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Mosquitoes and Seasonal Temperature Shifts: Backpacking the High Sierra. | High Sierra Backpacker

Mosquitoes and Seasonal Temperature Shifts: Backpacking the High Sierra.

Alex Wierbinski's picture

By Alex Wierbinski - Posted on 20 April 2018


Skeeter Video One
Attack in the Emigrant Basin, Emigrant Wilderness


  Backpacking the High Sierras 



2017-2018 News of Note


Mosquitoes and Seasonal Temperature Shifts

Two important things about the changing seasons in the High Sierras
    Two deeply interrelated realities strike me as being very important to average backpackers about the seasonal temperature changes in the High Sierras. The first concerns shifting your gear with the changing temps. The second is the High Sierra Mosquito Cycle.

Both shifts follow the annual temperature rise in Spring to Summer temps, and the subsequent drops in temp at the end of Summer to Fall and Winter conditions.

The most critical points in this annual cycle are when the emergence of Spring Temps out of Winter allows us to lighten up our gear, and when the onset of Fall into Winter demands increaslingly heavier gear.

For Summertime Backpackers, and especially PCT & JMT hikers tracking the shifts from Winter to Spring temps, and finally to Summer conditions, are vitally important to ascertain when the mountain passes clear of snow, when the rivers become fordable, when the trails have stopped flowing like creeks, finally drying out and hardening up.

The character of each year's transition will determine our early season gear requirements, of which our clothing selection for mosquito protection, as well as cold and wet conditions,  will be a significant consideration in our final selections.

Each of these seasonal steps of rising temps releasing greater amounts of water also marks points in the annual rise and fall of mosquito populations. The greater the heat and moisture available with the advent of the Spring Thaw, the more mosquitoes we will experience. How long the skeeters last through Summer is determined by how much water they started with, this year's wind, sun, and temps through Spring and across Summer, and especially how much more moisture is supplemented by Summer storms. The balance of these three factors determines each year's unique mosquito profile.

We are currently tracking these changing conditions through the Backpacker Alerts, and earlier, through Tracking the Spring Thaw reports. Please post reports from your early-season experiences there.

Each year's "early" is different!

 Factors Always at Play

Of much more importance to Summertime Backpackers, if we actually experience Summer conditions during any particular year, is the trajectory of the High Sierra Mosquito Cycle. That we are here is indicative it is again that time of year to pull out and feature information about High Sierra Mosquitoes.

These efforts began by specifically tracking mosquites each year, using myself as the test dummy. We've focused on tracking and identifying the weather patterns and snopack status predicting mosquito population trajectories. We use the Backpacker Alerts to follow and identify points along each season's specific trajectory.

The most fundamental pattern is that mosquito populations are tied to the availability of the interrelated sources of water and heat. Without sufficient water and heat mosquitoes populaitons are lighter and die more quickly even than during seasons with abundant heat and water. The Summer of 2015 was just such a mega-drought year, with minimal skeeter populations incapable of making it through Summer.

During the past few years the Sierra has experienced wild and wide swings in its weather. The phenomonal wet year of 2010-11 has been bracketed by a series of profoundly dry years, finally being crowned by the record precipitation Winter of 2017. 2015 provided us with an example of a season with minimal snow, water and mosquitoes, while 2017 provided record precipitation to the Sierra fueling heavy skeeter populations through the whole Summer from the Spring Thaw until the first Fall freezes finally killed them off.

Mosquitoes need water, and these wet and dry years serve to provide us with the "bookends" of mosquito population possibilities, from very low populations during dry years to huge populations during wet. And, these radical extremes happend over a three year period...

The bottom line is that the Sierra rarely has mosquito problems when it has no water (it has other problems!), and has lots of mosquitoes when it has lots of water. Nonetheless, we will only underestimate the mosquitoes at great risk to our comfort and safety, even during dry years.
Always carring basic mosquito protection gear is mandatory.

Every year has a sweet spot for the mosquitoes, when their populations peak. Some are very short, others very long.

Tracking the Weather and Snopack
are vital to identifying each year's specific character.


Evolution of the Weather in 2018: High Sierra Backpacker's Calendar


The critical period of mosquito innudation will be shorter in duration and they will have less "density" during dry years, but there will still be a "sweet" spot during early Summer when the skeeters will dominate the Sierra even during the driest years.

Your observations and scouting reports are vital to our understanding. Post your observations about Trail and Fording conditions on the Spring Thaw page, and post your updates about mosquito conditions here.



