Identification | Concerns | Control
Larvaecide and Pesticide Applications
As the warm summer months approach we are once again confronted by the yearly emergence of midges and mosquitoes. Even without the health risks that come with the bites from the mosquitoes the inconvenience and the resulting complaints would still necessitate efforts at controlling these emergences. The concerns that we now have about the West Nile Virus only serve to reinforce the need to reduce or eliminate the presence of these species before they become a problem.
The purpose of this article is to enable a wastewater operator to identify and address a midge or mosquitoes problem in their wastewater lagoon or other treatment facility, hopefully before they become a public nuisance.
The midge is a fly that often alarms people because of its resemblance to the mosquito although they differ from mosquitoes as they are not equipped to bite and do not have scaled wings. On this note, although midges do not bite some of the treatments for their control will also provide reduction or even elimination of mosquitoes also, a dual benefit in areas with concerns related to the West Nile Virus, which is, along with Encephalitis, a mosquito born illness.
The adult midge is slender, usually less than 5mm long with long slender wings and legs. They lay their eggs on water and are usually found in quiet waters, with a preference to polluted waters that contain excessive algae growth, as this is their food source. They are bottom feeders, which makes lagoon systems and trickling filters particularly vulnerable, and increases the likelihood of excessive midge populations.
Most of us can recognize mosquitoes; they differ from the midge in that they have scaled wings and in the females; elongated mouthparts that enable them to bite. The mosquito lays its eggs in any standing water and the "wrigglers", as they are called because of their swimming motion, feed on algae and other organic matter. They can be found anywhere from a water filled tire or basin to lakes and ponds. As they prefer still water, non aerated lagoons provide an ideal habitat.
During peak emergences large numbers of midges can form clouds that are not only irritating but can even block visibility, making them a potential threat as they group around well lit areas, such as street lamps. They are also very invasive entering everywhere from homes and businesses to peoples eyes, ears, etc. Large populations of midges are not only a hindrance to employees trying to do routine maintenance they are also irritating to local residents. As any City Manager will attest to, wastewater treatment facilities are subject to enough concerns without additional reasons for complaints.
Mosquitoes, although they may not swarm, have the ability to bite and spread diseases making them an even greater concern than the midge. The presence of mosquitoes in a populated area can result in numerous complaints and greatly impact the tourist business when they are present as well as posing a health threat to both people and animals.
As we have already mentioned control of midges may also address concerns with mosquito populations. The most probable cause of the presence of these insects is poor circulation and maintenance of the lagoon system. Clearing shoreline plants to increase wind action on the pond and/or recirculation will improve the health of the lagoons as well as hindering the development of the larvae. The reduction of vegetation also reduces the available habitat for the larvae to form and grow as well as improving the appearance of the lagoon area. Wave action from the wind may also reduce some of the larvae population.
Controlling the growth of algae will reduce the amount of available food for the midge and mosquito larvae, although the obvious limitation is that the algae is also beneficial in wastewater treatment because of the oxygen exchange that it provides. Ideally, the use of surface aerators set to stir heavily during the daylight hours will help control the algae levels in the lagoon system and still maintain adequate levels of dissolved oxygen in the water. As the mosquito larvae also tend to inhabit the water surface it may also help reduce their numbers.
There are several other methods of algae reduction, including the use of copper sulfate. This chemical is very effective in killing algae but can also be toxic to fish and many of the beneficial organisms that provide treatment of the wastewater so must be administered with caution and in compliance with state and federal regulations. Other methods include the introduction of duckweed to the lagoons as it effectively reduces the sunlight and limits the growth of the algae, covering the lagoons is also an option. As a precaution, consideration should be given to the fact that all of these solutions alter the existing treatment process, thereby creating new challenges.
Perhaps the most effective method of reducing the algae is the use of HTH (High Test Hypochlorite), better known as granular chlorine. As it is in granular form and easily measured, careful calculations can and should be made prior to its use. Caution should be taken not to apply too high a dose which could kill off the beneficial "bugs" that the lagoons rely on to provide treatment of the influent wastewater. There are also established limits in the NPDES Permits issued to systems that discharge to receiving waters and these must be considered before this method is used.
Stocking of the lagoons with Gambusia (Mosquito Fish) is another alternative that can be utilized to reduce the number of larvae present in the lagoons as they are a food source for the fish, thus the name. At the same time consideration should be given that this method can also create its own problems. Introduction of fish could hinder mechanical processes, worse case scenario perhaps is the possibility of the fish dieing out due a shock load or loss of adequate available oxygen and creating an additional impact to an already compromised system. There is also the added concern of the potential release of these fish into the receiving waters with the effluent discharge. As these fish are considered a pest in some areas, systems with a NPDES Permit should definitely consult with the agency overseeing their permit prior to introducing these fish into their system.
As a final alternative to controlling or attempting to eliminate midge and mosquito population chemical applications may be considered. The two most important considerations of using this remedy are the financial and environmental impacts. Chemical applications can be very costly and do not provide long term protection; as long as a food supply is available the insects will reappear. Careful consideration must also be given to the impacts on receiving waters; once again any system that has a NPDES Permit must first receive approval before making any changes to their system operation.
There are several different products available on the market for this purpose; they can be found on the Internet or by consulting your local Environmental Department for more information. Before purchasing any products make sure that you have thoroughly researched the application methods and found the best one for your needs. Proper planning can also be very helpful, starting a treatment program prior to the insects yearly cycle could reduce the amount of treatment required and certainly reduce the inconveniences.
In the event that there is already a major emergence of insects or that you are unable to treat the lagoon waters, fogging may be the only feasible method of reducing the population. Once again the cost must be taken into consideration as well as the fact that relief will only be provided in the areas that are treated. An additional consideration must be taken as to the impacts to the neighboring residents who should be consulted prior to exposing them to these products.
Information for this article was obtained from the following online sources:
1. MSN Encarta - Mosquito
2. Midges, Midge Elimination and Biology
3. Midges and Crane Flies, HYG 212
