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CHEMICALS USED TO CONTROL MOSQUITOES ON REFUGES DIFFER IN TOXICITY TO TADPOLES

Donald W. Sparling and Peter T. Lowe
USGS Patuxent Wildlife Research Center



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Photo of Constructed Pond at Patuxent

Two chemicals widely used by National Wildlife Refuges to control mosquito populations, AltosidŽ and AbateŽ (trade name disclaimer); differed in their toxicity to amphibian larvae in field and laboratory trials. National Wildlife Refuges are instrumental in protecting a large number of fresh, brackish, and saline wetlands for migrating and wintering waterfowl and for other functions inherent to wetlands. Unfortunately, when these refuges are in proximity to areas populated by humans, they can be potential sources of nusiance or disease to humans and livestock due to breeding populations of mosquitos. This concern is especially high along the western and eastern seaboards. The potential conflict between maintaining the natural functions of refuge wetlands and the concern for public health has led to a search for integrated pest control methods that are both effective on mosquitos and relatively safe for non-target organisms. One method involves application of chemicals to either directly kill larvae or inhibit their emergence as adults.

We tested AbateŽ (44.6% temephos), an organophosphate insecticide, and AltosidŽ (5% methoprene), a juvenile growth inhibitor on tadpoles. AbateŽ is the most widely applied mosquito abatement chemical in National Wildlife Refuges whereas AltosidŽ and the bacterium, Bacillus thuringensis, are used less frequently.

AbateŽ Depressed Growth of Gray Treefrog Tadpoles

In one experiment we determined that AbateŽ reduced growth of gray treefrog (Hyla versicolor) larvae in constructed ponds located at Patuxent Wildlife Research Center, MD. The ponds averaged 202 m2 in area and 105 m3 in volume. Each chemical was applied to six ponds with a hand sprayer following manufacturers' label instructions - 1.5 ounces/acre (21.6 mL.L-1) for AbateŽ and 3 ounces/acre (42.8 mL.L-1) for AltosidŽ. Six control ponds were sprayed with water but no chemical. Spray dates were 24 May, 13 June, and 6 July 1995 which were selected to simulate actual application times at Prime Hook National Wildlife Refuge, DE. On 12 June, 13 very young (< 2 days old) or 8 older (7 - 10 days old) tadpoles were placed into plastic containers within each pond. The containers were screened to allow food (algae) and water to enter freely but block predators. On 13 July, the containers were removed and the length of each tadpole from snout to vent, their collective displacement volume, and their collective weight were measured.

The mortality rate of tadpoles is reportedly very high under natural conditions and there was no difference in mortality among treatments for either age class. However, the very young tadpoles experienced a higher mortality rate than the older tadpoles (Table). By the end of the experiment, older tadpoles showed a significant difference in size due to treatment with those in AbateŽ-treated ponds smaller than those in AltosidŽ-treated or control ponds. Body size of very young tadpoles was marginally different among treatments.

AbateŽ was More Toxic than AltosidŽ in Laboratory Tests

In a second set of experiments we attempted to determine the 96-hr median lethal concentration (LC50) of both AltosidŽ and AbateŽ for green frog (Rana clamitans) tadpoles using a static renewal protocol in synthetic soft water. The assays were conducted in an environmental chamber at 21 oC and 16:8 light:dark cycle. Concentrations ranged from 1.86 to 10 mL.L-1 for AbateŽ and 18.6 to 100 mL.L-1 for AltosidŽ. Four tadpoles were used in each 2-L bell jar and each concentration was run in duplicate. At the end of the study, survivors were euthanized and frozen. Butyrlcholinesterase (BChE) and acetylcholinesterase (AChE) assays were run on whole bodies of control and AbateŽ-treated tadpoles.

The dose-response curve for AbateŽ was:

% Mortality = -4.408 + 1.046*Concentration, LC50 = 4.21 mL.L-1 (95% confidence interval = 3.61 - 5.10 mL.L-1), standard error of slope = 0.295, p = 0.0004 (Fig. 1). After 5.5 hr of treatment on the first day, three of the eight tadpoles at 10 mL.L-1 and one at 5.1 mL.L-1 were unable to right themselves. By 24 hr, half of the tadpoles at 10 mL.L-1 were dead. The estimated 48 hr LC50 was 7.70 mL.L-1. BChE activity (mmol/cm/sec) declined sharply with the lowest dose of AbateŽ and remained low in all treated tadpoles (Fig. 2). However, AChE activity did not decrease with dosage.

