Larvicide Treatment Tips: How And When To Use Larvicide

By: Mary Ellen Ellis

There are many ways to deal with pests in the yard orgarden. Mosquitoes,in particular, may be handled through a variety of strategies. If you havestanding water, larvicides can be a good option in addition to preventativepractices. Know the pros and cons before you use larvicides in your garden.

What is Larvicide?

A larvicide is a product that kills insects at the larval stage,when they are active but still immature. You’ll find these products in multipleforms in garden stores and nurseries: briquettes, tablets, granules, pellets,and liquids.

You can use a larvicide to manage mosquitoes which lay eggsin standing water. The larvicide goes directly into water. Mosquito eggs aretypically found in buckets of water, gutters, fountains, ponds, puddles thatdon’t drain quickly, septic tanks, and even on the tops of pool covers thatcollect water. You don’t have to worry about mosquito eggs in chlorinatedwater.

How Do Larvicides Work?

Different larvicide treatments work in different ways. Thosethat contain the spores of the bacterium called Bacillusthuringiensis israelensis, or Bti, kill the larvae of flies andmosquitoes only. They do so by acting as a poison in the larvae when ingested.The benefit of Bti larvicides is that they won’t kill predatory beneficialinsects.

Another type of larvicide contains methoprene, which is aninsect growth regulator. It has a broader spectrum and may kill larvae of allkinds of aquatic insects. It acts by interfering with the molting stage. Asidefrom being harmful to aquatic insects, neither larvicide is toxic to otheranimals, pets, or people. They will not harm plants either.

It’s best to try to prevent mosquito formation first. Tryusing more natural methods to control mosquitoes, such as by draining standingwater when possible, cleaning ponds, fountains, and bird baths regularly, andencouraging predators. When those fail or are inadequate, try an appropriatelarvicide. Always follow the directions on the product and it shouldn’t causeharm to plants or other wildlife.

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Feed-Thru (Oral Larvicides) for Pasture Fly Control

Altosid (methoprene) and Rabon (tetrachlorvinophos) are insecticides that are available as minerals or feed supplements for beef and dairy cattle fly control. These oral larvicides are consumed and pass through the animal’s manure at concentrations that kill fly maggots. With this approach, the only sources of horn flies and face flies in pastures are treated.

There are some important things to keep in mind when using this approach:

What pasture flies are targets?

Horn flies and face flies are key pests of cattle in Kentucky. They torment herds every year and often are present in damaging numbers: more than 200 horn flies per head and more than 10 face flies. Altosid products are labeled for horn fly control. Rabon larvicides are labeled for both horn flies and face flies. Supplemental fly control measures may be needed, depending upon fly pressure.

How do these products work and why is that important?

Altosid is an insect growth regulator (IGR). It interferes with normal development of horn fly maggots, preventing them from emerging as adults. Rabon, a Group 1 insecticide, attacks the maggot’s nervous system so that it dies before completing development.

Knowing that these products work differently is important when planning an insecticide resistance management program for horn flies. Altosid IGR only affects the larval stages of insects, so it is not included in insecticide ear tags, pour-ons, or sprays that control adults. However, the organophosphate insecticide Rabon is effective against both maggots and adults. It and other Group 1 insecticides are available in some brands of insecticide ear tags and fly sprays. This must be considered so a total herd fly control program is not based on products from a single insecticide group. Rotation among insecticides that attack pests in different ways is part of resistance management.

How important is dose?

All animals in the herd must eat the minimum daily amount of larvicide specified on the label. This ensures that a lethal concentration of insecticide is always present in all manure. It means keeping up with consumption and taking steps to increase or decrease it as needed. This may be done by relocating feeders to increase or decrease availability. Over-consumption increases control costs unnecessarily. Under-consumption can mean reduced fly control.

How can I tell if the product is working?

The best way is to keep a record of average fly numbers per head and compare the numbers with what is considered acceptable levels of flies. Write down a weekly count of horn flies and face flies per 10 randomly-picked animals in each herd. Face fly counts are straightforward but horn fly counts usually have to be estimates. Ten or fewer face flies per head is generally considered to be satisfactory control. Horn fly numbers should be less than 100 per side.

Counts on your cattle may be above the target values even if the oral larvicide is doing its job. Flies can move in from nearby herds (within 3 to 5 miles) where fly control is not practiced or is insufficient. In these cases, supplement fly control with dust bags, oilers, or some other means.

What conditions increase the effectiveness of oral larvicides?

The faster manure piles dry, the sooner they become less suitable for fly breeding. Extra rain can allow the development of greater fly numbers. The better the fly control is on nearby herds, the less pressure you will have from flies moving into your herd.

