Although I have never (in 20+ years of growing) seen an aphid on a lily growing in the ground here, I certainly have seen them occasionally on potted lilies. I do not consider them much of a threat at our location, but I realize there are many people who do consider them one of the worst pests of lilies.
Myself, I only feel threatened when they get into my roses - and that too is rare on those planted in the ground. I suspect Mother Nature has a good handle on controlling them under normal growing conditions as lady bugs are in abundance here, and the best natural control available. I also suspect the reason potted lilies are susceptible to attack is their proximity to the greenhouses and other potted plants.
To battle aphids, you should first have a little knowledge on how they work, and most importantly, what to watch for. There are a number of different types of aphids, the most common is the green aphid, shown on this page. The most dreaded of all aphids is the black aphid, for it is persistent and resistant to many chemicals and sprays. Black aphids reproduce very quickly, as do all aphids, and if you spot a few you must take care of them quickly or you will soon have an epidemic on your hands, and you will have the same problem the next year! Aphids usually appear by the end of May, especially after a period of cloudy, cool weather. Aphids like those conditions, and that is when they reproduce the most it seems. If you had just a few before the cloudy weather, give them a week of optimum conditions and they will infest your plants to the point where you will be ready to throw them out! At this time of year aphids cannot fly and concentrate on reproducing themselves. Later in the year, usually mid August around here, the aphids go into another cycle where they can fly, and this is definitely the time to kill them before they spread even further and settle for winter to repeat the cycle again for next year.
Aphids are feared by many lily growers as they are concerned about the transfer of virus from plant to plant, which can occur with aphids. Aphids are a sucking insect, inserting their piercing little mouths into the plant, then moving onto the next one. Example: Lily #1 has a virus, even though you cannot see any symptoms. Aphids settle on this infected lily for a few days, enjoying their steady meal and coating their mouths and piercing parts with plant juices that contain the transmittable virus. One day they decide it's time to move onto lily #2, which is perfectly healthy. First thing they do is sink their teeth into it, which immediately transfers the virus as the plant juices already on their mouths are absorbed by the new lily.
This picture illustrates clearly just how well they blend into the stems and leaves of plants, making them hard to spot - see the green of their body is exactly the same shade!
The first telltale sign of aphids is the little white 'fuzz'' that appears, usually at the tips of new growth and flower buds. The fuzz is actually the aphid midge (what it is in the first cycle of its life). If you can spot this stage early, you should be able to control them before they get totally out of hand.
It is often difficult to spot the midges when they first appear here, as they appear about the same time the poplar fuzz starts to fly outside, and it sticks to everything until we get a nice rain to wash it off. Close inspection is necessary, and observing the plant in question up close for a few minutes will show whether the fuzz moves or not - if it does, it is the midge rather than fuzz. If you can see the fuzz clearly as in this picture, you can be sure you have an established colony on your hands already.
Aphids prefer to be cool, and this is why you will find so many of them hiding under the leaves of your plants instead of on the surface.
Effective control of green aphids and a small amount of black aphids can be gained by using a product called END ALL, produced by the Safer's company.
BUY SAFER'S END-ALL ON AMAZON
You could also use a bio-control method, such as ordering lady bugs from the BUG LADY and let them loose in your garden. They will make a quick lunch of those nasty aphids for you! Years ago I worked at a daycare, and I would give the children jars and pay them a penny for every ladybug they caught and brought to me. I would take them home and let them loose in the greenhouse. What a fun way to spend play time for the children, and a cheap and effective control of aphids for me! Lady bugs are voracious eaters and very hungry when they wake up in spring.
If you find black aphids, cover your head and moan, then run for the department store to find a chemical pesticide called SEVIN. Staff at the Alberta Agricultural Research Station in Brooks, AB tell us it is the only pesticide available to the consumer which will be effective in controlling black aphids. Although not found on most store shelves any longer due to deregulation of so many pesticide chemicals, you can still find it at the local farm supply store such as UFA or Peavey Mart.
Please read the note below, added after this article was first published. It is correspondence between myself and a fellow regarding his experiences using Safers' Soap.
LEN G.: I just read your article on aphid control. I've used Safer's End All and have wound up with deformed leaves on the lilies, in particular the narrow-leave variety. (My lilies are all potted.) I've often seen, as well, the flower petals growing deformed from Safer's after they open up. (Whether or not the deformities are from the Safer's, I'm unsure, but it seems to happen to the lilies I use the Safer's on.) I have found an alternative to using Safer's. In roughly a litre of water in a spray bottle, I put in a tablespoon of dishwashing liquid then spray liberally on the leaves. After about a day or so, I take a different sprayer with just plain water and spray the leaves and buds off, washing the soap and dead aphids away. I repeat the process again in a few days after any eggs have hatched. No aphids or at the least very few that are left can easily be taken care of by using a small hobby paint brush to brush them away. It seems to work on the black ones, too. Be sure to do this process well after the warmest part of the day, preferably just prior to sunset, so the liquid will remain long enough to work. Another tip when looking for aphids: do it at night using a flashlight under the leaves. You will see any that are hiding under there. It works better than bending the leaves up and possibly causing stress to the plant.
Lynnette: Thanks for your email and your information. I haven't used it myself on any lilies so I wasn't aware it may damage them. I did know that the aphids themselves can cause new leaves to form deformed, without using any soap beforehand. With your permission, I would add your info to the article for others to benefit from. Thanks!
EDIT: Since this was first posted I have sprayed some lilies with Safer's End-All just to clean the leaves as it also does a wonderful job of shining up the foliage on many plants and I needed some really clean potted lilies for an event display. I did not notice any deformity after the fact so I'm wondering if something else, or perhaps too-strong of a mix was used. Especially considering John Rempel's steadfast recommendation to use it on lilies.