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There’s no easy fix for aphids on chilli plants. Neem oil kills aphids, no argument there. The trouble is it also kills off the ladybirds and hoverflies that would keep them down for you. So I’ve stopped using it, and here’s what I do instead.
There’s no easy fix for aphids. I get asked about this just about every week, and I’m not going to pretend there’s a magic spray that sorts it in one go. What I can do is show you what actually works for me, and why the first thing most people try might actually be making it worse.
I’ll be straight with you. I’ve recommended neem oil in older videos, and I’ve used plenty of it myself. If you’re using it, I’m not here to tell you to stop. It works, it kills aphids, no argument there. Over time though I’ve moved away from it, and I want to explain why so you can make your own call.
These days I don’t try to spray my way out of an aphid problem. I’d rather set the greenhouse up so the insects that eat aphids do the job for me.
Table of Contents
What Neem Oil Does Well, and Where the Problem Begins
Neem oil comes from the seeds of the neem tree. It contains a compound called azadirachtin that messes with insect feeding, moulting, and reproduction, which is why it works on such a huge range of pests, aphids included. It’s organic, and it breaks down fast, often within a day or two in sunlight, so it doesn’t hang around in the environment.
Which all sounds great. The catch is that last selling point, the one that says it’s safe for beneficial insects. That one’s only half true.
The usual argument is that beneficials don’t eat leaves, so they’re fine. Ladybirds eat aphids, not foliage. And that’s true, up to a point. A ladybird walking across a treated leaf will probably be okay, because an adult beetle can handle a bit of incidental contact. But that’s not the whole picture. The real problem is what happens to their eggs and their larvae, and what happens when they eat treated prey.
Why Neem Oil Can Backfire in a Greenhouse
This is the part that changed my mind. When you look at what neem does to the beneficial insects, the numbers are grim.
A study in the Journal of Applied Entomology tested a few different neem formulations on beneficial insects. When the eggs themselves were sprayed:
- ladybird egg hatch rates dropped by more than 40 percent.
- green lacewing egg hatch rates dropped by nearly 28 percent.
The worst finding was about secondary exposure. Ladybird, lacewing and hoverfly larvae that ate aphids treated with neem died at rates as high as 100 percent at some concentrations. Every single larva. Separate research found around 80 percent mortality in treated ladybird eggs, with only about 13 percent making it through to adulthood.
Think about what that means in your own greenhouse. You spray neem to kill the aphids. A ladybird larva hatches nearby and starts eating those aphids, which is exactly what you want it to do. But the aphids have neem in them, so the larva takes it in through its food, and it dies. You’ve killed off the next generation of your own pest control.
I’ll be fair here. Used carefully, at the right dilution, at dusk when the beneficials are least active, neem can keep adult ladybird losses below about 5 percent. So it can be done more responsibly. The problem is what it adds up to over a season. Every application chips away at the beneficial population, and aphids breed far faster than their predators, so they’re the ones that bounce back first every time.
I’ve watched this happen for myself. In the seasons where I used neem more often, I saw fewer ladybirds and fewer hoverflies around the greenhouse. Once you’ve got those predators working for you, putting the neem down is the smarter move, and it’s the one I’ve made.
Why Aphids Are There in the First Place
Before you go treating anything, it helps to know why you’ve got aphids at all, because they don’t just turn up at random.
Aphids feed on sap, and what they’re really after is amino acids. The single biggest thing that pushes the amino acid content up in your plant’s sap is nitrogen. So if you’re feeding heavy on nitrogen, you’re basically laying on a feast for them. The research backs this up over and over: more nitrogen means more aphids, breeding faster and living longer.
Stressed plants get hit harder too. A plant that’s been underwatered or heat stressed while it’s being fed hard is about the worst combination going. And all that soft, sappy new growth that high nitrogen pushes out is exactly what aphids go for first.
There’s one more thing to check. If you’ve got ants about, they’re very likely farming the aphids and protecting them, because they feed on the honeydew the aphids produce. So the ants are actively keeping your infestation going.
So before I treat anything, I ask myself a few questions:
- Am I overfeeding nitrogen?
- Are the plants stressed from heat or watering?
- Is there a load of soft new growth for them to pile onto?
- Have I got ants farming them?
My First Line of Defence: Ladybirds
Ladybirds are still one of the best aphid predators you can get, and for good reason. A single adult can eat 50 to 60 aphids in a day, and somewhere in the region of 5,000 over its lifetime. I collect them whenever I spot them, bring them into the greenhouse and drop one straight onto a plant that’s covered in aphids. Come back a couple of hours later and they’re gone. It’s brilliant to watch.
They’ve got one catch though. They fly. In an open greenhouse on a warm day with the vents open, a lot of them will just leave. You might put 50 in and have five left a week later. Releasing them in the evening helps, because they’re less likely to take off straight away, but there’s no guarantee they’ll stick around.
So ladybirds are a good start, but they’re not the whole plan.
The More Reliable Greenhouse Ally: Hoverflies
Hoverflies don’t get anywhere near the attention ladybirds do, and honestly most people have no idea what they actually do for you.
