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Aedes aegypti on the comeback

When the rains returned after the long (ten week) drought we had starting at Christmas, it took a couple of weeks for the mosquito populations to return – and return they did in force. Part of the reason for this is how mosquitoes lay their eggs – on the sides of breeding sites, so that they only hatch after a rain. This means that in a drought, the eggs build up in huge quantities – and when the rains come, they all hatch at once.

Aedes aegypti
Front view of Aedes aegypti showing the characteristic white dots in front of the antenna.

Importantly, some of us taking part in the iNaturalist project are observing a return of the Aedes aegypti. You may know that this mosquito is a very aggressive biter, and a very efficient vector of disease, much more so than the more common Aedes albopictus. In Kehena, I only caught one aegypti in the months from November through to March when the rains returned. During that time, I caught dozens of albopictus. Now, I can catch aegypti regularly. Clearly, the drought has tipped the scales towards aegypti, at least for a while. In fact, this is kind of what is expected based on their biology.

Another volunteer, Don Kephart, up in the Kohala area lives in an area dominated by aegypti (there are no albopictus!) and he says the population is the largest he’s seen in years.

This is important information, because aegypti are much better vectors of diseases like dengue, zika, chickungunya, etc. If we have established populations of aegypti we are at a greater risk of disease outbreaks than if we do not have these populations. Making sure the DOH knows where these populations are will help them in their response to future outbreaks. Who would have guessed there was a resident population of aegypti on the “wet” side? There were reports of aegypti here last in 2002. And we have also seen them this year near Kalapana (another volunteer, Devin has caught them there).

It would be great to get more observations of these mosquitoes on the leeward side of the island. If you are interested in helping, we can teach you how to ID mosquitoes and maybe even give you some equipment to help (such as a macro lens for your smartphone).

Here is a link to my recent aegypti observation on iNaturalist.

Just contact me!   mosquitotraps at hotmail dot com or on FB.

Bromeliads: 200 mozzies per day

A friend of mine has a beautiful garden – he and his partner are very talented gardeners. Outside their property along the road is a 6 foot wide bed of plants about 100 feet long, many of them bromeliads. So, I went over there today with a turkey baster and did some sampling. In one small bromeliad, I found just over a dozen larvae. In another, I found a similar number. To get these larvae, I sucked up as much water as I could from the center, and a few of the leaf wells. So it’s reasonable to assume that I got maybe half of them – the small larvae were probably hiding deeper than I could reach, and the pupae are fast and so could escape the suction. (With the turkey baster, you never get all the water).

So let’s just say for the sake of argument there are 25 mosquitoes in larval or pupal form in each plant. Along the road, I estimate there are at least 100 bromeliads of this type. So, that of course means there are something like 2500 mosquito larvae and pupae there. Since on average it takes about 12 days to go from egg to adult, this means at least 200 adult mosquitoes per day are hatching from that one bed of bromeliads. And even though we’ve had little rain recently, the leaf wells had lots of water in them. So these bromeliads are going to keep producing mosquitoes even when it’s dry.

Food for thought.  I’d encourage everyone to grab a turkey baster, and maybe a fat straw to stick on the end, and go out to your nearest bromeliad and see if you can find larvae.


So what can you do?

Assuming you don’t want to pull all the bromeliads out, you can sprinkle them with BT (Mosquito Bits, lasts 14 days). There are also
liquid forms of BT that you can spray that might be more effective at reaching the hard to get broms.  Be aware that BT is harmful to caterpillars (larval stages) of both good and bad insects, so it might be best to use the Bits.

Personally, I would pull 99% of the bromeliads out.  Keep a few choice specimens that would be easy to treat and monitor.  We have so many beautiful plants here, we can choose some that are not mosquito factories and still have a fabulous garden.

What about soapy water or bleach water?

These don’t work.  The DOH was telling people to use soapy water until the CDC told them to stop saying that.  Now, they are telling us to use bleach water, but research shows the concentration of bleach required to kill larvae is really, really high.  So it isn’t going to work!  Just use BT.


