I have (had?) two hives in my backyard in NSW. Winter here has been pretty mild this year, and there have been plenty of bees moving in an out of my hives. Until sunday when I suddenly noticed there were no bees at all. On closer inspection there was a huge pile of dead bees in front of each hive. I opened the hives up, and there were no live bees at all, just a pile of dead bees an inch deep at the bottom of the hive.
I can’t see any obvious signs of disease ( a few SHB, and my last alcohol wash showed about 8 mites per hive), and I might be being a little simplistic but I wouldn’t expect disease to kill both hives at the same time?
Any thoughts? Could this be a pesticide/herbicide issue?
Sorry for your loss, Lee.
How long ago did you do the wash and did you treat?
We are in the unstable phase of varroa spread, so there are high levels of external Infestation from feral and unmanaged or poorly managed hives.
That looks very much like varroa collapse to me. I’m seeing it a lot and expect plenty more this spring.
There’s also many absconding colonies as they struggle with varroa. They are often incorrectly reported as swarms. In the heavily infested areas, I recommend washing at least every four weeks. I’m washing every time I open the hives this spring, so every 10days. It’s easier to knock down a low count than high. I’ve seen loads in the hundreds in several hives in our area.
Mike
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Thanks Mike
I washed about 10 weeks ago, and treated with formic pro.
I’m just surpised that both hives appear to have collapsed at the same time
Lee
Pre varroa, I would have suspected poisoning. Now, I’m regularly seeing varroa collapses of multiple hives.
Did you wash after the treatment? I’ve seen counts of 50 before treatment, then 30 within a week of removing treatment. The treatment worked, but the reinfestation is so high.
We’ve had such a mild winter, that bees forage at any opportunity and bring home more mites. We’re in for a rough time for up three years until it stabilises.
Mike
I agree with @aussiemike, the most likely cause is varroa, despite your treatment. The treatment was a little late in my experience. I do my last one around September in the northern hemisphere, which would be March for you. Then I repeat the counts and treat again if they are not below 2 mites per 100 bees.
So sorry for your loss. Varroa is a very steep learning curve 
Thanks Dawn
The DPI suggests washing at least every 16 weeks, so I was trying to avoid washing during Winter.
Thanks - next time I’ll wash more frequently. The problem is if this leads to more frequent treatment, there aren’t that many available in Australia to avoid repeated treatment with the same chemicals.
Thanks Mike
No, I didn’t wash after treatment - I was trying to avoid opening the hives during winter.
Lee
Good reason to do your last treatment in early autumn! 
Washing at 16weeks was more for initial detection as varroa spreads through the country. Once you have it, at least every four weeks is necessary. The levels explode exponentially during the unstable phase.
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So sorry for your loss, @lmsmith27, your pics are heartbreaking
There are more discussions on this forum but here’s one that includes a link to a YouTube video by Randy Oliver, an authority on varroa management:
I use oxalic acid sponges, made using Randy’s instructions. I place them on top of the frames early in spring. The bees walk on, chew and carry away OA-impregnated sponge bits over the the course of the season and in doing so, disperse OA throughout the hive. Very effective in my experience.
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Unfortunately, OA is not yet registered in Australia. ApiBioxal and AluenCap are going through the registration process with no indication of how much longer it will take.
Many beekeepers are illegally using OA anyway, of course. Caring for the bees comes first.
Amateur Beekeepers Australia(ABA) with support from the Australian Honey Bee Industry Council(AHBIC) and other amateur associations are lobbying for a New Zealand style “Own Use Exemption “ clause in our legislation to allow beekeepers to use unregistered or off label non-synthetic compounds to treat our own bees. This would allow us to embrace the latest research for better results without waiting for expensive and time consuming registration and label changes.
It would also allow clubs and associations to offer training in the safe, effective use of potentially dangerous compounds. An uphill battle against the behemoth bureaucracy for sure, but worth it I in my opinion.
Mike
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