Spiders: Natural Biological Control Agents Against Insect Pests in Philippine Rice Fields

Abstract

Spiders are economically significant arthropod predators. Their most important use is in agriculture and forestry, where they serve as natural biological control agents against insect pests. Of the 35,000 described species all over the world, the checklisted Philippine spiders make up just 1.47%, consisting of 517 species and 225 genera belonging to 38 families, grouped into two infraorders (Mygalomorphae and Araneomorphae) of the suborder Opisthothelae. In Philippine rice fields, the teeming spider fauna consists of 337 species under 28 families, while the surrounding habitats have 70 species in 50 genera under 19 families. The Philippine spider record is the highest in the entire Asian tropical rice fields.

An ecological study of Philippine irrigated rice fields reveals that 3,098 spiders belonging to 42 species are largely grouped into two functional guilds: the web builders and the hunters. Among the web builders, the most diverse are the comb-footed spiders (Theridiidae), long-jawed spiders (Tetragnathidae), garden spiders (Araneidae), and dwarf spiders (Linyphiidae). The diverse hunters, on the other hand, are the jumping spiders (Salticidae), crab spiders (Thomisidae), and the wolf spiders (Lycosidae). In irrigated rice fields, spiders are visible on the rice canopy and above water environments during or immediately after transplanting in both wet and dry seasons. The wide array of insect prey for spiders includes collembolans, dipterans, moths and butterflies, rice bugs, leafhoppers, and planthoppers. The entire prey spectrum consists of 198 species belonging to 91 families in 14 orders of Class Insecta.

Predation rates of spiders are known only in 14 taxa, comprising four hunters and 10 web builders. The hunters are Lycosidae (3 species, 2 genera) and Oxyopidae (1 species), while the web builders are Araneidae (2 species), Linyphiidae (1 species), Theridiidae (1 species), and Tetragnathidae (6 species). A community assemblage of these 14 taxa at any one time consumes 65 leafhoppers, 72 planthoppers, 26 rice stem borer and leaffolder moths, 51 rice whorl maggot flies, and 146 collembolans in a day. In turn, spiders serve as hosts of 15 hymenopteran parasites, two pathogens, a nematode, as well as prey to bullfrogs, toads, birds, ants, and wasps. Natural enemies of spiders limit their effective and efficient use in the natural biological control of rice insect pests.

To conserve spiders and optimize their innate potentials in the regulation of rice insect pests, the following provisions are recommended: 1) Maintain refuge areas with diverse plants and landscapes. 2) Maintain files of rice straw around rice fields. 3) Avoid burning habitats surrounding rice fields. 4) Avoid overgrazing rice bunds and surrounding grassy habitats. 5) Avoid using herbicides on grasses around rice fields. 6) Use judiciously selective insecticides against rice insect pests.