Deborah A. Dritz

STRAW AND WINTER FLOODING BENEFIT MOSQUITOES AND OTHER INSECTS IN A RICE AGROECOSYSTEM

Rice fields are widespread agroecosystems that provide wetland habitat for many species, including pests like mosquitoes and beneficial insects. They can be used as models to understand how basal resources affect food web dynamics in seasonal wetlands. Rice field management may also influence adjacent communities by affecting mosquitoes, wildlife, and air quality. Rice straw incorporation and winter flooding have become common methods used to prepare seedbeds, largely replacing burning of straw. These methods could affect aquatic insects, including mosquitoes, because they increase nutrient availability during the growing season. We studied 16 fields where straw was either burned or incorporated into soil after the previous growing season; these treatments were crossed with either winter flooding or no winter flooding. Algae, mosquitoes, other herbivorous insects and predatory insects all responded positively to one or both treatments that increased nutrients (straw incorporation and winter flooding). While the overall increase in insect production could benefit wildlife, mosquito abatement personnel may need to monitor unburned fields more closely. The issue of mosquito production adds to the complexity of agricultural and environmental concerns bearing on rice field management. Straw incorporation and winter flooding reduce particulate pollutants caused by burning, reduce fertilizer needs, and increase densities of beneficial insects. However, these techniques may increase mosquitoes, methane production, and fungal diseases of rice. Further improvement of straw management practices could minimize these problems.

Control of Mosquito Larvae In Seasonal Wetlands On A Wildlife Refuge Using Vectomax™ CG

Abstract There is a great need for novel insecticides to control mosquitoes. VectoMax™ is a new mosquito larvicide that combines toxins from Bacillus thuringiensis subsp. israelensis (Bti) and Bacillus sphaericus (Bs), and is designed to provide extended mosquito control. We tested the initial efficacy and longevity of control of mosquitoes using one of the formulations, VectoMax CG, in a full-scale study conducted in seasonal wetlands. VectoMax CG was applied by air at 8.9 kg/ha to 3 wetlands and 3 others were untreated controls. VectoMax CG controlled Culex tarsalis through day 28 and showed activity against Aedes melanimon. Use of this dual-material, extended-action formulation could minimize inspection visits and reduce application costs compared to Bti and Bs alone, and its combination of toxins may forestall resistance development.

Does synergized pyrethrin applied over wetlands for mosquito control affect Daphnia magna zooplankton or Callibaetis californicus mayflies

BACKGROUND Public health agencies may apply aerosolized synergized pyrethrin over wetlands repeatedly to control mosquitoes. This concerns wildlife managers because studies have shown the accumulation of pyrethroids, which are chemically similar to pyrethrin, in sediments in amounts that can be toxic to invertebrates. The authors tested whether repeated applications of synergized pyrethrin over wetlands caused mortality of two aquatic invertebrates: the zooplankton Daphnia magna Straus and a mayfly, Callibaetis californicus Banks. Fifteen wetland mesocosms were either exposed to repeated pyrethrin sprays or were protected by lids. Invertebrates in screened cages were placed in mesocosms before the fifth and eleventh spray, and directly into wetlands before spray 11. Six mesocosms were exposed to spray deposition. Caged adult mosquitoes were used to verify that sprays drifted over mesocosms. Sediments were analyzed for insecticide residues. RESULTS There were no detectable effects of synergized pyrethrin on 36 h survival of Daphnia or mayflies, but most exposed adult mosquitoes died. Some exposed sediments yielded pyrethrin (< or =34.5 ng g(-1)); most showed piperonyl butoxide (PBO) (< or =14.9 ng g(-1)). CONCLUSIONS Deposition of aerosolized 25% pyrethrin + 5% PBO may contaminate wetlands, but its application at rates used for mosquito control did not produce detectable effects on indicator species.

Efficacy of Spinosad in Control of Larval Culex tarsalis and Chironomid Midges, and Its Nontarget Effects

Abstract Formulations of spinosad for mosquito and midge control (e.g., Natular®) are derived from the Saccharopolyspora spinosa bacterium. They offer a different mode of action from other larvicides. We tested the effectiveness of single-brood (2EC) and extended-release pellet (G30) formulations against larvae of Culex tarsalis and chironomid midges, at mid- to high label rates, in wetland mesocosms. We also monitored survival of mayfly nymphs and other nontarget insects. Both formulations were effective against mosquitoes for >4 wk and yielded excellent midge control. Spinosad caused mortality of mayflies and other nontarget insects. Spinosad was less toxic to mayflies than to targets, and effects on mayflies were undetectable after day 21. The higher toxicity for mosquitoes indicates that future research could identify rates that reduce nontarget effects while maintaining high efficacy.

Effects of Rice Straw and Water Management on Riceland Mosquitoes

Abstract Rice fields are important sources of mosquitoes in many regions, and rice (Oryza spp.) growing practices can affect mosquito populations. Rice straw incorporation and winter flooding have become common methods to prepare seedbeds, largely replacing burning of straw. These methods increase nutrients during the growing season. We sampled mosquito larvae during 1999–2001 in 16 0.72-ha plots where straw was either burned or incorporated into soil after the previous growing season; these treatments were crossed with either winter flooding or no winter flooding. In 2000, all fields were drained mid-season for an application of herbicide, and then they were reflooded. Mosquitoes responded positively to straw incorporation and winter flooding, especially in combination. The mid-season reflood in year 2 was associated with an order of magnitude increase in Culex tarsalis Coquillett larvae. Results confirm that rice straw and water management can strongly influence mosquito populations.