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Mosquito-Proof Your Garden

Overview:

Mosquitoes are a significant health hazard.  Unfortunately, the winter rains that we need so badly in California will also create an environment that is perfect for mosquitoes to quickly multiply.  However, changing a few things in your garden can protect you from dangerous mosquito borne illnesses.

Stop mosquitoes in your garden

Stop Mosquitoes. Picture courtesy of the CDC http://www.cdc.gov/features/stopmosquitoes/

El Niño +/-

This year’s powerful El Niño is predicted to do a lot of things. On the plus side, it is hoped that the winter storms will begin to replenish the drought stricken water table in California.  On the negative side, El Niño is also expected to cause flooding and landslides in the Western United States as well as cause drought and food shortages to other parts of the world.

However, one of the other more sinister dangers of the storms that have not been addressed in as much detail is the very real increased risk of mosquito-borne disease.  That issue, as well as important information about how to mosquito-proof your garden and other prevention measures, is the topic of this article.

 El Niño: Forecasts

El Niño Forecast
Image source: National Oceanic and Atmospheric Administration

 

The mosquitoes are coming:

The main ingredient that mosquitoes need to reproduce and multiply is stagnant water.  However, it doesn’t take much stagnant water to cause problems because those miniature flying vampires can develop in just a small amount of water hidden in any corner of your yard.  With the constant rains and cooler temperatures, you can expect that there will be more pockets of water available for those suckers.

 

Mosquitoes and disease:

Mosquitoes can be vectors for several different serious diseases. This basically means that the flying menace can carry around microscopic pathogens that will make us very sick.  Although not all mosquitoes carry infection, you can’t really tell which ones are the trouble makers in the seconds before you attempt to swat them… if you see them at all.

 

What diseases do mosquitoes carry?

Mosquitoes can transmit a variety of different diseases caused by bacteria, viruses and parasites.

Worldwide: more than 700 million people a year are infected with mosquito-borne illnesses.  Some of the terrible illnesses include malaria, dengue, filariasis, West Nile virus, chikungunya, yellow fever, Japanese encephalitis, Saint Louis encephalitis, Western equine encephalitis, Eastern equine encephalitis, Venezuelan equine encephalitis, La Crosse encephalitis and Zika fever.

In the United States, common mosquito-borne illnesses include Eastern Equine Encephalitis (EEE), St. Louis Encephalitis (SLE), LaCrosse Encephalitis (LAC), Western Equine Encephalitis (WEE), and West Nile virus (WNV).  WNV is the most common of these in the United States and in 2014, 48 states as well as the District of Columbia reported cases of WNV.   In my clinical/medical work, I have seen too many patients with brain injuries from these terrible infections.

eastern equine encephalitis Brain MRI

Brain MRI showing areas of severe damage from eastern equine encephalitis.
Image courtesy of the journal article “Mosquito bites and eastern equine encephalitis” qjmed.oxfordjournals.org/content/107/5/397

 

With global warming and globalization, more illnesses are arriving in the United States.  For example, the mosquito-borne disease Chikungunya (the disease makes your feel like your bones are being crushed) has recently made its way to the United States. There are now confirmed reports of this disease in Florida and Arizona.

Another concerning example is dengue fever, also known as “breakbone fever,” and in severe cases, it can cause the life threatening “dengue hemorrhagic fever.” In 1780, dengue fever was found as far North as Philadelphia.  However, in the modern era, dengue is rare in the US.  Unfortunately, this is changing.  Today Hawaii is experiencing an outbreak of dengue fever that is thought to have been introduced into the state by a visitor.  As of New Years Day 2015,  200 people were sickened by the virus on the big island of Hawaii. Other warmer states such as Florida and Texas have also seen cases of dengue fever in this century.  Disturbingly, about half of the continental United states has mosquitoes that are capable of carrying the dengue fever virus (see map below).

