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Mosquitoes and Seasonal Temperature Shifts: Backpacking the High Sierra.

Alex Wierbinski's picture

By Alex Wierbinski - Posted on 28 February 2010

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Mosquitoes and Seasonal Temperature Shifts: Backpacking the High Sierras 


Two important things about the changing seasons in the High Sierras

    Two things 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 light gear, and when the onset of Winter out of Fall demands heavy gear.

For Summertime Backpackers 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.

We are tracking these changing conditions in the Tracking the Spring Thaw report. Please post reports from your early 2001 experiences here. This year "early" is during mid-July!

 Of much more importance to Summertime Backpackers, if we actually experience Summer conditions this year, is the trajectory of the High Sierra Mosquito Cycle. So it is again the time of year to pull out and feature this article about High Sierra Mosquitoes.

The bad news is that the mosquitoes are going to be a heavier and last longer than last year's incredibly heavy mosquito season.

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



The Season so Far

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.

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.

The dry Winter and Spring conditions ended last year, and this year has provided an even heavier Winter Snowpack accentuated by heavy Spring snowstorms even later in 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 this year, 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



#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. As the snow melts higher up the mountain, the mosquitoes follow 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, the mosquitoes literally dominate the physical environment.

During an "average" season trails are generally snow free from 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 Sierras are generally dried out, and mosquitoes are limited to diminishing zones of  moist terrain. This is just a general time line which differs with each season's specific weather. 

2010 was a massive mosquito year. Heavy Spring snows maintained moisture deep into Summer, perpetuating very heavy mosquito populations through the middle of August. 2010 was a banner year for mosquitoes.

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.

    I stopped one unfortunate backpacker, 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, 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.

Hat: Keep them off my head and bald spot.

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


DEET: 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 until 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 thinging 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, 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.


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.

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.

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.

For a very informative article on the various insect repellents, see Insect Repellent Active Ingredients on Wildbackpacker.


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I went for a day hike up to Twin Lakes, in Crystal Basin off Hwy. 50, on 7/26/11. The mosquitoes were bad lower down in the wooded or marshy areas but once up on the granite near twin lakes they were not too bad.
Repel 100 is only thing I've ever found that works in the Sierra Nevada, at least in my experience. The squeeeters are unforgiving and so should be your protection! Getting ready to head up there soon, how wet and snowed in is the Marsh area?? Bad enough to have to skirt the edge at base to the south to get to Twin lakes or is trail under water? I want to get to the falls and Island Lake above Twin Lake.
Alex Wierbinski's picture

Hey Eric, Bridgeport's twin lakes? Naw, must another of the Twin lakes. Peeler lake is above Bridgeport's Twin Lakes.

I agree about the DEET. I use 100% too, but they frkn love me and my blood. Some people do not attract them as powerfully, and could get by with a weaker solution.

Give me another reference to your trailhead, and I'll see what info I can get to you.


Alex Wierbinski
Nope..Twin Lakes in Crystal Basin, I know that area in Bridgeport too, Peeler Lake is a nice destination :) I used to guide into Hoover Wilderness down to 20 Lakes Basin. I'm going to get up into Twin and Island Lakes within the week I'm hoping anyway.
Alex Wierbinski's picture

I was raking my brains! I should have known. Sweet. I love looking at the range from Round Top...It just frkn glows in twilight.

I'll bet the ground moisture is way up down low, even sloppy, even if the creeks have gone down. Recent reports have indicated few skeeters up high due to the continuing snow, but the thickness of low skeeters must be moving up the mountain by now.

And at this point in the season the skeeters are usually in full retreat! Not this year.

The 100% solution would be wise even for people with icewater in their veins.

Shoot me some insights when you get back!


Alex Wierbinski
Great information - thanks. I plan on hiking into Bishops Pass, south on the JMT, hitting Whitney, then out to Whitney Portal - in mid-September. Sounds like mosquitoes won't be a concern, which is great. Wondering how worried I should be about early snow?
This is an extremely helpful page. Thank you for sharing your wealth of information -- I really appreciate it. I'll be leaving August 21st to hike the JMT solo for the first time, and you cleraed up a lot of my concerns about mosquitos. SOunds like I'll be (thankfully) missing out on the worst the skeeters have to offer. Take care and travel on, my freind!
Alex Wierbinski's picture

Thanks dude. I'm really sorry I've not got the JMT sections of the trail guide up, but I'll tell you what I know.

If you want some ideas (campsites) and feedback on your hiking plan, I'd be glad to share some fine spots I know about.
I know real cool campsites at Donohue and Island Passes. Two great scrambles between TM and Reds.

I generally run into Fish Valley South of Red's-a facinating alternative to the JMT-but campsites at Selden Pass are top notch...

See why I'm itching to write the southern section of the guide? I can't wait to get there again soon, and dude, you're going to have an experience of a lifetime.

August 21 will put you past the mosquitoes, and with that date you don't have to concern yourself with the Spring Thaw & serious snow and fording issues: smart!

But...the risk of early dusting of snow really increases after Sept 1, and is really always a threat, so don't skimp on insulation, shell, or a proper tent.

Have a blast dude, and I'm here to help with resupply, hiking plans, campsites, alternative routes, and side trips.


Alex Wierbinski
Where I got the shit kicked out of me by mosquitoes. My sister and I were supposed to be at Lyons Lake for two night, but...there was no way. I have easily 100 bites on me. I have never experienced anything like it.
Alex Wierbinski's picture
Hey Dude, When did you and your sis hit the Desolation? How wet was it, ie, how do you see the heat drying out the terrain? Thanks for the comment, and keep us posted on your experiences!
Alex Wierbinski
We were there this week, Thursday July 15th and Friday the 16th. We hiked out Saturday morning. It is WET. No drying out yet and maybe not for weeks. The Lyons Lake/Sylvia Lake trail is blocked by patches of snow after the signpost for the turnoff to Lyons...and the trail to Sylvia is partly under a creek which is usually really low. It is wet wet wet. The wildflower display is off the charts, though. It's gorgeous. For a sort of neophyte like myself, it was tough. My sister is an environmental scientist who has worked in Alaska's outback for ten years. She said that it was as bad as AK if not worse.
Alex Wierbinski's picture
Hey, Thanks for the informative report. I cut and pasted it into the July open thread: I was supposed to leave for a 100 mile trip from Walker (Hwy 395) to Tuolumne Meadows, and I was prepaired for 'skeeters. But transportation problems have pushed back my departure. Thanks again for the info, Alex
Alex Wierbinski
Sure, no problem. Some people may have no trouble- mozzies apparently don't bite everyone- but they love me. Hope your trip is good!

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