Sep 26, 2011 – Mosquitoes – Timing Is Everything
QUESTION:
When is the right time of year to start applying larvicide for mosquitoes? I am located in Alabama.
ANSWER:
Good question, and given that larvicides can have a significant cost factor it is important not to place them when they are not doing the most good. There are essentially 4 different basic kinds of larvicides. Surface oils and films are designed to coat the water surface and prevent the larvae and pupae from accessing oxygen, smothering them. This will not work on all species of mosquitoes as a few kinds use their syphon to penetrate aquatic plants to access oxygen. IGR's (insect growth regulators) such as methoprene act to inhibit proper development of the larvae to the adult stage, causing their death in the pupa or earlier. The bacteria such as B.T.I. (bacillus thuringiensis israelensis) is ingested by the larvae and causes a toxic reaction in their own gut, causing death in this manner. The 4th group would be normal contact insecticides that may be labeled for use in these aquatic sites, but for this answer we'll stick with the first 3 choices.
It is very important to know which species of mosquitoes you are dealing with in your area. Some kinds are much more likely to breed within small, temporary containers of water such as tree holes, tires, buckets, etc., and if you do not find these and treat them in some manner you miss killing them. If you assume that all the mosquitoes must be coming from the local marsh or pond or irrigation ditch it may not be correct, so capture some adult mosquitoes and get that positive ID so you know what kinds of breeding sites to look for. You also need to know some quirky habits of even the larvae, such as the tendency of some species larvae to gather in masses when feeding at certain stages of their development. For this you might need to use the highest Label rate possible when treating in that location.
Whenever possible and legal you should consider simply eliminating the breeding source. Why treat the tree hole if you can drain it or fill it with sand? Why treat the old tires if you can drill holes in them and drain the water, or empty the bucket, or drain the tarp, or fill in the low areas in the soil? Source reduction is by far the best mosquito control that reduces the need for any pesticides. With today's environmental movement many breeding sites cannot be manipulated, so these may need to be treated in some manner.
Once you determine the potential breeding sites it is important to sample them when the weather becomes warm enough for the larvae to begin development. In some cases the eggs may already be present in that site, and they hatch when the water warms sufficiently. The presence of adult mosquitoes does not signal the need to begin larviciding, but the presence of larvae does. Again, it depends heavily on the species of mosquito in your area as to when this may begin. Some species are well adapted to developing in much colder water than other kinds can tolerate. Using a dipper to sample the water and count the mosquito larvae will tell you when the activity is beginning.
The most susceptible stage of the larva is when it is close to pupation, and for most species of mosquitoes the larva might be between 5 and 10 mm in length when you decide to treat. Too early and you may miss later emerging larvae and too late and many of them may already have pupated. Fortunately, some of the larvicides such as the BTI have formulations that last at least 30 days and as much as 60 or more, so you have a pretty wide window. But, begin your sampling in early spring with the first warmish days and treat once you determine larvae are present at the proper stage of development. In many regions mosquito presence and breeding continues all summer long and into early fall, so repeat treatments may be necessary once you feel the active ingredient has run its course in the sites you have treated.
View past Ask Mr. Pest Control questions.
When is the right time of year to start applying larvicide for mosquitoes? I am located in Alabama.
ANSWER:
Good question, and given that larvicides can have a significant cost factor it is important not to place them when they are not doing the most good. There are essentially 4 different basic kinds of larvicides. Surface oils and films are designed to coat the water surface and prevent the larvae and pupae from accessing oxygen, smothering them. This will not work on all species of mosquitoes as a few kinds use their syphon to penetrate aquatic plants to access oxygen. IGR's (insect growth regulators) such as methoprene act to inhibit proper development of the larvae to the adult stage, causing their death in the pupa or earlier. The bacteria such as B.T.I. (bacillus thuringiensis israelensis) is ingested by the larvae and causes a toxic reaction in their own gut, causing death in this manner. The 4th group would be normal contact insecticides that may be labeled for use in these aquatic sites, but for this answer we'll stick with the first 3 choices.
It is very important to know which species of mosquitoes you are dealing with in your area. Some kinds are much more likely to breed within small, temporary containers of water such as tree holes, tires, buckets, etc., and if you do not find these and treat them in some manner you miss killing them. If you assume that all the mosquitoes must be coming from the local marsh or pond or irrigation ditch it may not be correct, so capture some adult mosquitoes and get that positive ID so you know what kinds of breeding sites to look for. You also need to know some quirky habits of even the larvae, such as the tendency of some species larvae to gather in masses when feeding at certain stages of their development. For this you might need to use the highest Label rate possible when treating in that location.
Whenever possible and legal you should consider simply eliminating the breeding source. Why treat the tree hole if you can drain it or fill it with sand? Why treat the old tires if you can drill holes in them and drain the water, or empty the bucket, or drain the tarp, or fill in the low areas in the soil? Source reduction is by far the best mosquito control that reduces the need for any pesticides. With today's environmental movement many breeding sites cannot be manipulated, so these may need to be treated in some manner.
Once you determine the potential breeding sites it is important to sample them when the weather becomes warm enough for the larvae to begin development. In some cases the eggs may already be present in that site, and they hatch when the water warms sufficiently. The presence of adult mosquitoes does not signal the need to begin larviciding, but the presence of larvae does. Again, it depends heavily on the species of mosquito in your area as to when this may begin. Some species are well adapted to developing in much colder water than other kinds can tolerate. Using a dipper to sample the water and count the mosquito larvae will tell you when the activity is beginning.
The most susceptible stage of the larva is when it is close to pupation, and for most species of mosquitoes the larva might be between 5 and 10 mm in length when you decide to treat. Too early and you may miss later emerging larvae and too late and many of them may already have pupated. Fortunately, some of the larvicides such as the BTI have formulations that last at least 30 days and as much as 60 or more, so you have a pretty wide window. But, begin your sampling in early spring with the first warmish days and treat once you determine larvae are present at the proper stage of development. In many regions mosquito presence and breeding continues all summer long and into early fall, so repeat treatments may be necessary once you feel the active ingredient has run its course in the sites you have treated.
View past Ask Mr. Pest Control questions.