Dengue mosquito map

Where mosquitoes are found that have the ability to carry dengue fever (in blue)
Map credit via the journal article, “Dengue in the United States of America: A Worsening Scenario?” BioMed Research International

Your pets are also in danger from these little flying plasma thieves.  Heartworm is another one of the infections transmitted by mosquitoes which is a life-threatening disease for dogs and sometimes other animals such as cats… and even, under very rare circumstances, humans.

 

Mosquito life cycle in a nutshell:

Mosquito eggs are laid on water, or areas that will likely become wet. Upon hatching, the young vermin go through several stages of development and emerge as adult mosquitoes in about one week (yes that is a fast turn around time).  Stagnant water is a critical ingredient for their development because they need it for the larval and pupal stages of their life-cycle. Without it they cannot develop into the airborne bloodsuckers that cause so much havoc.  However, they do not need much water (and yes, they can actually develop in a little water as what you would find in a nutshell). The females are the biters in the family and they need the nutrients in your blood to develop their eggs. When mosquitoes bite, they can also inject infectious diseases into your body.

Mosquito Life Cycle

Mosquito Life-Cycle. Image courtesy of the CDC: http://www.cdc.gov/Dengue/entomologyEcology/m_lifecycle.html

 

Where do mosquito’s breed?

Dangers in your garden: 

Unfortunately our gardening supplies can be a potential haven for developing mosquitoes… and when you live in a normally dry climate such as San Diego, it is easy to forget about theses possible breeding grounds.

Therefore, it is important to remember that anything that can hold water in the yard needs to be addressed.  Even tiny containers have the ability to support the growth of hundreds to thousands of mosquitoes. Mosquitoes only need one half (1/2) inch of standing water to breed and turn into biting adults. Common garden related items for us to be aware of include container trays/saucers, flower pots, wheelbarrows, yard carts, mixing bins, garbage cans, garbage lids, buckets, waterfall basins, etc.

pot saucer mosquito

Planter saucers can easily hold enough water to breed mosquitos

 

Other areas for mosquitoes to develop: 

Other non-gardening items that have been a problem for mosquito growth include tires, clogged roof gutters, pet dishes, children toys, bird baths, bottles, cans, boats, barrels, tarps/covers, and even tree holes.

Control Larval Mosquitoes

Magnification image of the larval stage of mosquitoes.
Image courtesy of the EPA: http://www.epa.gov/mosquitocontrol/controlling-mosquitoes-larval-stage

 

Mosquito illness prevention:

Prevention measures can be split into three basic categories:

1. Remove their nurseries, 2. Mosquito avoidance, 3. Mosquito repellents

 

1. Remove/destroy their nurseries:

Remove their water:

Basically, try to get rid of anything in your yard that can hold standing water (see examples above).

If the items cannot be removed from your yard then re-position those items so that they cannot collect water.  Cover trash bins with lids to keep the inside dry.  Pull tarps tight so they cannot collect water. Turn buckets upside down. Clean drains and gutters. Fix leaking faucets. Put holes in the bottom of garden hose containers. Fill tree holes that can hold water with sand or mortar… or call a tree service to remove. Be diligent and inspect your grounds regularly.

Yard mosquito prevention

Garden hose containers should have holes drilled in the bottom to allow drainage.

Larvicides:

Of course some areas are not easy to empty out such as waterfall basins.   Siphoning the water out of these areas will work but it must be done regularly… after every rain.

Placing one of the many available mosquito larvicides in the standing water is also a popular and perhaps more convenient option for these problem areas.  Commonly used larvicides can be broken down into 4 basic categories

  • Bacterial Insecticides (Bacillus species)
    • These products contain a type of bacteria that specifically kill mosquito larva.  They are considered organic and can kill mosquito larva within hours. All professional references I have seen state that this insecticide is harmless to beneficial insects, pets, birds, fish and wildlife. However, some people have reported to have skin/eye sensitivity to this product.  Therefore, best to avoid direct contact and (as always) follow the directions on the packaging.
    • Mosquito Dunks are dried wafers that contain these beneficial bacteria. You basically put the wafer in areas of stagnant water, it dissolves and releases the bacteria to kill the mosquito larva. Each small wafers has the mosquito killing power that last for up to 30 days.
    • Mosquito Bits are the same idea but they are tiny little pieces of that dried material. This smaller granular size allows it to be more easily sprinkled into random areas of standing water.

