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

Biodiversity of arthropods was assessed in cabbage fields of seven local partners in three upland barangays [villages] of Don Victoriano, Misamis Occidental [Philippines]. These included Nueva Vista, popularly known as Mansawan, Gandawan, and Lake Duminagat. Cabbage fields varied in size (60-836 sq m) and slope (20-40 deg). 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 comprising 10 orders belonging to 60 families. Detrivores include various flies, gnats and their relatives, collembola, termites, sowbugs, and mellipedes. The diamondback moth or DBM, [Plutella xylostella (Linn.)], was the major pest of cabbage, which limited production and reduced yield. Populations from the three sites, however, did not differ significantly. Spiders dominated the predatory guild. Spider numbers were significantly more abundant in Gandawan and Lake Duminagatamong treatments, the farm near the forest harbored significantly more spiders than the sprayed and unsprayed cabbage farms. Other insect pests observed 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 (Gmelin)]. Hymenopterous parasites and predators, such as block ants, sphecid, and braconid wasps were minimal. Tachinid flies (Tachinidae) parasitized cutworm larvae, while a single cabbage looper larva was parasitized by a braconid wasps, Cotesia sp. (Braconidae). Very few adults of this wasps, 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.