A Very Wet Year

The Season so Far 
<I am leaving this 2011 discussion of conditions here because it describes the mosquito population trajectory during a very wet year. 2017 now offers us another example of a very wet year. 2015 was the driest year over the span of human experience in the Sierra.>


May 22: We have had a VERY heavy Winter snow pack, and this deep snow has been supplemented by some late-season snow storm activity in May.

June: Series of Storms add to heavy Winter snowpack, including an incredibly powerful storm that brough snow in late June.

July: Amazing deep snow pack still sitting high on the Sierra Crest, deadly fording conditions, and incredibly wet conditions generally. 

Mid-July: Classic early Spring Conditions. Deep wet snow at elevation, deep water flowing at all major river fords. The white-water intense flows appear to have receded, but deep water is common in all main drainages. Conditions on the terrain itself are very wet, from the soft snow at high elevation, through the saturated meadows below the snow line, to the soils in shaded areas at lower elevations. All of the terrains except the wet snows at the highest elevations are harboring dense populations of mosquitoes.

Mid-July 2011 Analysis

I can now say that we are having as heavy of a Spring, even a bit heavier, than we did last year. We are experiencing both an ultra-wet and a very delayed Spring Thaw in terms of snow and the subsequent mosquito populations that follow the thaw. 

The late season storms and the huge amount of Spring Snow in the mountains has delayed the mosquito population explosion. The heavy Spring Thaw and runoff are being complimented by heavy mosquito populations following the thaw upmountain.

The very snows that delayed the mosquito population are now providing the very moisture that fuels their population explosion, The huge amount of snow still sitting on the Sierra Crest so late in the season will provide the mositure that will sustain incredibly high mosquito populations through the middle of August.

When this much water is warmed by this much mid-Summer heat, extremely dense and long duration mosquito populations are the result. 

  This is because such a heavy snowpack so deep into the Summer translates into huge amounts of moisture avaiable to fuel and sustain heavy mosquito populations much deeper into Summer than normal.   

This level of snowpack in mid-July assures that the heavyiest period of mosquitos this year will be from the third week of July until the second week of August, in my opinion. Normally, at least for the last 8 years, the heaviest mosquito period has been in late May through late June.

Drying-Warming Pattern in Motion over Time
The stregenthing pattern of weaker Winters and earlier and earlier Spring Thaws that has been establishing itself during the last decade has been completely altered during the past two years by heavy Spring snows falling on top of deep Winter snowpacks.

Prior to the last decade Spring Thaw would typically allow access to the mountains by mid to late June, depending on the season. During the last decade Sierra access has been possible as early as late May.

Wet Year among Dry
The dry Winter and Spring conditions ended last year (2016), and 2017 provided an even heavier Winter Snowpack accentuated by heavy Spring snowstorms even later into Spring. 

For Summer backpackers this means that early access to the mountains was blocked by snow, the Sierra's "normal" opening date of July 4 only offered snow-free access to low elevations which were already deeply inundated by mosquitoes, and the majority of this already seriously shortened Summer backpacking season is going to feature heavy mosquitoes, and heavy backpacker traffic.

  We are, and will be experiencing extra-heavy mosquito populations again during the Summer of 2017, and these conditions will be sustained even later into Summer than last year, which was a very heavy mosquito season.

None of this means anything to the properly prepped backpacker. We can deal with swimming in mosquitoes. You can deal with the little buggers on the trail and in camp with a little forethought, the right setup of gear and chemicals, and a reasoned approach.

Check out Tracking the 2011 Spring Thaw

The Wet Bottom Line
The Bottom-Line is that we can expect heavy skeeters through August during a
Heavy Snowpack Year.


A Very Dry Year

We have been experienceing an unprecedented drought in the High Sierra from after the heavy snow year of 2011 until the opening of the brutally dry 2015 Summer Backpacking Season.

I had not yet begun recording the trajectory of seasonal weather on the annual backpacking 2015 calendar, but we are now in 2018!

Unlike the complexity of a snowy Spring as seen above during 2011, the Spring of 2015 has brought a much weaker level of the "unsettled" weather which typically characterizes the transition of Spring to Summer Conditions in the Sierra Nevada Mountains.

We have been seeing the remnants of tropical moisture sucked North and East into the Sierra Nevada  from the Central Pacific by a series of low pressure zones over the West and Southwest of US, bringing light rains, rather than the typical Spring snowstorms coming out of the Northwest from the Gulf of Alaska.

End of an Era
The days of Winter Storms out of the Northwest are slowly coming to a close, which means the era of Winter snowpacks in the Sierra is coming to a close. If I am correct this is going to significantly drive down mosquito populations into the future.

