parts of the world. Usually, the species are highly polymorphic, with the larger individuals possessing powerful mandibles capable of cracking open seeds. Seed fragments are fed to larvae, but probably many harvested seeds escape destruction either by being abandoned in stores or by germinating quickly within the ant nests. Thus, seed harvesting by ants, which could be viewed as exclusively detrimental, actually may carry some benefits to the plant through dispersal and provision of local nutrients to the seedling.

An array of beetles (especially Curculionidae and bruchine Chrysomelidae) develop entirely within individual seeds or consume several seeds within one fruit. Some bruchine seed beetles, particularly those attacking leguminous food plants such as peas and beans, are serious pests. Species that eat dried seeds are preadapted to be pests of stored products such as pulses and grains. Adult beetles typically oviposit onto the developing ovary or the seeds or fruits, and some larvae then mine through the fruit and/or seed wall or coat. The larvae develop and pupate inside seeds, thus destroying them. Successful development usually occurs only in the final stages of maturity of seeds. Thus, there appears to be a “window of opportunity” for the larvae; a mature seed may have an impenetrable seed coat but if young seeds are attacked, the plant can abort the infected seed or even the whole fruit or pod if little investment has been made in it. Aborted seeds and those shed to the ground (whether mature or not) generally are less attractive to seed beetles than those retained on the plant, but evidently stored-product pests have no difficulty in developing within cast (i.e. harvested and stored) seeds. The larvae of the granary weevil, Sitophilus granarius (Box 11.10), and rice weevil, S. oryzae, develop inside dry grains of corn, wheat, rice, and other plants.

Plant defense against seed predation includes the provision of protective seed coatings or toxic chemicals (allelochemicals), or both. Another strategy is the syn- chronous production by a single plant species of an abundance of seeds, often separated by long intervals of time. Seed predators either cannot synchronize their life cycle to the cycle of glut and scarcity, or are over- whelmed and unable to find and consume the total seed production.

Chapter 11