Start early. Consumption of oral larvicides should begin before the flies appear so that the only fresh manure available is treated.

Oral larvicides are a tool for pasture fly control. Success depends on careful monitoring of consumption and keeping track of fly numbers to assess the need for extra control measures.

Feed-Thru (Oral Larvicide) Insecticides for Pasture Fly Control

Eliminating or treating a pest’s breeding site is often right at the top of the list of pest management options. It can work if the breeding site is very specific – like freshly deposited cow manure, the egg-laying site of choice for female horn flies and face flies. This allows the option of feed-thru larvicides (insecticides that control fly larvae or maggots) that pass through the digestive tract and are present at toxic levels in manure.

Figure 1. Mineral feeders can be modified with a face wipe to provide control of adult face flies and horn flies (Photo: Lee Townsend, UK).

Oral Larvicides

Active ingredients used as oral larvicides include the insect growth regulator methoprene (Altosid) and the organophosphate insecticide tetrachlorvinophos (Rabon). Insect growth regulators affect the development of larvae and prevent them from emerging as adults. In contrast, organophosphate insecticides kill by disrupting normal function of the nervous system. In either case, these chemicals must be present in the manure at or above levels that are toxic to the larvae. Either active ingredient can be formulated in different ways and is available under several different brand names.

Methoprene – Altosid 0.5% Premix Dose or Altosid 0.1% IGR Block – to prevent the breeding of horn flies in the manure of treated cattle.

Tetrachlorvinophos – Rabon 7.76 Oral Larvicide Premix – to prevent development of horn flies and face flies in manure of treated beef and lactating dairy cattle.

Diflubenzuron – ClariFly Larvicide – for horn flies and face flies feed additive

Dose rates are based on milligrams of insecticide active ingredient per 100 pounds of body weight per animal per day.

Other Types of Mosquito Control

Of course, there are other ways of killing mosquito larvae that don’t involve chemicals or bacteria.

One easy way to get rid of unwanted mosquito larvae is by adding mineral oil to standing water (not used for drinking). It works in the same way as larvicidal oils, drowning the larvae and pupae.

All you need is 1/4 teaspoon of mineral oil per gallon of water. You can achieve the same results by substituting vegetable oil or neem oil for the mineral oil.

You can also try the same trick using dish detergent, but you’ll only need 1/16 teaspoon per gallon of water.

A few other DIY ways to reduce larvae/adult mosquitoes include:

  • Eliminating any areas of standing water (including draining birdbaths) to prevent mosquito larvae from having any place to survive.
  • Regularly maintaining outdoor pools by making sure the pump is in good working order and proper levels of chlorine are used.
  • Adding goldfish or mosquito fish to decorative water features, ponds, or lakes since they eat mosquito larvae.

If you’re concerned about mosquitoes in your yard, start by removing all areas of standing water you can. For the rest, your best option is to use a larvicide, and there are many effective options available!

Bacillus thuringiensis israelensis

The most effective biolarvicide for mosquito control on the market!

Bacillus thuringiensis israelensis (Bti) has been used for decades by backyard gardeners and commercial growers to control mosquitoes, fungus gnats and black fly. A bio-rational control, Bacillus thuringiensis israelensis is naturally present in the environment and controls the larval stage of certain Dipterans – the aforementioned mosquitoes, fungus gnats, and black fly. It is target specific, rivals S-methoprene in efficacy and is safe for use around mammals, birds, fish and amphibians – keystone species within an ecosystem.

Bacillus thuringiensis israelensis works similarly to Btk in that it must be ingested by the target pest. Once the bacterium has been ingested, it produces crystalline toxins which disrupt the pest's digestive system, thus stopping continued development and killing the pest larvae before they reach adulthood. This method allows you to reduce disease vectors and egg populations while simultaneously disrupting the pest's breeding cycle. For best results using Bti, applications should be made early in the target pest's life cycle while there are still high numbers of larvae present. Trapping measures are also important when setting up the most effective integrated pest management program for mosquitoes that you can.

Please give us a call at 1-800-827-2847 if you have any questions about Bti, its uses, targeted pests, etc.

Related Articles:

  • Bti for Mosquito Control FAQs – Frequently asked questions regarding Bacillus thuringiensis israeliensis and its use in mosquito control. Published by the EPA.
  • Efficacy of Bacillus thuringiensis var. israelensis against Mosquitoes in Northwestern Burkina Faso - Test results and report for various field experiments conducted in 2012. Published by BioMed and NIH Central.
  • Bacillus thuringiensis israelensis (Bti) In Drinking-Water - Background document for development of WHO Guidelines for Drinking Water Quality. Published by the World Health Organization.

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