 

  • Insect Growth Inhibitors

These products contain chemicals with names such as S-Hydroprene and S­-Kinoprene. I am personally less excited about these non-organic solutions.

 

  • Organophosphate Insecticides

These are basically toxic nerve agents. Although the name of this class of chemicals has “organio-” in it, they are definitely not the kind of organic product that we think of for gardening.  The EPA says that “No harmful effects to humans are expected from using products containing these active ingredients”  However, I am not so confident in the safety. My main concern is that this class of chemicals has been linked to human birth defects as well as the indiscriminate killing of other insects including bees.

 

  • Oils and Films

Oils and films disperse as a thin layer on the surface of the water which cause larvae and pupae to drown.

  • For more information about larvicides:

The EPA has a nice fact sheet outlining these options called;  Controlling Mosquitoes at the Larval Stage.

Free fish for neglected pools:

Pools that are clean and suitable for swimming do not breed mosquitoes.

However, neglected-stagnant pools and ponds can quickly breed millions of the bloodsuckers.  Government agencies are tasked with addressing neglected pools (or other large areas of standing water) that can become a health hazard.  As a result, some counties conduct annual aerial surveys to monitor for the presence of neglected swimming pools.  Those government  agencies should also quickly follow-up on any complaints from anyone about neglected pools. For example San Diego County encourages reporting of neglected pools and they have an online reporting form as well as contact numbers & emails:  (858) 694-2888  vector@sdcounty.ca.gov

Many counties (including San Diego County) will also stock neglected swimming pools with mosquitofish for free.

Mosquito Fish government

Free Mosquitofish. Image from San Diego County. http://www.sandiegocounty.gov/deh/pests/wnv/prevention/chd_wnv_mosquito_fish.html

Chlorine:

I have read conflicting reviews about the use of chlorine.  Some personal reports say it works. However, several government sites specifically state that dumping chlorine in neglected pools is not an effective way to control/kill mosquito larvae. Perhaps the active killing power of chlorine just doesn’t last long enough for the many cycles of egg laying over time.

 

More Info:

The state of California also has a webpage dedicated to numerous ways of fighting West Nile Virus (which is basically going to fight mosquitoes in general.

2. Mosquito barriers/avoidance: 

Covering up:

Covering up your body with long-sleeved clothing and pants will also make it harder for mosquitoes to tap into your bloodstream. Loose fitting clothing is preferred because the suckers can sometimes needle their way through tight garments to the nearby skin just beneath.  Sorry yoga pants.

I have also heard that mosquitoes are more attracted to dark clothing… so go with the lighter colored fabrics.

Keep out:

Keep mosquitoes outside of your home:  Make sure that you repair and use window/door screens. Keep doors closed that don’t have screens on them.

Danger times:

Anytime can be mosquito time.

  • Dengue, yellow fever, and chikungunya carrying mosquitoes bite mainly in the daylight hours (from dawn to dusk).
  • Malaria, West Nile, and Japanese encephalitis carrying mosquitoes mainly bite in the dark (mainly from dusk to dawn).

 

3. Mosquito repellents:

Repellent overview:

According to the CDC, wearing insect repellent is the best way to prevent diseases spread by mosquitoes. Here is a direct quote from the CDC website: “Yes! It is safe. When used as directed, insect repellent is the BEST way to protect yourself from mosquito bites—even children and pregnant women should protect themselves.”

The CDC also reminds us that the Environmental Protection Agency (EPA) regulates repellent products in the United States and recommends the use repellent products that have been registered by EPA.

