My 2 Year Plan to Control Agapanthus Gall Midge

This year, I cut off all my Agapanthus flowers before they could open. Why?

Agapanthus bring so much to the garden with their bold spherical heads of trumpet flowers, beloved by pollinators. For me, they have been perfect container-plants: showy, low maintenance and easily overwintered in my cold greenhouse. I have 7 varieties, including small and large growers in various shades of blue, purple and white; most are deciduous, one evergreen. They all grow quickly, needing potting-on every couple of years. Their huge root systems eventually destroys even terrazzo container, so in recent years I have been dividing and repotted them regularly, leading to my current collection of 30+ large pots.

This year should have been their best…

This year should have been their best, with the garden full of blooms in July and August. But it’s not. There are no flowers. I have cut them all off. And now I’m planning to ditch up to half of them, and I’ll not be giving them away. Why? Agapanthus Gall Midge – that’s why.

Agapanthus Gall Midge

This ‘new to science’ species was first identified in 2014 (at RHS Garden Wisley in Surrey, England), but its natural habitat and country of origin remain a mystery to this day. The pest has now spread across the UK and is a serious problem for the horticultural and floral trades, and us lowly gardeners. If you grow Agapanthus, and don’t have Gall Midge yet, I’m sorry to tell you, it is just a matter of time.

Life Cycle

The Agapanthus gall midge lays her eggs in the emerging flower buds of Agapanthus plants between June and September. The 1mm long grubs hatch and live inside the growing buds, feeding on the flower tissue. This causes flowers to become distorted. Where the infestation is severe the whole flower bud may die, or grow in a distorted shape. Lighter infestations may affect individual flowers within a head, causing them to fail to open, or shrivel. The effects range from discolouration of a few individual flowers, to the entire flower-head becoming distorted, and in severe cases the flowering stems become distorted, or die before opening.

If not removed promptly, the grubs in infected flowers will eventually burrow out and fall onto the soil below where they overwinter in the top two inches. In the spring the adult midges emerge and fly off in search of young Agapanthus buds to reinfect.

Currently, there are no reliable chemical or biological controls available, and none on the horizon. And so far, no resistant varieties have been identified, so it looks like we are stuck with this new pest for the foreseeable future.

The only viable option once you get an infection, is to break the life cycle. This requires cutting off ALL agapanthus flowering stems — whether infected or not — before they open everywhere in your garden (and ideally the neighbours if you can get them on side), then burning or binning them (not composting them at home.) This prevents the grubs getting back into the soil, breaking the life cycle.

If you only have a few plants in pots, you could try removing all of the soil around their roots in autumn (or at least the top two inches) to remove the hibernating grubs, but this will be laborious if you have many plants, and is impractical for plants growing in the ground.

The Symptoms

The above shows the kind of damage that moderate and heavy Agapanthus Gall Midge infection can cause. Mild infections can be much less notable, with just a few individual buds failing, so the infection may easily go unnoticed. Over several years, if nothing is done, the population of gall midge builds up until every flower head is damaged and distorted, ruining the display.

The Cause

When infected buds or flower spikes are cut open, tiny grubs can be found. These larvae are only 1-3mm long and are the same colour as the stamens in the buds, so can be difficult to see.

The 5 Stages of Development & Risk Level

Stage S (Flower head sheath developed): The green protective sheath is still closed around the main head. The adult midge pierces this to lay eggs. The larvae hatch inside but cannot escape yet.

Stage EF (Early Flowering): Individual flower buds begin pushing out of the splitting sheath. The maggots are small and feeding heavily inside the buds. At this stage, they are still sealed inside, meaning de-budding now provides 100% eradication.

Stage MF (Mid-Flowering): Buds have fully emerged from the sheath. Infected buds look swollen, deformed, or show brown/purple discoloration. Larvae are reaching full size (1–3mm). This is when the very first mature larvae begin to exit the buds and drop to the soil below.

Stage F (Flowering): Healthy buds open into flowers, but infested buds remain closed, look like mush, or dry up completely. Larval dropping peaks during this phase as they complete their feeding cycle.

