Until 2016 the species of midge causing this problem was undescribed (i.e. new to science). Consequently, very little is known about the biology and lifecycle of this insect. The Plant Health team at RHS Garden Wisley have been studying the midge since its discovery in 2014, and are asking for help from gardeners who have seen agapanthus gall midge or damaged flowers.

​Please send photos of symptoms on the flowers and buds, plus opened buds showing larvae where possible, to [email protected]. Please include postcode of location of the plant, to help us to map how widespread the midge is in the UK.

In previous years we have asked for samples to help with research; many thanks to those who have contributed these have helped further our understanding of the midge. We no longer require large quantities of material so please do not send samples unless individually requested.

Submissions to our pest and disease surveys are stored permanently in an anonymised form in order to monitor the spread of the pest or disease. We may contact you within 2 months of your submission in order to verify your sighting but your personal data will not be permanently stored in connection with your submission and will be deleted after 1 year. We publish and share only non-identifiable data from survey submissions (such as a six figure grid reference) with third parties and the public for the purposes of scientific research and advancing understanding among gardeners.

If the foliage of Agapanthus appears healthy but the flowers are abnormal in the ways described below, then agapanthus gall midge is most likely the cause:

• Affected flower buds are deformed in shape and may have patches of brown discolouration 
• Affected buds fail to open and either dry up or rot
• If the infestation occurs as the flower spike is developing, the entire flower head may collapse or fail to develop
• Numerous creamy yellow or orange maggots, up to 3mm long, may be found inside the buds, crawling around in a watery liquid

Currently control measures attempt to interrupt the insects life cycle:

• Monitor closely for symptoms as soon as flower buds start to develop
• Remove and destroy infested flower heads
• Destroy badly infested plants
• Re-pot container grown plants, replacing growing media to remove pupating or overwintering larvae
• Consider applying a mulch or pot topper to the area where the larvae are likely to fall to when they leave the flowers to pupate. Specifically Strulch (a straw based mulch) and MelCourt EcoBark (a bark-based pot-topper were effective at reducing or delaying the emergence of pupating midges in our recent laboratory study, but this needs validating on plants in outdoor conditions.

The RHS has been researching potential cultural, biological and chemical controls and this has directly informed the advice given above. The results of the most recent project, completed in collaboration with ADAS and funded by the AHDB can be downloaded from: https://horticulture.ahdb.org.uk/development-of-novel-control-options-for-agapanthus-gall-midge-2

The tiny gall midge lays eggs on the plant and the larvae develop inside the individual flower buds, inside the flower head sheath or in the petals of flowers that have gone over. The larvae can then cause the bud to be deformed and discoloured and often fail to open, as their feeding activities convert the plant material into a gall. The severity of the damage can range from a couple of buds failing to collapse of the entire flower head.

Infestation can be confirmed by opening the buds or flower heads and looking for the presence of small maggots 1-3mm in length which are a creamy yellow colour. The midge larvae leave the flower head to pupate in the soil, which takes around ten days. It is likely that they also overwinter in the soil and pupate the next spring.

The larvae can live in any stage of flower development, including in senesced flowers. Larvae can most commonly be seen inside individual flower buds, but if infestation occurs before the flower head sheath opens then the larvae can live and feed between the developing flowers and cause complete failure of the flower head.

Our research so far has shown that there may be multiple overlapping generations of the midge, as active larvae have been seen between mid-June and early October.

So far no biological or pesticide treatments have proved effective at targeting the larvae in the flowers. This is unsurprising as they develop inside the plant tissue and the long active period of the adults makes targeting egg-laying females difficult. The underground pupation and overwintering life stage is likely to be the most useful target for control.