Two tadpoles died at each of the middle concentrations of AltosidŽ but none at the highest level. Because this mortality was not correlated with concentration, we were unable to determine a dose-response curve for AltosidŽ.

Management Considerations

Of the two mosquito abatement chemicals, AbateŽ clearly presents the greater risk to amphibian larvae. AbateŽ appeared to inhibit the growth of young gray treefrog larvae in the pond experiments. Whether this was a direct physiological response or due to a secondary effect such as chemically-induced decreased food availability is unknown. In addition, the median lethal dose for AltosidŽ, although undetermined, must be at least 10 times greater than that for AbateŽ and AltosidŽ is applied at only twice the concentrations as AbateŽ.

One of the problems in relating the LC50 value to actual conditions is the calculation of field concentrations. Label directions on AbateŽ and AltosidŽ prescribe application rates on an ounce per surface acre basis. Although this is practical for surface-dwelling mosquito larvae, interpretation of concentration is difficult for organisms such as tadpoles which live at indeterminate depths. Based on our pond study, the calculated LC50 is well within expected field concentrations. However, care must be taken in extrapolating the laboratory study to field conditions because of the absence of ligands such as dissolved organic carbon (DOC), the inability of the tadpoles to escape exposure, and the length of the study. Under field conditions temephos is known to bind with DOC and to degrade within a day or two. In a related experiment, we found that full strength pond water decreased the toxicity of AbateŽ compared to synthetic soft water. Thus our laboratory experiment could be interpreted as a worst case situation for either compound. Further work on additional species and formulations of AbateŽ are required.

For further information contact: Don Sparling or Peter Lowe, Patuxent Wildlife Research Center, Laurel MD Ph: (301) 497-5723, FAX: (301) 497-5744, Email: Don_Sparling@usgs.gov or Peter_Lowe@usgs.gov  
or Fred Pinkney, U.S. Fish and Wildlife Service, Chesapeake Bay Field Office, 177 Admiral Cochrane Dr., Annapolis, MD. Ph: (410) 573-4521, Email: Fred_Pinkney@fws.gov

Table. Survival and body size (x- + SD) of gray treefrog tadpoles exposed to AltosidŽ and AbateŽ for 31 days in constructed wetlands, 1995.
Treatment and Initial Age of Tadpoles* Percent Survival Average Weight (mg) Average Displacement Volume (ml) Length (mm)
Control 2 d 83.4 +/- 18.0 0.29 +/- 0.21 0.28 +/- 0.17 6.77 +/- 1.48
Control 8 d 51.2 +/- 28.1 0.56 +/- 0.37 0.50 +/- 0.35 7.00 +/- 1.16
AbateŽ 2 d 75.4 +/- 19.8 0.19 +/- 0.17 0.16 +/- 0.14 4.82 +/- 2.54
AbateŽ 8 d 47.4 +/- 26.4 0.26 +/- 0.15 0.24 +/- 0.17 5.58 +/- 0.68
AltosidŽ 2 d 80.3 +/- 13.2 0.25 +/- 0.23 0.19 +/- 0.18 6.70 +/- 1.08
AltosidŽ 8 d 45.6 +/- 23.3 0.34 +/- 0.21 0.36 +/- 0.22 6.25 +/- 0.48

* n for 2d tadpoles=26 per pond, for 8d tadpoles=16 per pond.

Figures

Figure 1: Response Curve for Green Frog Tadpoles

Fig. 1. 96-hr dose-response curve for green frog tadpoles exposed to AbateŽ in synthetic soft water.

Figure 2: Butyrlcholinesterase activity in green frog tadpoles

Fig. 2. Butyrlcholinesterase activity (as percent of control mean) in green frog tadpoles exposed to AbateŽ for 96-hrs.

For more information, contact:
Donald W. Sparling, E-Mail: Don_Sparling@usgs.gov
Merriam Lab 1st Floor
11510 American Holly Drive
Laurel, MD 20708-4017
(301)497-5723

Peter T. Lowe, E-Mail: Peter_Lowe@usgs.gov
Merriam Lab 3rd Floor
11510 American Holly Drive
Laurel, MD 20708-4017
(301)497-5705

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