The bottom line is the size of the Winter snowpack, the temps, and the "storminess" of the Spring to Summer transition determine the unique level of annual moisture available for mosquito propagation.
During 2015 the feeble snowpack created a weak thaw, which offered water sufficient for a small percentage of mosquito populations during an average snopack year. The period of intense mosquito activity and insane densities following the thaw line will be significantly shortened, and of significantly lower intensity across the Summer of 2015. The lack of snow will limit how many mosquitoes rise out of these boggy conditions saturating meadows, and how long these "mosquito machines" will be "in production." The truncated Winter this year assures mosquito populations will be much lower than normal, and die out much earlier than the "normal" drop-dead date of August 15. Significant skeeter populations will be lucky to make it to late July this year.

The Dry Bottom Line
The Bottom-Line is that we can expect heavy skeeters through June during a
Ultra-Light Snowpack Year.

In-between these extremes of wet and dry we find that the "average" snowpack year in the Sierra supports heavy Skeeters through July, with populations mercifully tapering-off through August, to their "average" drop-dead date as mountain meadow soils finally dry-out around August 15.

Each Summer's unique Storm and T-Storm activity will alter the trajectory of each Summer's drying and associated mosquito population levels.



#1> The High Sierra Mosquito Cycle
         Of first importance for Summer time backpackers is the mosquito cycle. The mosquitoes follow the melting snow. As soon as the snow melts, there are mosquitoes. Lots of mosquitoes. As the snow melts higher up the mountain, the mass of mosquitoes follows the melt up the mountain. As soon as enough ground is exposed, a critical mass is achieved, and from that time until the ground once again dries out a bit, the mosquitoes literally dominate the physical environment.

The "Average" Year
During an "average" season the high trails are generally snow free at some point in time between the end of June to mid-July. As the snows clear, mosquito populations continue to rise as high altitude meadows clear of snow and fill with water. By the middle of August the Sierra and its meadows are generally dried out, and mosquitoes are limited to diminishing zones of  moist terrain. This is just a general, "average" timeline which differs with each season's specific weather, with each year's differing heat and precipitation profiles. 

Dry Year
2015 Season
A very brief period of Spring Thaw and Spring Saturation will limit mosquito numbers and duration this year.

Wet Year(s)
2010-11 Season
(added: 2017)
2010 was a massive mosquito year. Heavy Spring snows maintained moisture deep into Summer, perpetuating very heavy mosquito populations through the middle of August. The Summer of 2011 was a banner year for mosquitoes.

Every Year
Serious measures are required to tolerate the mosquitoes every year. I have seen people literally crying in frustration and pain while stumbling down trail from inadequate mosquito protection.

    Crying Backpacker
Hiking up towards Carson Gap from Round Lake I could hear the sounds of muffled sobbing, frustration, and anger coming down the trail towards me. Listening carefully, I determined that it was not injury, but likely blisters, rash, or mosquitoes torturing the backpacker I was hearing, but had not yet seen. In a moment the backpacker came down stumbling-trotting down the trail, waving hands around face, running as fast as possible from the cloud of mosquitoes engulfing/pursuing her.
No problem. I can fix that.
I stopped one unfortunate backpacker being eaten alive by mosquitoes, calmed her down, DEETed her up, gave her proper directions, the exact mileage, and the estimated time to her trailhead. Reasonably recovered, she headed out, steeled against the onslaught.

Don't underestimate the mosquitoes.

High Emigrant Wilderness under FULL mosquito ATTACK. Tahoe to Whitney.

above: Under Full Mosquito Attack, late Spring, High Emigrant Wilderness. Note the mosquitoes visible on the net! I ate, drank, and breathed mosquitoes at one time or another during this trip. Note the navy peacoat!

Travel Measures against mosquitoes

      You must have the proper protection to hike through the mosquito high season in comfort. Walking into a light breeze is generally sufficient to out-run the mosquitoes every now and then, if you are so lucky.

On calm days, or when walking with the wind, the mosquitoes can and will follow you, and bag on you, for miles. Do not unnecessarily brush grasses, bushes, or pine boughs as you pass by. Mosquitoes bed down on the shade side of all three during the day, and brushing the foliage as you hike by will bring them out.

Mosquitoes do not like the sun. During bright days you are generally OK. But passing clouds and passing through shaded meadows and forests brings them out on even the brightest days. Be ready to quickly deploy the sleeves, pant legs, or the DEET on the trail as the situation and your preferences require.

