INTRODUCTION

This paper is the culmination of over 3 years of research by various mosquito control agencies throughout NJ. The idea for the research came from questions raised by the NJ Mosquito Control Association (NJMCA) Research and Development Committee about Gambusia affinis stocking rates and effectiveness of the fish in various habitats. The effectiveness of Gambusia depends on many variables including temperature, fecundity, fish density, and habitat type (Sholdt 1972). The NJMCA Research and Development Committee coordinated the research efforts.

Gambusia have been stocked for mosquito control purposes in NJ since 1905 (Smith 1908). Since that time there have been varying degrees of activity with respect to their importance in the New Jersey mosquito control community. During the past 10 years there has been a renewed interest in Gambusia, and they are probably stocked in more areas throughout NJ than ever before. Gambusia are an integral part of the integrated pest management strategies employed by mosquito control agencies.

The NJMCA Research and Development Committee concentrated on investigating the effectiveness of Gambusia in the following areas: woodland pools, mine pits, stormwater management facilities, ornamental pools, abandoned swimming pools, ditches, brackish marshes, and freshwater swamps.

GAMBUSIA EFFECTIVENESS IN UNUSED SWIMMING POOLS

In Warren County an abandoned swimming pool stocked with Gambusia for 4 years in succession experienced effective larval reduction. An average of 35 fish were stocked in early spring. The population quickly increased to hundreds of fish by the beginning of summer. Follow-up dipping revealed fish ate all the Culex and Aedes larvae in the swimming pool. They can effectively control Anopheles larvae when emergent and floating vegetation is absent or removed. In situations where there was enough vegetation on the surface of the water to afford the Anopheles larvae harborage, Gambusia could not effectively control them. Gambusia offer mosquito control personnel an effective nontoxic solution to this backyard mosquito problem.

GAMBUSIA STOCKING GUIDELINES FOR SWIMMING POOLS

• 50 fish are sufficient for an initial early spring stocking.
• For the fish to be effective against Anopheles spp. mosquitoes, emergent and floating vegetation should be removed.
• Gambusia must be stocked every year in northern climates because oxygen levels become depleted under the ice and the fish die.
• Pool chemicals should not be introduced while fish are present.

GAMBUSIA EFFECTIVENESS IN ORNAMENTAL PONDS

Gambusia use in ornamental ponds and large bird baths gave similar results to Gambusia use in swimming pools. Because of size and nutrient limitations, only limited fish reproduction was observed.

Between 15 and 50 fish should be stocked, depending on the size of the pond. In ornamental ponds containing other fish, check for mosquito larvae before introducing Gambusia because the fish already present could be effectively controlling the mosquito larvae.

Due to winter die off in most ornamental pond situations, Gambusia will need to be stocked on a yearly basis.

GAMBUSIA EFFECTIVENESS IN BRACKISH SWAMPS

In a 3 year study of brackish swamps in Monmouth County, areas slated for stocking never retained enough water for initial stocking to occur.

Conversations with other coastal counties in NJ with brackish swamps revealed that similar conditions occur elsewhere in the state. In the instances where a brackish area did contain stockable amounts of water, there frequently were other fish (e.g. Fundulus heteroclitis) present that provided mosquito control.

GAMBUSIA USE IN FRESHWATER SWAMPS

In freshwater swamps, Gambusia generally worked quite well when there was sufficient water. In some large areas the presence of predatory fish (bass, pickerel, sunfish, etc.) or an outflow into streams and lakes precluded Gambusia stocking (Kent 1993). Comprehensive statewide studies are needed to fully explore Gambusia use in this habitat.

FRESHWATER SWAMP RECOMMENDATIONS

• Use a site that is semi-permanently wet.
• Pick a small site where the boundaries and water flow patterns can be ascertained.
• Monitor the progress closely.

GAMBUSIA EFFECTIVENESS IN DITCHES

The Middlesex County Mosquito Extermination Commission placed Gambusia in several ditches. Ditches were stocked from mid-May to mid-June with an average of 1,800 fish/acre. Ditches studied ranged in size from 3-6 ft wide and from 100-200 ft long.

In most ditches the fish did a good job of controlling immature mosquitoes. In some instances when water levels receded and puddling occurred, fish could not access all areas and eat the larvae. Also fish occasionally did not spread out into the entire ditch but stayed concentrated near a culvert pipe outfall. The depth of the water at this location tended to be deeper than the remainder of the ditch and possibly afforded the Gambusia some degree of protection from water evaporation or a greater supply of food.

Most of the larvae encountered were Culex spp. Gambusia did a good job of controlling the larvae in most ditch situations. However, even at the stocking rate of 1,800/acre, control in some ditch situations was not observed until the fish population noticeably increased.

