Assessing the diversity of selected arthropods in cabbage-growing areas in Mt.Malindang, Misamis Occidental [Philippines]

The main hypothesis is that farms nearer the forest would have higher diversity compared with those farther away. Moreover, parasites and predators would be more abundant in farms closer to the forest. Species richness, measured using Margaleff's index, did not significantly differ among treatments for the three sites. Correspondence analysis also showed general uniformity of species richness among sites and treatments. Several classes of arthropods were found associated with cabbage. The more numerous included insects, spiders, sowbugs, and amphipods. Insects dominated these arthropods belonging to 60 families and 10 orders. Detrivores included various flies, gnats and their relatives, collembola, termites, sowbugs and millipedes. The diamongback moth or DBM (Plutella xylostella (Linn.)), was the major pest of cabbage. Populations from the three sites however, did not differ significantly. Spiders dominated the predatory guild. Spider numbers were significantly higher in Gandawan and Lake Duminagat, among treatments, the farm near the forest harbored significantly more spiders that the sprayed or unsprayed cabbage farms farther away. Other insect pests included the cabbage looper (Trichoplusia ni (Hubner)), cabbage worm (Crocidolomia pavonana (Fabr.)), cutworm (Spodoptera litura (Fabr.)), and the green peach aphid (Myzus persicae (Sulzer)). Two leaf-feeding beetles were also found associated with the cabbage agroecosystem, but their populations were very low: flea beetle (Psylliodes sp.) and squash beetle (Aulacophora indica (Gmelia)). Hymendopterous parasites and predators, such as black ants, sphecid, and braconid wasps were minimal. Tachinid flies (Braconidae) parasitized cutworm larvae, while a single cabbage looper larva were parasitized by a braconid wasp, Cotesia sp (Braconidae). Very few adults of this wasp, however, were collected in cabbage fields. Species richness and DBM population were not correlated with yield. Soil nutrients, especially phosphorus, affected yield. There was a strong correlation between average cabbage yield (kg) and the amount of phosphorus in the soil (r=0.92). Moreover, results indicated that average cabbage yield was correlated with spider number.