Arthropod fauna of conventional and organic rice fields in California. (SA Fall 1993 (v5n5))

Fall 1993 (v5n5)

Arthropod fauna of conventional and organic rice fields in California.
Louis S. Hesler; Albert A. Grigarick, Michael I. Oraze and Andrew T Palrang
Econ. Entomol. 86(1):149-158. 1993

This study assessed the composition and abundance of pest and nonpest arthropods inhabiting conventional and organic rice production systems in California. The study was conducted in four pairs of conventional and organic rice fields located in the Sacramento Valley: three paired sites in 1988 and one paired site in 1989. Conventional and organic rice fields differed in two key respects: 1) Organic fields were free of synthetic pesticides or fertilizers during the growing season, and for at least 12 months prior to rice planting; and 2) Organic fields underwent less intensive disk plowing and harrowing in preparation for rice planting; this reduced tillage resulted in greater amounts of coarse plant material that was not as extensively incorporated into the seedbed. All fields were subdivided by levees into six or more discrete basins. Sampling was restricted to three basins within the interior of each field.

Rice fields were sampled for major arthropod pests at the early seedling stage, early tillering stage, and just before or during the reproductive phase. Populations of each pest species were compared using a paired t test. Additional sampling measured the relative abundance and activity of other aquatic arthropods in the rice fields. Populations of non-pest species in each system were compared using the paired t test, and also by determining the degree of taxonomic overlap between treatments, termed the "quotient of similarity," with values ranging from 0 (no taxa in common) to 1 (all taxa in common).
Results

The results of this research concur with findings of other studies comparing arthropod populations in organic and conventional farming systems. First, although differences in the numbers of pests can be found, the levels of most species remained below treatment thresholds in both systems. For the seven major pests in this study, the differences in abundance or in level of damage between conventional and organic treatments were not significant (P > 0.05). A summary of the data for two major rice pests is shown in table 1. Rice water weevil, and aster leaf hopper are the principal arthropod pests in rice during the early tillering and early reproductive stages. Pest damage in the seedling stage appeared slightly higher in organic systems, but this did not affect final plant densities in the field.

Table 1. Infestation levels of rice water weevil and densities of aster leafhopper in rice fields in California.

No. aster leafhoppers

Early sample

Site Treatment % plants ^a scarred by weevil feeding No. per 0.073 m² No. per plant

Pleasant Grove Conventional 16.8-9.4 6.78-4.20 0.49-0.51

(1988) Organic 6.0-7.3 1.11-1.44 0.12-0.18

Erickson Conventional 8.2-4.7 15.78-10.65 2.75-1.71

(1988) Organic 1.8-1.5 1.22-1.71 0.28-0.39

Gage Conventional 3.7-5.5 no data no data

(1989) Organic 8.7-5.1

^a Differences between treatments were not significant (t=.83, df=2, P> 0.05).

^b Differences between treatments were not significant (per area t=3.23, df=1, P> 0.05). Differences in densities recorded for a second sample taken later in the season were not as marked as the early samples.

Second, populations of the nonpest species were generally higher in organic fields [notably three predatory taxa-a giant water bug (Belostoma flumineum), back swimmers (Notonecta spp.), and an adult predacious diving beetle (Thermonectus basillaris)]. Where this study differed from previous ones is that the variety of taxa collected did not differ significantly between conventional and organic rice fields. The quotient of similarity between the two treatments was 0.923 indicating that many of the same species were present in both organic and conventional systems. Previous studies in other crops showed that organic systems had greater diversity (Dritschilo and Wanner, 1980; Brown and Adler, 1989; Goh and Lange, 1989; Kromp, 1989 and 1990).

Some preventive control measures are available for the rice pests observed in this study. Damage from rice water weevil, for example, can be reduced by using tolerant varieties, draining fields, disking grassy levees and other pest habitat, and delaying the time of planting. Good weed control is an effective means of controlling aster leafhopper and army-worms. Preventive measures are not always sufficient, so more research is needed to determine how to control outbreaks of these key pests in organic systems.

References

Brown, M.W. and C.R.L. AdleL 1989. Community structure of phytophagous arthropods on apple. Environ. Entomol. 18:600-607.

Dritschilo, W. and D. Wanner. 1980. Ground beetle abundance in organic and conventional corn fields. Environ. Entomol. 9:629-631.

Goh, K.S. and W.H. Lange. 1989. Arthropods associated with insecticide-treated and untreated artichoke fields in California. J. Econ. Entomol. 82:621-625.

Kromp, B. 1989. Carabid beetle communities (Carabidae, Coleoptera) in biologically and conventionally farmed agroecosystems. Agric. Ecosyst. Environ. 27:241-251.

Kromp, B. 1990. Carabid beetles (Carabidae, Coleoptera) as bioindicators in biological and conventional farming in Austrian potato fields. Biol. Fertil. Soils 9:182-187.

For more information write to: L.S. Hesler, Northern Grain Insects Research Laboratory, USDA-ARS, Rural Rt. 3, Brookings, SD 57006.

(DEC.51 5)

Contributed by David Chaney

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