STOCKING GUIDELINES FOR DITCHES

• Only stock ditches that do not have outlets to streams or lakes.
• Stock 2,500 fish/acre.
• Stock ditches that remain wet for extended periods of time.
• Continue monitoring area after stocking.

GAMBUSIA EFFECTIVENESS IN WASTEWATER SETTLING BASINS

The effectiveness of Gambusia within abandoned wastewater settling basins has been studied by the Burlington County Mosquito Extermination Commission. The site under investigation consisted of 5 settling basins, each approximately 0.10 acres in size. Three of the 5 basins held water consistently and contained a fairly heavy growth of emergent vegetation. These basins contained Anopheles and Culex larvae. The remaining 2 basins (basins 2 and 3) were subject to drying, especially during periods of above-average temperatures and below-average precipitation. Larvae present within these 2 settling basins included Aedes, Anopheles, and Culex spp. No other species of fish were present within any of the 5 basins.

The settling basins were stocked with Gambusia in the spring of 1980. Water levels were mechanically manipulated so that all 5 contained approximately 5 ft of water. Between May 2 and May 16, 1980, each basin was stocked with approximately 250 Gambusia; this translates to a stocking rate of 2,500 fish/acre. Of the Gambusia stocked, 67% were gravid females. Excellent larval control was achieved at this stocking rate. In addition, the fish were able to overwinter successfully, and restocking has never been necessary. The Gambusia thrived within this habitat, and their population became so large that the site has been used since 1980 as the principal source of Gambusia for stocking purposes throughout Burlington County. To date, excellent larval control continues within the site.

As was mentioned previously, the water within basins 2 and 3 would drop to dangerously low levels during dry seasons. In 1992, the 5 basins were modified, creating 2 larger basins, each approximately 0.25 acres in size and 10 ft deep at their deepest point. This ensured that basin dry-down would no longer be a problem; even during the severe drought experienced in the summer of 1995, the basins contained sufficient water for the survival of the Gambusia.

WASTEWATER SETTLING BASIN RECOMMENDATIONS

• Stock 2,500 fish/acre.
• Monitor the site closely.
• Physical manipulation of the site to enhance fish accessibility and survivability should be considered

GAMBUSIA EFFECTIVENESS IN MINE PITS

The Morris County Mosquito Extermination Commission investigated the effectiveness of Gambusia in a small abandoned mine pit over a 3-year period. After 3 yr of observation it was determined that Gambusia were very effective at controlling larval mosquito populations while sufficient numbers of fish were present.

The mine pit was stocked with 25 Gambusia each June. This translated to 900 fish/acre. Fish sampling each spring revealed the fish did not overwinter, primarily due to low water levels. At this stocking rate, it took until mid-July for the Gambusia population to increase to a mosquito control level. On 1 occasion when water levels were receding, larvae were observed in a puddled section of the mine pit inaccessible to the fish. In 2 out of the 3 years of the study, the mine pit was dry by the beginning of August which resulted in a loss of the fish.

Prior to the introduction of Gambusia this site received repeated insecticide treatments. After stocking, mosquito larvae were only present in the mine pit when fish densities were low or the area became puddled.

MINE PIT RECOMMENDATIONS

• Stock 2500 fish/acre for quick control.
• Continue periodic monitoring to assess water levels and fish viability.
• Yearly stocking will probably be required

GAMBUSIA EFFECTIVENESS IN WOODLAND POOLS

Gambusia have been found to be marginally effective at controlling early season mosquito production in woodland pools. At stocking rates of up to 500 fish/acre, larval density was not noticeably reduced. Gambusia are not particularly good at larval consumption when cold water temperatures prevail. Due to the temporary nature of woodland pools, Gambusia (at traditional stocking rates of 300/acre) rarely have a chance to increase in population sufficiently to control larval mosquito populations. Experimental stocking rates of 1,500 fish/acre have been used on a limited basis in woodland pools in Warren County. This stocking rate has produced some favorable results.

An ideal woodland pool situation for Gambusia stocking is one in which the woodland pool is subject to sufficient sunlight to warm the water. The maximum water depth should not be more than 3 feet because on cold evenings the fish descend to the bottom of the pool and are slow to react to re-warming of the water. Stocking rates should be 2,500 fish/acre to enhance larval predation.

Woodland pools regularly dry down during summer. Therefore, the problems aquatic vegetation and predatory fish pose for permanently wet sites do not exist for the woodland pool situation. Predatory fish can decimate Gambusia populations, while emergent aquatic vegetation can allow Anopheles populations to build up to unacceptable levels.