The basics

The travel basics against mosquitoes are a combination of clothing and chemical protections.


A Long Sleeve shirt that is mosquito-proof is helpful. It should be loose fitting with a tight fabric weave sufficient to resist mosquito bites. Remember, you also need to keep cool, so a loose fitting shirt will ventilate better. Your long-sleeved shirt should also have a collar stiff enough to secure the bottom of your head net.

Long Pants that are mosquito-proof, and long enough to be tucked into your socks.
Zip to Shorts pants work really well.


On the Trail Guide
PCT hiker wearing SKEETER PANTS

Keep them off my head and bald spot.

I do not recommend gloves while hiking. DEETing the backs of your hands is generally sufficient. 


2017-2018 News of Note

The only effective insecticide. A non-toxic highly effective insecticide is in the pipeline, but it is not yet on the market.(Isolongifolenone) Deet will protect all exposed skin not covered by your mosquito-proof clothing. As per the label warning, do not put clothes on over skin you have DEETed.

  The measures above also protect against the Golden Biting Flies in Northern Yosemite, which are little bastards all Summer long. Well, their populations do fade late in the season, at the end of Summer. In any case, you will never forget their bite. The hair on the back of my neck tightens up just thinking about it.

I don't pre-emptively kill anything, except Yosemite's Golden Biting Flies. When they start hunting me, I start hunting them back. Everything else has to bite me first, except for golden biting flies. When I see one coming for me, I try to kill it.

Mosquitoes are stupid. They crave blood. Their specialty is the Kamakazi Plung. Golden Biting Flys are smart. They stalk you with the goal of sneaking up on your softest exposed skin from behind, and laying a piercing bite into your most sensitive parts.

I hate the little bastards. Which means that I fear their bite, which is likely correct.

I have had some real adventures with these guys, as they attempt to stalk me down the trail. They are generally targeting the salt around the back of my armpit between the pack and body. This is the hardest place for me to defend. They try to come in from angles where you cannot see them. They also target the back and sides of my neck.

When they come in, it is not to collect salt, it is to bite. Not just any ordinary bite, but a pinching, shrieking, piercing, frkn bite. To say I don't like it is an understatement.

I've experienced many degrees of discomfort. But there is something about these frkn little bastard's bite that really pisses me off. I will kill them when they begin tracking me.

No mercy to the Golden biting Flys.

The smart ones I encounter on the trail figure that I'm not worth the risk. The stupid or daring ones and I battle, pitting their lives against my peace.

  I have tried everything to deter Yosemite's Golden Biting Flys. Most compounds work poorly. Only DEET is reliable. Even when they penetrate my defences, which always happens, DEET on the neck deters a brutal bite.

I bring no other insecticide except 100% DEET into the mountains with me. That's my experience. What's yours?


Recent Scientific Research on DEET

A fascinating array of scientific papers and research.

Impact of insecticides on the cognitive development of 6-year-old children,
INSERM, July 10, 2015 

For a very informative article on the various insect repellents,
see Insect Repellent Active Ingredients on Wildbackpacker (removed-not found).

Skin-Applied Repellent Ingredients,

Camp Measures against mosquitoes

 Clothing and Gear 
    When you make camp the mosquitoes will zero in on you. You must have the proper mosquito gear to be comfortable in camp during Spring's full mosquito inundation . I prefer to use the proper clothing and gear protection in camp rather than DEET.

  A full coverage tent is mandatory. Unless you want to be eaten alive all night long. Be my guest! There is no question about putting up my tent during mosquito season. I don't use the fly cover unless weather threatens. Not only do I put up my tent, but I also have an entry system that scrapes most of the mosquitoes off my clothes when I enter the tent.

I only open the door zipper a crack, then sit down into the cracked zipper, pushing it open with my weight. My body falling through the unzipping door scrapes all the mosquitoes off my clothes. I then flip my legs into the tent, quickly zip the door, and then begin the hunt for all the skeeters that did get into the tent.

  If your tent is not securely set up, or your zippers are weak, be careful about using my skeeter-scraper technic of sitting into a partially zipped door. You could damage your tent, break your zipper, or just bring it down. 

  Before entering the tent I brush all the mosquitoes off my clothes that I can. Remember, each time you enter the tent you must take the time to kill off all the mosquitoes that entered with you. Or they will eat you. And search well. Mosquitoes know when they are being hunted, and they will try to hide. 

Mosquito Netting: I am talking about the hat-brim to shirt-collar nets that protect your head. Though these nets restrict vision, they are a welcome relief from the constant use of DEET in camp or on the trail. 