NOTE: to be viable, larvae need to drop onto soil, where they can burrow and overwinter. Flying adult gall midges will emerge in late spring seeking freshly emerged Agapanthus flower-spike buds in which to lay their eggs and repeat the cycle. Successful treatment strategies must break this cycle.

Treatment Plan

1. Rigorous De-budding of all Agapanthus flower spikes

The only way to get rid of Agapanthus gall-midge in a garden is to break the cycle by removing all flowers before the grubs can fall onto the soil. This means removing flowering spikes before flowers start to open. Even healthy-looking flowers need to be removed, as adult midges may still be active and lay eggs on the unopened buds in the centre of the flower head. The only way to guarantee removal of all the grubs is to remove all agapanthus flower stems before they open.

2. Repeat for second year

It is recommended to repeat this rigorous flower-removal for a second year. This is not only because of how easy it is to miss a flower head, allowing infection to linger, but because it is understood that some grubs may remain dormant in the soil only emerging in their second year. Consequently, it is best to commit to two years of thorough Agapanthus de-budding. This is what I am going to do.

Following this two-year plan one might reasonably expect 2-5 years of perfect agapanthus flowers before reinfection.

3. Additional Steps & Prevention

Re-infection can happen at any time if the adult midge returns. However, the adults are weak fliers, so most infection comes from one’s own garden. Even after getting rid of my own midges using the two-year method above, there is a constant risk of midges returning.

The main sources of re-infection

• New nursery plants
  I think the infection in my garden arrived in 2024 when I purchased Agapanthus “Poppin Purple”. It always had distorted flowers and I didn’t know why. The nursery trade cannot guarantee that their plants are free from Agapanthus Gall Midge. If you purchase any agapanthus it is best to put them in isolation, and inspect carefully to see if they are clear before adding them to your garden. I won’t be buying any more Agapanthus.
  
• Adjacent gardens
  Being weak fliers, the adult midges tend to stay close to the garden they emerge in. Inevitably, though, they will gradually spread to adjacent gardens that have Agapanthus, attracted by the scent of the plants. The more Agapanthus you have around you (in neighbouring gardens), the sooner you will be re-infected. If you have a lot of Agapanthus in your garden, then the scent will attract more adult midges. Even if no Agapanthus are being grown in your neighbourhood, some adult midges can be carried many kilometres on the wind, so will eventually find their way to your ‘clean’ plants.

Reducing the number of agapanthus plants

For the above reasons, I am planning to reduce the number of agapanthus plants in my garden. I will probably dispose of about half of my pots, keeping just one or two of each variety. In doing so I will make the de-budding and care easier, and reduce the ‘scent-pool’ that attracts adult midges.

Also, although agapanthus foliage is attractive in its own right, I don’t really need so many plants for foliage alone. After two years, my plants will probably need dividing again, so reducing the number now may not be much of a loss.

Mulching the pots

I am planning to add a one inch (2.5cm) layer of horticultural grit as a mulch on top of the soil of the Agapanthus plants that I keep. Such a mulch has been shown to inhibit emerging midges as the adults are weak and find it hard to make their way out of the soil through a compact layer of angular grit. The only problem is that many of my Agapanthus pots are filled with large crowns, so have little space for adding mulch.

Placing pots on paving or gravel

If pots of Agapanthus are placed on impermeable surfaces, such as gravel or paving, grubs falling onto these surfaces will not be able to get into the soil and will dry out and die. I have a few pots in such locations, so instead of cutting off all the flowers this year, I have left a few stems that hang out well over the edge of their pot. I have moved other plants away, so any grubs will fall onto the paving and not onto soil.

The Long Game

Sacrificing my blooms for the next two summers is a bitter pill to swallow, especially with over 30 pots currently sitting flowerless in the garden. However, it is the only way I know to reclaim the spectacular summer displays I’ve come to love. By shrinking my collection, layering on the horticultural grit, and strictly breaking the midge’s life cycle, I hope to be able to grow them pest-free again… for a few years at least.

It is a patient, tactical strategy, but hopefully worth the effort. Like much of gardening, there is little point in worrying. Take action, and see how it turns out. In a couple of years, I may be looking at completely clean and healthy flowers once again.

[I’ll update this post in future years to share how well my plan worked!]