WOODLAND POOL RECOMMENDATIONS

• Use woodland pools that stay flooded well into the summer.
• Deep woodland pools (over 5 ft) do not allow for sufficient warming in early spring to stimulate Gambusia activity.
• Stock at 2,500 fish/acre.

GAMBUSIA EFFECTIVENESS IN STORMWATER

MANAGEMENT FACILITIES

Gambusia have been used in Ocean County since the early 1970s. With the proliferation of stormwater management facilities in the 1980s, Gambusia was used to control larval mosquito populations in ponded stormwater management facilities. Since that time a system has been developed for stocking and maintaining Gambusia in stormwater facilities throughout Ocean County.

Stocking Gambusia can be an effective method to control mosquito breeding in stormwater management facilities. Some 51% of all Gambusia stocked stormwater facilities are no longer inspected on a routine basis in Ocean County. This is because of the effective larval control Gambusia provide. Gambusia stocked at rates approaching 1,000 fish/acre gave both immediate and long-term control for as long as the fish were present.

Maintaining an abundant fish population is the biggest problem for adequate larval control. Unfortunately, 59% of the stocked facilities that are routinely inspected needed periodic insecticide treatment over the past 3 mosquito seasons. Native fish can cause a problem in maintaining adequate Gambusia population densities. The pre-stocking survey that should be performed before any Gambusia introduction can identify any problems that may exist.

While a small number of stormwater management facilities remain wet enough to maintain a fish population (only 17% in Ocean County), Gambusia can be an important tool in reducing the number of facilities that need frequent inspection and treatment. Ocean County has some stormwater management facilities that have not needed pesticide treatments for years following the introduction of Gambusia. Gambusia can be a valuable asset to any larval mosquito control program for stormwater management facilities.

GAMBUSIA GUIDELINES FOR STORMWATER MANAGEMENT

FACILITIES

• All types of stormwater management facilities should be considered (retention, detention, infiltration).
• Prior to stocking, monitor water levels for at least 1 yr to determine the ability of the facility to maintain a fish population.

• Stock Gambusia at a rate of 1,000/acre.

• Once the facility is stocked, periodic inspections should still be performed to monitor fish activity and ensure effective control.

Occasional restocking may be necessary

• Keep accurate records of stocking

information (i.e. locations, number of fish, dates etc.).

GAMBUSIA STOCKING RATES

Habitat Number of fish

Bird bath (large) 10/site

Ornamental pond/swimming 35-100/site

pool (depending on size)

Stormwater facility 1,000/acre

Ditches (1-2 yd wide) 1/every yd of ditch length

Mine pits 2,500/acre (immediate control)

Freshwater swamps 1,000/acre (inoculation rate;

Woodland pools control not needed for 2 or

Sedimentation ponds more month) .

GAMBUSIA STOCKING OBSERVATIONS

· Immature Gambusia are not as effective predators as mature Gambusia (Meisch 1985).

· Drydown is the biggest cause of fish loss and subsequent mosquito production from a Gambusia - stocked area.

· Emergent vegetation is the greatest problem associated with the inability of Gambusia to control Anopheles spp. larvae.

· Low fish density is the primary cause for the failure of Gambusia to control Aedes spp. and Culex spp. larvae.

· In NJ a large percentage of the stockable sites will dry down during the summer. Control levels of fish should be stocked initially to achieve the best larval control.

· The stocking recommendation of 2,500 Gambusia/acre is supported by findings in the American Mosquito Control Bulletin No. 6, Biological Control of Mosquitoes (Meisch 1985).

· Although mosquito fish do a good job of controlling larvae in many situations, periodic monitoring of the fish and larval populations should be done.

LITERATURE CITED

Kent, R, M Boriek and K Powers. 1994. How to use the state bio-control (mosquitofish) program for mosquito control in New Jersey. 1O Pg. Bull. of N.J. Dept of Envir Prot and Ener, Trenton, NJ.

Meisch, NV 1985. Gambusia affinis affinis. pp. 3-17. In: HC Chapman, (ed.), Biological Control Of Mosquitoes. Am Mosq Control Assoc Inc, Fresno, California.

Sholdt, Lt LL, MSC, USN, Lt DA Ehrhardt, MSC, USNR and AG Michael, 1972, Guide to the use of mosquito fish, Gambusia affinis,, for mosquito control. Navy Environmental and Preventative Medicine Unit No. 2, Norfolk, Virginia.

Smith, JB. 1908. Report of the mosquito work in 1907. N.J. Agric Exp Station, Entomology,

Report for 1907. pp. 541-543.

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