I found these little black solid spots on the back on my strawberry leaves. They look like seeds but they are very soft.
What causes them, and should I do anything to treat them?
These are aphids whose lifestyle is to live off the plant juices in the leaves. They can be found almost everywhere during the growing season and prefer plants with soft leaves. You will rarely find them on plants with hard waxy leaves as the cuticle is too hard to bite through.
Control is easy with 5 ml dish soap to 1 litre of water. Agitate and spray the underside of the leaves. For plants with edible fruit you may wish to wait a few minutes and then wash the soap solution off with more water.
Repeat this at five to seven day intervals until control is achieved.
Going off of what Kevin said (I can't make comments yet :). I used to work in a greenhouse for a year in high school. There is another way other than dish soap and water as well as picking them off yourself which I used to do, however it depends on where you plants are.
You could release lady bugs as they love eating them. Lady bugs tend to get ridd of them rather rapidly, faster than the aphids reproducing anyways. However the downside is that if you have too many, they will start to eat your plants like the aphids. So ventilation definitely helps get them out if it's in an enclosed area.
Dish soap and water helps, just keep applying just like Kevin said every 5-7 days. Usually we did it every weekend and don't do too much as you don't want your plants absorbing soapy water. I used to take cotton swaps and wipe the areas where they were infested at, mostly under leaves and on the stems until they were gone.
@JStorage and any other passersby,
First I wanted to address "Organic": "Organic" is a somewhat flexible term. Science would define it as an object or substance with a carbon-based molecular structure, like a human or petroleum. The USDA/NOP/OMRI certification of "Organic" hinges on a laundry list of various arbitrary requirements and is more subjective. An apple is sill an organic object regardless of how it was grown, and I may be inclined to eat it either way. However, I would not chase it down with "organic" toilet cleaner under the assumption that it would be healthy somehow. Don't get too caught up in all the hype of labeling. Pay more attention to ingredients and keep things simple but logical. If you want to get old-school, call up a vendor like Ferilome or your local County Extension Office and have a chat. I have known producers of great organic products that did not go through all the hassle of obtaining official "organic" certification. A lot of it is smoke and semantics. And don't think for one minute that companies like Monsanto will ever stop trying to muddy the waters further.
As for the dish soap: For years people have recommended a dishsoap mixture to combat soft-bodied insects in the garden, but this tradition probably began when dishsoap was a more simple concoction. Nowadays, dishsoap can contain a wide variety of fragrances, lotions, antibacterial agents, etc. It's somewhat of a crapshoot whether or not the dishsoap will kill or damage your plants. People using the dishsoap method are actually trying to reproduce a commercial product referred to as "Insecticidal Soap." True "Insecticidal Soap", has a far more predictable ingredient list. It's active ingredient is generally the same across all brands, potassium salts of fatty acids. This stuff is super cheap, especially if you purchase the small bottle of concentrate and mix your own. It may even be cheaper than dishsoap.
So to answer JStorage's question, the homebrew dishsoap insecticide will only be "organic" if the dishsoap is "organic", but even then you still run the risk of killing your painstakingly tended crops.
Other popular "organic" insecticides include the bacteria known as BT, or bacillus thuringiensis, Neem oil, and even petroleum oil based products which effectively smother the insects to death. Neem oil is a favorite of mine as it also works on hard-bodied insects like scale, and it's said that the smell of the oil continues to deter insects after application. Oils can also supresses the spread of fungal spores.
For the bonus round, I try to stick to calcium, copper and sulfur for my dedicated fungicides.