Gloves: I use gloves against the mosquitoes in camp.

    Properly set-up, your clothes will protect you from mosquitoes in camp and on the trail. The only problem is during heat waves, when you really don't want to wear long pants and long sleeves in camp or on the trail. In that case you can use DEET in camp, but you will have to wash it off before you get into your sleeping bag, and I prefer to wash up when I make camp, rather than after dark when it has cooled off and all the mosquitoes are out. 

From Trail to Camp 
  If I am rolling down the trail wearing shorts and a tank top while completely DEETed out, I will have to make the transition to long pants and long sleeves for warmth and mosquito protection in camp. That's another reason I carry my lightweight plastic water jug. Remote baths.

Best Backpacking Water System

I fill my jug up and walk a safe distance from the water source, and sit on a rock for my de-DEETing. I rinse off all DEETed skin with water. Then I am ready to put on my camp clothes.

  Don't wash chemicals, be it DEET, suntan lotion, or moisturizer off anywhere near the water source. If you want to swim, wash the chemicals off first, (including suntan lotion) then jump in! DEET is already contaminating American Streams.

Safe Application of DEET
I only apply DEET to the backs of my hands, and then rub the DEET from the backs of my hands onto my arms, face, legs, and neck.

 Never apply DEET in a location where it will run into your eyes, mouth, or membranes when you sweat. I never apply DEET to the fronts of my hands, as I want to prevent contamination of my food and water. By keeping my hands DEET-free, I am able to avoid eating and drinking it.  

The Heat and the DEET
Hot mosquito-filled early Summer days demand light clothing and lots of DEET. In 2009 the high mosquito season in the Sierras spanned the mid July heat wave that brought temps up to the low 90s at 8000+ feet in the Northern Sierras. Thus I was hiking in shorts and a tank top, slathered with DEET.

But you must make your own hot weather decision. You can wear long pants and long sleeves to protect yourself against mosquitoes, and sweat like a pig. Or you can use DEET, and wear shorts and a tank top to keep cool in the heat. In that case you will still become a mosquito target as you sweat the DEET off. And when the DEET is on you are subject to becoming a victim of your own self-induced chemical warfare.

For a very informative article on the various insect repellents,
see, Skin-Applied Repellent Ingredients,

The Annual End of the Mosquitos
  The mosquito domination begins to seriously diminish at the beginning of August as the ground dries out. By August 15 the mosquitoes have generally receded into a minor irritation. This will be different for different parts of the Sierras, and differ with each season's weather variations. The overall decline of the mosquitoes is dependent on the each season's overall weather conditions and character, but local weather, drainage and soil moisture levels will determine how these factors play out in each location you encounter.

  Wet meadows will hold mosquitoes in any month. Especially bad are the High Sierra granite basins holding lakes and meadows. These granitic basins tend to hold wet soils late into the Summer. A good example is the Emigrant Basin in Emigrant Wilderness. This bowl tends to hold moisture and mosquitoes thicker and longer than well-drained areas. 

  Once things dry out and the mosquitoes decline, tents are no longer required for protection, shorts and the tank top can be worn without chemical protection, and your mosquito net can be stowed until next Spring. These are the glories of late-season backpacking in the High Sierras. But in late or early Summer backpacking, be sure that the temps will not unexpectedly drop to Winter levels, or be prepared.

#2>The Warming Weather
  This leads to the  Second Important thing about Seasonal Change in the Sierras: The basic seasonal temperature changes. It is these changes which drive the mosquito populations, and it is these changes which should dominate your gear selection.

The Temp changes from Winter to Spring, and then Spring to Summer are important because they determine the weight of your gear selection. The most important changes concern when temperatures rise from Winter levels during Spring, and when they again drop in Fall.

  It is important to observe these changes accurately, as you do not want to get caught in an early Winter storm in your mid-weight Fall and Spring gear when your heavy Winter gear is required. 

Check out the Gear and Weather sections of the trail guide. As of today, Veterans Day, 2010, both sections, as well as the rest of the guide are under construction.

I hope you find this informative, and come back to check on the trail guide and this Backpacker's Forum as they grow. Feel free to ask questions, add your own experiences, and post your comments through the comments link below.

Register to post your own pages on this, and the other Backpacker's Forums.



Recent Scientific Research on DEET


 Pyrethroid Danger for Kids
Impact of insecticides on the cognitive development of 6-year-old children,
INSERM, July 10, 2015 



Skeeter Video Two
Under Attack at Stubblefield Canyon, Yosemite.

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