The adults live on nectar and pollen, so they’re pollinating your plants while they’re at it. But the useful part is the larvae. Female hoverflies lay their eggs right in or next to an aphid colony, and when those larvae hatch they start eating straight away. One larva can get through up to 400 aphids over a week to ten days of feeding, and some species manage even more.
Trials in greenhouse crops back this up. On melons, hoverflies cut aphid numbers by anywhere from 54 to 99 percent; on strawberries, 50 to 70 percent, depending on the crop and conditions.
The big advantage under cover is that the larvae don’t wander. They stay put, right where the eggs were laid, on the plant where the aphids already are. That’s what makes them more dependable than ladybirds in an enclosed space. They don’t just fly off somewhere else.
Why I Grow French Marigolds Among Chillies
So how do you get hoverflies to show up and stay? This is where the marigolds come in. I grow French marigolds through my greenhouses and polytunnel, more of them this year than I ever have, and I do it for two reasons.
They Help Deter Certain Pests
French marigolds give off volatile compounds, including one that researchers at Newcastle University identified as limonene. Whiteflies can’t stand it, and in a greenhouse where the air is fairly still and those compounds stay concentrated, the marigolds put a few pests off.
They Feed the Adults That Produce the Predators
This is the bigger one. The flowers give adult hoverflies, ladybirds and parasitic wasps the nectar and pollen they need. The hoverflies come in for the flowers, spot the aphids on your chillies, and lay their eggs right there. That’s pest control that keeps feeding itself.
How to Plant Marigolds for Better Aphid Control
Where you put them matters. I don’t just grow a few marigolds off in a corner and hope. I get them in among the chillies, so the nectar is sitting right next to the problem. That’s what makes a hoverfly settle on the right plants and lay where the aphids already are.
Here’s roughly how I do it:
- In the greenhouses I pot the marigolds up and stand them on the benches, right alongside the chilli pots.
- In the polytunnel I’ve got them in the raised beds and dotted among the pots as well.
- If you’re only growing a handful of plants, you can be even more targeted, one marigold to every two or three chillies.
- Go for single bloom or open centred varieties rather than the tightly doubled ones. The doubles look lovely, but the insects can’t get at the pollen.
It sounds almost too simple to make a difference, but it’s one of the best changes I’ve made.
The Counterintuitive Rule: Do Not Wipe Out Every Aphid
Here’s the bit that feels wrong.
Don’t try to kill every last aphid. If you clear the lot, you’ve just taken away the food supply for your ladybirds and hoverflies, and they’ll either leave or starve. Then the next time aphids blow in, and they will, because they travel on the wind, you’ve got no predators left in place to deal with them.
I had exactly this last season. The ladybirds in my germination area were so good at their job there were no aphids left, and I actually started worrying about what they were going to eat. That’s the right instinct. A few aphids left alone is the price of keeping an army of predators around.
So the goal was never zero aphids. It’s a setup where they never get the upper hand.
A Better Aphid Strategy for Chillies Under Cover
If I boil the whole thing down, it looks like this:
- Ease off the things that draw aphids in, especially heavy nitrogen and stressed plants.
- Watch for ants, and deal with them if they’re protecting the aphids.
- Use ladybirds as a quick hit where you can see an infestation building.
- Get hoverflies and other beneficials established so control keeps ticking over on its own.
- Grow French marigolds in among the chillies to keep those predators fed.
- Lay off the repeated spraying that kills beneficials and sets you back to square one.
- Live with a few aphids if that’s what keeps the whole thing running.
For me that’s turned into a smarter way to grow, and honestly a lot less work over a season. I’m not chasing every outbreak any more. I’ve set the greenhouse up so they struggle to get going in the first place.
FAQ
Does neem oil kill aphids?
Yes, it does. It disrupts the way aphids feed and breed, so it works. The catch is that using it again and again can also hit the ladybirds and hoverflies that would keep aphids down for you the rest of the season.
Why stop using neem oil if it’s organic?
Organic doesn’t always mean harmless in a greenhouse. Neem might leave the adult beneficials alone if you’re careful, but their eggs and larvae are far more exposed, especially when the larvae eat aphids that have been treated.
What attracts aphids to chilli plants?
Feeding heavy on nitrogen is the big one, because it loads the sap with what aphids are after. Soft new growth, heat stress, underwatering and ants farming them all make it worse.
Are ladybirds or hoverflies better for aphid control?
They do different jobs. Ladybirds are fast and great as a first response, but they’ll fly off. Hoverfly larvae are more reliable under cover, because they stay put on the plant where the aphids are.
Why use French marigolds with chillies?
They put some pests off, whiteflies especially, and their flowers feed the adult hoverflies, ladybirds and parasitic wasps you want around. Grown in among the chillies, they help the whole thing run itself.
Should I aim for zero aphids?
No. Clear out every aphid and your predators have nothing to eat, so they leave or starve. A few aphids ticking along is what keeps your predators around for when more blow in on the wind.