Mosquitoes

Most mosquitoes are not a threat to health; nevertheless, certain types of mosquitoes can carry

viruses capable of producing encephalitis, an infection of the central nervous system.

A mosquito-borne disease of concern in Saskatchewan is Western Equine Encephalitis

(WEE). This disease has occurred in modest epidemic form on the prairies affecting

many horses and several people on sporadic occasions. Only one or two of the many

kinds of mosquitoes in Saskatchewan can carry WEE. A surveillance and control

program is in place to ensure that outbreaks of WEE are prevented. For further

information on this disease and its prevention, contact Vikram Misra, Department of

Veterinary Microbiology, University of Saskatchewan at (306) 966-7218.

West Nile Virus (WNW) is also spread by the bite of an infected mosquito. WNV can

infect people, horses, many types of birds and some other animals. Most people who

become infected with WNV will have either no symptoms or only mild ones. However,

on rare occasions, WNV infection can result in severe and sometimes fatal illnesses.

There is no evidence to suggest that WNV can be spread from person to person or from

animal to person.

Mosquitoes develop through four stages in their life-cycle: egg, larva, pupa and adult (see

Figure 19). Hatching of the egg and development of the larva and pupa can only occur in water.

These pre-adult stages of the mosquito obtain oxygen directly from the air . They can inhabit

sluggish, poorly aerated waters, such as the edges of swamps, temporary pools of melt water or

floodwater, weedy shallows of ponds, and small volumes of water in catch basins in storm sewer

systems, at the ends of culverts, garden ponds, and water trapped in discarded tires and unpunctured,

unburied containers. These are, in fact, preferred breeding places for most mosquitoes.

Mosquitoe development unsuccessful in open, well-aerated bodies of water or in running streams.

The larvae are popularly called "wrigglers", from their characteristic motion as they travel to and

from the surface where they breathe through their "tail". After three moults, the mosquito is in the

pupa stage. This has a relatively large "head", and breathes at the surface without the "wriggler"

motions. Finally, at the end of development, the adult emerges from the split pupal skin.

The details of the mosquito life-cycle vary considerably from species to species. They include: preferred host, preferred breeding habitat (temporary or permanent water bodies), preferred water temperature, emergence time, duration of each life stage and number of generations per season.

Consequently, effective control of larvae (larviciding) requires considerable knowledge of the biology of the particular mosquito species.

 Figure 19: Mosquito Life Cycle

4.3.1.2.1 Mosquito Control Measures

A) Learning to Live With the Problem

This is the most environmentally friendly measure. This means wearing light coloured clothing, not

using perfumes, buying over the counter repellents, avoiding going out during peak periods like the

cooler parts of the day and other measures which will lessen your chances of being bitten.

B) Preventing the Mosquito Problem

This pest breeds in small aquatic habitats which are rarely of use to man, control by habitat

reduction is a viable option (providing these modifications are environmentally sound in the

affected location):

·  puncture, crush, and/or bury "dump" items that can otherwise trap and hold snowmelt and

rain water; and

·  correct and prevent recurrence of erosion pits at the end of culverts, standing water .trapped

in culverts and seepage ponds in irrigated areas.

Another measure may be to stock fis h in suitable problem waters. Permission to do this must be

obtained through SE’s local Conservation Officer.

A combination of breeding area reduction and insecticide treatment usually provides the most

economical and efficient control of mosquitoes. The effects of habitat reduction are usually

long-term or permanent, while pesticides provide only temporary control.

C) Biological Control

Biological control is an environmentally friendly control measure. Biological pesticides containing

Bacillus Thuringiensis sp. have been proven to be effective in killing mosquito larvae (see Item D

below). Biological pesticides are quite target specific resulting in very little loss of other insects.

D) Chemical Control of Mosquitoes

For active chemical control of mosquito numbers, there are two basic approaches:

·  killing adults in the aerial or land environment when they are old enough to bite

(adulticiding). This method is not very effective. Also, terrestrial or aerial spraying of adult

mosquitoes is usually prohibited near aquatic systems; and

·  killing larvae in the water before adults emerge (larviciding). The low mobility of larvae

(in contrast with the adults) makes selective use of the pesticide quite feasible. The

remainder of this section focuses on mosquito larvicide programs.

(i) Planning

A good, cost-effective and environmentally sound larviciding program for mosquito control

requires careful planning. Larviciding is sometimes effective on a single parcel of private

property. But normally, due to the distances mosquitoes may fly or be carried by breezes,

effective control calls for joint action by a number of neighbours or by a municipality. Key

factors in planning and implementing a successful program are:

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·  carry out a mosquito survey in the area where control is desired and for several

kilometres adjacent to this area, concentrating on areas up-wind;

·  determine and map the mosquito breeding sites, paying special attention to the places

flooded for only a few weeks in the spring. Such a map must accompany the application

for a permit. This can be a section of a topographical map. A contour map (1:50,000) or

a Township Photomap is recommended. These are available from Saskatchewan

Geomatics Division, Mapping Branch, Information Services Corporation of

Saskatchewan, #10 – 200 Research Drive, REGINA SK S4P 3V7, telephone

(306) 787-4880

·  work out in advance the routes for control equipment to follow into and through the

treatment area;

·  if possible, arrange for workers from the area to carryout the observations and treatment

because they will be most familiar with the terrain and its problems and are more likely to

be available to continue control work in future years;

·  collate the information on all the above points before formulating the mosquito control

program; and

·  update the information on the area each year with an early survey of breeding sites since

these depend upon factors that can change from year to year.

(ii) The Survey

Effective control depends upon an accurate biological survey. An accurate survey should:

·  start in early spring;;

·  locate the actual breeding areas, map the abundance of larvae;

·  determine how developed the larvae are; and

·  identify and map the wet areas where larvae are present and also where they are absent.

(iii) Treatment Timing

Mosquito larviciding is best carried out when the larvae are about half-way into their

development, toward the pupal stage. They should be roughly 5 to 10 millimetres long. These

guidelines are adequate for the more conventional larvicidal chemicals. For insecticides that

work by altering development, more precise identification of the stage of development is

essential. The target is usually a stage near pupation, as indicated on the product label. If the

severity of the problem warrants, second generation mosquito larvae may be treated later in the

season. In any case, the effectiveness of the program should be determined through regular

follow-up surveys. Such surveys may indicate a need for repeat treatment.

a) Method of Application

·  treat only the documented breeding areas;

·  carefully follow the instructions on the label;

·  use only the recommended means of application. Larvicidal formulations may be spray

concentrates, granules or capsules and need to be distributed in different ways; and

·  treatment by spraying from aircraft requires special planning.

b) Restrictions and Precautions

·  wet areas not containing larvae should not be treated with insecticides;

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·  treatment of wet areas where larvae are not abundant would be inefficient, expensive and

may be environmentally damaging;

·  many mosquito larvicides work by disabling the insect's nervous system. These

chemicals may have similar effects on the nervous systems of other animals, including

man. Therefore, precautions on the labels must be rigorously observed; and

·  most products are only registered for use in standing waters wholly confined on users

property with no outflow to adjacent lands or watercourses.

c) Follow Up

The effectiveness of treatment and the need to repeat larvic iding should be assessed through

regular follow-up surveys. These surveys should be done:

·     24 hours after the treatment;

·     once a week throughout the breeding season; and

·    one to two days after heavy rains from midsummer to early fall.

If using insecticides that alter development, larval counts cannot be used as a criterion for

treatment effectiveness. In this case, numbers of adult mosquitoes is the criteria. An excellent

web site reference is http://www.mosquito.org/mosquito.html which has the most up to date