Spatial occurrence and hatch of field eggs of the tadpole shrimp Triops newberryi (Notostraca: Triopsidae), a potential biological control agent of immature mosquitoes.
The tadpole shrimp (TPS), Triops newberryi (Packard) (Notostraca: Triopsidae) is a potential biological control agent for immature mosquitoes breeding in ephemeral habitats. The occurrence of TPS eggs in soil and their hatch were investigated in 11 flood-irrigated date gardens in the Coachella Valley of southern California in 1999. Each garden was sampled several times after the rows were recently irrigated. All these date gardens harbored from very few to a large number of eggs in the soil. Overall, the average density of total eggs on ranches with clay loam soil was significantly higher than that on ranches with silt loam soil. The average densities of total eggs were significantly lower on the ranches that were disked compared to those on the ranches that were undisked before sampling. Two types of eggs were found and designated as "fresh" (yellowish to brownish) and "old" (blackish) eggs. This is the first time that these dimorphic eggs have been reported. The density of fresh eggs was lower than that of old eggs in most soil samples. The date gardens with high egg densities were sampled for determination of vertical occurrence, where soil was sampled up to 38.5 cm deep. Fresh eggs were recovered from soil in depths up to 25.6 cm, but the densities progressively declined with depth. The old eggs, however, were recovered from all soil depths studied, and there was no obvious relationship between soil depth and their density. This pattern of vertical occurrence of TPS eggs is the result of frequent disking for weed control and fruit harvest. Hatch of TPS eggs in surface soil samples ranged from 0 to 7.2 per 100 g dried soil. Hatch of viable eggs had an inverse relationship with soil depth. No TPS hatched out from the soil samples taken deeper than 15.4 cm. Fresh and old eggs distinguished by color were subjected to hatching tests. Fresh eggs exhibited high hatch, with hatching rates of 35.5-45.0% and 40.2-60.3% for the 1st and 1st plus the 2nd hydrations respectively. The old eggs, however, did not hatch at all. These findings provide quantitative information with regard to occurrence of natural TPS populations in flood irrigated agricultural fields, which could serve as a potential regulatory force of immature mosquito populations sharing ephemeral habitats with TPS.
Department of Entomology, University of California, Riverside 92521-0314, USA.
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D000818:Animals; D003445:Crustacea; D004531:Eggs; D004777:Environment; D009032:Mosquito Control / Q000379:methods*; D011157:Population Dynamics; D011235:Predatory Behavior; D012987:Soil; D013534:Survival
D012987:Soil