Mosquito eggs on landing strip

Today I pulled the red velour strip of paper out of a TrapNKill trap I set three weeks earlier close to a house with some kind of mosquito factory on it.  Lots of eggs are visible (though my eyes are not good enough to see them without a hand lens).  In case you are interested to see what they look like, here are some photos made by holding a hand lens in front of my phone camera…..

Mosquito Eggs
Mosquito eggs – the black elongated dots. You can see both edges of the red strip for an idea of scale. The eggs are supposed to be about 0.8 mm long.
Mosquito Eggs close up
Closeup of the mosquito eggs


How many infectious mosquitoes are there?

This is a fun little analysis to work out how many infectious mosquitoes are on the Big Island (Jan 2016).

Bottom line: between five hundred and a thousand infectious mosquitoes right now.

The Dept of Health regularly publishes details of current dengue cases.  They also publish an estimate of how many of those people are infectious to mosquitoes.  Fortunately, this is usually a small number, like 3 or 8 (though there is a bias to lower numbers because often DOH does not complete testing until after the person is no longer infectious).  Unfortunately, this doesn’t really tell us much because we can’t catch dengue from people.  So much more interesting would be how many infectious mosquitoes are there?

You might think this would be impossible to know unless we spent a whole lot of money trapping and testing mozzies.  Fortunately, we can make an estimate with some middle school math.

Here is how a scientist would start to estimate that number. It involves a few assumptions we can argue about, but what matters is the “order of magnitude” result.  Is it 1, or 10, or 100, or 1000, or 10,000??? This kind of analysis is a standard procedure in science.
We’ll assume:
  1. for every one confirmed case of dengue in humans there are three more that are not confirmed – ie not sick enough that they go to the doctor. (This number is from DOH, also WHO and numerous research papers). Instead, they go to work, to the beach, the market, to the farms, etc. We’ll conservatively assume the confirmed cases are so sick they stay in bed and don’t get bit.
  2. each infectious person is infectious for 5 days only (it could be longer). We’ll say 5 to be conservative.
  3. each infectious person who is out and about will get bit just twice per day on average. (some people will be much, much higher than this and some will be zero). Each bite produces one infectious mosquito. You can see that means on average that every asymptomatic infected person goes out and infects ten mosquitoes (order of magnitude) during the time they are infectious.
  4. But, these mosquitoes eventually die so we have to figure out how to take that into account by calculating, on average, how many days an infectious mosquito will live.  We’ll assume the average mosquito adult lives about 35 days (lab raised Aedes mosquitoes have survival rate of 50% at 60 days, some live to 100 days). We’ll assume that if they do bite an infected person, it happens half way through their life, ie 18 days. This is a pretty reasonable assumption for randomly occurring events.  Now, after the mosquito bites someone, it takes about 5 – 7 days to become infectious.  So we need to subtract this from 18 days to get the average number of days a mosquito will be infectious.  That leaves about 12.
So here is how we make the estimate:
We’ll call P the number of confirmed cases during the last 12 days.  As of January 14th, I’d estimate (based on DOH numbers of 218 people in 120-ish days, ie about 2 per day) that there were 25 dengue cases in the last 12 days.  So P = 25.
The number of infectious mosquitoes, M is given by:
M = P (confirmed cases) * 3 (infectious humans per confirmed case) * 5 (days a human is infectious) * 2 (mosquitoes that get infected per day per infectious human).
Then M = 25 * 3 * 5 * 2 = 750.  So this is the order of magnitude estimate for how many infectious mosquitoes are out there.  So, it’s reasonable to say, that there are between 500 and 1000 infected mosquitoes on the Big Island today.
Now, before you go into a panic and lather on the DEET, you have to also realize that not all infectious mosquitoes are going to infect someone, especially since they are mostly albopictus which are not as effective as aegypti.  Albopictus get half their meals from animals (DOI: 10.1371/journal.pntd.0003037), mostly pets, and none from birds.  So, there is some thankfully small probability that an infectious mosquito will infect someone, and if we can keep that probability low enough, then dengue will die out!
Mosquito traps (lethal ovitraps) also lower that probability by reducing the average lifetime of an infected mosquito.  If we can cut the 12 days down to 6 days, we just cut the number of infectious mosquitoes by a factor of 2. That’s why health authorities all over the world (but not DOH) respond to a confirmed case by placing 4 to 10 mosquito traps around every house within 200 meters of a confirmed case. In fact, mosquito traps deployed like this have been shown to eliminate 90% of the mosquito population (eg Thursday Island, Australia 2004).
Notes:  I have read dengue shortens the life of mosquitoes but not included it here (and I hope it gives them terrible joint pains, splitting head aches, fever, etc).

