There are few things as discouraging as raising a crop only to have it taken over by tiny insects that you can’t seem to control.
We have a few critters making an impact on our greenhouse right now; spider mites, thrips and new this week: flea beetles. But today I’m focusing on aphids—a pest that we most recently turned the corner on.
I find that we get three kinds of aphids in our greenhouse. Foxglove aphids, green peach aphids, and most recently, melon aphids. We’ve been fighting with these aphids for a few months now, and spent a lot of labour shaking them out of our lettuce while that battle was ongoing.
Of course there are a number of synthetic chemical sprays that will kill aphids, but the natural controls available are so good that I cannot imagine why one would not use natural pest control.
There are some soft chemical controls for aphids as well. Water with 2% insecticidal soap, for example, works decently well on many plants (soap can burn some plants, so always try this on a small area before doing everything). You have to get the aphid quite wet for this to work, so make sure you have a fine mist and that you hit the underside as well as the leaf tops.
There are also some fungi species that can be applied onto aphid outbreaks. I’ve had some success with one fungi spray called botaniguard, but nothing as good as using wasps.
The best control out there, in my opinion, is parasitic wasps. We use two kinds, depending on the type of aphid. Ervi and Colemari. When I say wasp, I am not talking about the yellow jackets that stung you as a child. I’m talking about very tiny insects–in the same size range as a fruit fly–and which do not have stingers that can harm you.
These wasps land on the aphids, lay an egg on it and leave. The egg develops, and the resulting juvenile parasitic wasp begins to grow inside the aphid, eating it from the inside out. It then pupates inside the hollowed-out wasp body, eventually bursting out as an adult which can lay more eggs if there are still aphids around.
The wasps are only effective if you can identify the type of aphid you want to control. (Here is a video outlining how to identify aphids and some info on controlling them.)
Using the parasitic wasps takes some patience. At the beginning you will only find a few aphids that are parasitized. But every week there will be a few more. Then after about 5 weeks, you will have about half the wasps parasitized and at that point you are on the home stretch. Parasitic wasps are one of the few natural control systems that I use where I expect perfect control–although this takes some time.
The big problem comes with the lettuce. Not only is the lettuce full of tiny ridges and places to hide (so the wasps seem to have more trouble) but we only take 6 weeks to grow the lettuce from seed to harvest, so when we get aphids on them, we are often harvesting aphids that are in some part of the process of being parasitized, so they are not around long enough to fully develop and let the aphids out. Also, the wasps do attach the dead aphid mummy to the leaf so that you have to pick it off or risk having customers complain about the bugs on their lettuce.
Biologically, a good answer is predatory midges. We do use them occasionally when we have aphids on some crops, but they are not great for lettuce because customers tend to be even more squeamish about the bright orange maggots that they produce than they are with aphid mummies.
Our go-to with aphid control on lettuce is first wasps which we introduce for a week to make sure they get established, then we throw in a pile of
lady bugs (or lady bird beetles, depending on where you’re from). The lady bugs will eat the aphids, both parasitized and not (though not as well as the midges do). The big advantage is that customers don’t mind lady bugs (well, not adult lady bugs at least). The lady bugs will set back the parasitic wasps quite a bit, but I find that the wasps will eventually win the battle and we end up not having to be paranoid that we might have missed removing a orange worm from the lettuce we’re shipping.
On fruiting crops like eggplant and tomatoes, I would never dream of using ladybugs—I don’t want any chance of setting the wasps back in their task. I just release the appropriate wasp and count the carnage.
Recently we’ve introduced banker plants for aphids in our greenhouse. We grow barley grass and raise grass aphids on it. Then we can introduce colemani parasitic wasps onto the aphids growing on the grass. Unfortunately there is no banker system developed for ervi yet, but I’m hoping someone figures one out in the near future.
In the house, many people might be squeamish about using beneficial insects to control pests. Soft chemicals (like soap) and biological fungi are possible answers. But if you can handle having tiny, non-stinging wasps around, nothing works as well. Whichever way you go, hit them early. Aphids spread very quickly and can do a lot of damage in a short time.