Recommended mosquito repellents:

CDC has evaluated information published in peer-reviewed scientific literature and data available from EPA to identify several types of EPA-registered products that provide repellent activity sufficient to help people reduce the bites of disease-carrying mosquitoes. Products containing the following active ingredients typically provide reasonably long-lasting protection:

  • DEET

(chemical name: N,N-diethyl-m-tolua-mide or N,N-diethyl-3-methyl-benzamide). Products containing DEET include, but are not limited to, Off!, Cutter, Sawyer, and Ultrathon.

 

  • Picaridin

(KBR 3023 [Bayrepel] and icaridin outside the United States; chemical name: 2-(2-hydroxyethyl)-1-piperidinecarboxylic acid 1-methylpropyl ester). Products containing picaridin include, but are not limited to, Sawyer Clothing Insect Repellent, Cutter Advanced,  Skin So Soft Bug Guard Plus, and Autan (outside the United States).

 

  • Oil of lemon eucalyptus

(OLE) or PMD (chemical name: para-menthane-3,8-diol), the synthesized version of OLE. Products containing OLE and PMD include, but are not limited to, Repel and Off! Botanicals. This recommendation refers to EPA-registered repellent products containing the active ingredient OLE (or PMD).

“Pure” oil of lemon eucalyptus (essential oil not formulated as a repellent) is not recommended; it has not undergone similar, validated testing for safety and efficacy, is not registered with EPA as an insect repellent, and is not covered by this recommendation.

 

  • IR3535

(chemical name: 3-[N-butyl-N-acetyl]-aminopropionic acid, ethyl ester). Products containing IR3535 include, but are not limited to, Skin So Soft Bug Guard Plus Expedition and SkinSmart.

Recommended application of bug repellents:

CDC recommendations/precautions regarding the appropriate application of repellent chemicals can be found on their website and some of them are highlighted below:

  • Obviously use the products as directed on the label.
  • Apply appropriate skin repellents only to exposed skin or clothing, as directed on the product label. Do not apply repellents under clothing.
  • Never use repellents over cuts, wounds, or irritated skin.
  • When using sprays, do not spray directly on face—spray on hands first and then apply to face. Do not apply repellents to eyes or mouth, and apply sparingly around ears.
  • Wash hands after application to avoid accidental exposure to eyes or ingestion.
  • Children should not handle repellents. Instead, adults should apply repellents to their own hands first, and then gently spread on the child’s exposed skin. Avoid applying directly to children’s hands. After returning indoors, wash your child’s treated skin and clothing with soap and water or give the child a bath.
  • Use just enough repellent to cover exposed skin or clothing. Heavy application and saturation are generally unnecessary for effectiveness. If biting insects do not respond to a thin film of repellent, apply a bit more.
  • After returning indoors, wash repellent-treated skin with soap and water or bathe. Wash treated clothing before wearing it again. This precaution may vary with different repellents—check the product label.
Spraying of mosquito repellent

Spraying of inspect repellent.
Image courtesy of the CDC. http://www.cdc.gov/media/dpk/2014/dpk-whd.html

About Thomas Osborne, MD

Dr. Osborne is a Harvard trained Radiologist and Neuroradiologist who loves to share his insight about medicine and gardening.

6 comments

  1. Awesome, Great information

  2. Just found your site while researching kumquat trees. Then I clicked to this article. Have you ever tried growing plants that deter mosquitos? I hear mint, for example, might be good for that. And if you have grown plants have you noticed a difference?
    Thanks!

    • Thomas Osborne, MD

      Hey Lori from TX
      Thanks for the note and good question.

      I also have heard and read nonscientific articles about some plants ability to deter mosquito’s.
      However, I have not seen any scientific proof that this works to keep your yard safe from mosquito’s.
      So sure, I have mint growing in the yard and I am hopeful…
      Unfortunately, my personal suspicion is that if the mint does anything to influence the bloodsuckers behavior, it would be very very minimal… and not good enough to keep you safe.

      Best,
      Tom

  3. Great article. Thank you. We received our first bites last week. I expect it will be a bad season for those critters.

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