Why two poisons?

Commercial traps like the TrapNKill and others just use one poison.  This poison diffuses from the landing pad (or air in the case of TrapNKill) into the water bath and so both adults and eggs and larvae are killed by it.

So why do I recommend two poisons?  It’s much more hassle.

The answer is pesticide resistance.  This is a real threat world-wide, where mosquito populations are rapidly becoming resistant to key pesticides used to control them.  It’s a major headache, and countries that are on top of this problem regularly monitor pesticide resistance among their populations.

According to numerous research papers, pesticide resistance is increasing rapidly, and the pressures put on a mosquito population by a successful trap program are very large.

If you use just one pesticide, and a resistant female lands on the pesticide coated landing strip, she’s not too much affected by it, so she can lay eggs.  The eggs also have the same mutation that allows the female to have resistance – and therefore, if the same pesticide is in the water bath, the larvae will be immune and go on to develop into adults.  Other, non-resistant mosquitoes will die, leading to a rapid increase in the population of resistant mosquitoes.

So, use a different class of pesticide in the water bath – the chances of multiple-resistant strains is very small, and if the female can lay eggs, the eggs will die.

Kill dem mosquitoes dead!


Mosquito Eggs on BIF strips

Today I found mosquito eggs on a landing strip.  🙂

Mosquito Eggs

It was one of my early strips made with bifenthrin. Being paper, it was starting to rot in the water, but it still reached the surface. This is the first time I’ve found eggs, so this is great news.  For one thing, it shows mosquitoes will enter a mosquito trap and land on a BIF treated strip and lay eggs.  That means they should have gotten a lethal dose of poison.  Yeah!

I took this picture by holding my phone up to a hand lens so the quality is not so great.  But they are clear!

How to make BIF sticks

Bifenthrin coated landing sticks are a key part of the Four Month Mosquito Trap.  Making them takes a bit of work, but fortunately, you can make a lot at once and save them for long periods, or share them with neighbors. Use two or three in each trap.

BIF Sticks
BIF Sticks, ready to use
What you need
  • You need a lot of sticks (say 300!) that are the right length to fit in your buckets (and about 1 or 2 inches wide).  Cedar shims, paint stirring sticks, or  lath strips from the hardware store work well. Chop them into suitable lengths to fit diagonally inside your containers.
  • Bifenthrin concentrate. This must be water based, not oil based. For these instructions, the concentration must be 7.9%. Commercial products: Talstar or Bifen.
  • Red food color, at least a Tablespoon (0.5 oz) or better yet, an ounce to get a nice red tint.
  • Water and an 8 ounce measuring cup.
  • Disposable paintbrush.
  • A permanent ink marker (Sharpie).
  • A place to dry lots of sticks. This must be somewhere you can clean well – the bifenthrin will drip. Try laying a few 2x4s on the ground or on saw horses. You do not want the sticks to drip – instead, lay them flat so the solution stays on them until it dries. If possible, do this in the shade.
Make the Bifenthrin dilution

Bifenthrin comes as a liquid suspension with a concentration of 7.9%. You need to dilute this to 1%:

  1. Get an 8 ounce measuring cup.
  2. Add one ounce (six teaspoons, or two tablespoons) of the concentrate (you MUST vigorously shake the concentrate for a couple of minutes before pouring it out).
  3. Next, add two to four tablespoons of red food color – this is important so you can see immediately if a stick is treated.
  4. Next, add water until the volume is 8 ounces.

Viola! You have your 1% solution. 8 ounces should cover a few hundred sticks. Don’t let it settle, stir as you use it.

Next, label the sticks.

On the end of each stick, write “BIF” and draw a line about one or two inches from the end. This is where you can write the date when you make a trap. And BIF tells your friends what the stick is when you give it to them.

Next, paint the sticks.

Lay them where they can dry and coat them liberally with the solution, keeping the BIF labelled end of the stick clean. Paint one side, let it dry and then paint the other, and let it dry.

When they are done drying, you can store them in a dry cool place for several months. You must protect them from sunlight!

Making BIF sticks
Making BIF sticks

A mosquito trap full of wrigglers

How do we know mosquito traps attract mosquitoes and kill the adults and larvae?

Wrigglers taken from the methoprene mosquito trap
Wrigglers in my mosquito trap

These wrigglers were found on 1/5/16 in a mosquito trap I set 26 days earlier. There were hundreds of wrigglers and pupae!

This trap used a bifenthrin landing strip and Altosid (methoprene) in the water bath – see the Advanced Mosquito Trap. Methoprene is a hormone that stops larvae turning into adults.  They just die.

So, if the mosquitoes are laying eggs in a trap made with Altosid, it should end up with a lot of wrigglers and pupae in it, including lots of dead ones. And that’s what I found!

Dead pupa from mosquito trap
Pupa found in the mosquito trap. It was dead.

The final test was to put some wrigglers in jars, with bug netting over them, and see if they hatch. I’ve done this before many times with larvae collected from an untreated papaya trunk (I do evil experiments on them). The hatch rate is pretty high.

After several days, none of the Altosid treated larvae hatched into adults. (Pupae typically molt into adults within two days). But there are dead larvae and pupae floating on the surface. Fabulous!

This shows the females do visit the mosquito traps to lay eggs, and that the water bath made with Altosid does prevent adults from forming.  Kill dem mosquitoes dead!

What we still don’t have direct evidence of, is that the landing strip kills the adult mosquitoes that land on it.  Until we come up with an experiment to test that, we just have to take the word of the researchers in the scientific literature, that it does kill them. After all, they were right about the methoprene.

For now, I’m really happy that the mosquitoes visit our traps, and their offspring die.

The Four Month Trap

Today I’m publishing directions for making a trap that does not need to be refreshed for four months.  To me, this is a big deal.  I’m managing about 25 traps, and the number keeps growing!

It’s an experimental trap, based on research papers that have studied longer lasting chemicals – bifenthrin and spinosad.  Bifenthrin is another pyrethroid (man-made pyrethrin, which is a chemical produced by chrysanthemum plants), and spinosad, which is a chemical produced by bacteria in soil and which is approved for use in organic gardening.

If you make one of these traps, you will only need to check it every few weeks, mostly to see that the water has not evaporated, and that no animals have knocked it over or climbed in.

Please send feedback if you build these traps, to


Mosquitoes are not the only vectors

There is another funny thing about dengue fever.  Everyone says that mosquitoes are the “vector”. But it was not a mosquito that brought dengue fever to Hawaii.  Or transferred it from the Kona side to Puna. We are the vectors!  Do you suppose mosquitoes get splitting headaches and aching joints from the virus? Do they call it “proboscis breaker”?

Once you look at it like this, “vector control” takes on a new meaning.  You know, only about one in four(1) people who get it become sick enough that they seek medical help, but they may still be infectious.

These unknown vectors form a pool of infected people who might eventually make dengue endemic here.  So how to minimize that pool?  During this epidemic, anyone with symptoms fitting the typical dengue infection – fever, aching joints, etc – should be provided with a free medical checkup if they are uninsured.  This is cheap – a few dozen uninsured people a month at a cost of a few hundred dollars for a visit to a clinic or urgent care, and some lab tests.  Compare that with the economic impact of dengue becoming endemic.