Research

Hemerocallis gall midge ( Contarinia quinquenotata)

Introduction

Hemerocallis gall midge prevents the normal development of flower buds and has been causing problems for gardeners in the UK since the late 1980s. The RHS entomologists have investigated the biology and host range of this pest, resulting in a list of Hemerocallis species and cultivars that are less likely to be damaged.

Biology

Observations have been made on the biology of the hemerocallis gall midge, Contarinia quinquenotata . This tiny fly lays its eggs in the developing flower buds of day lilies ( Hemerocallis species and cultivars). Small white maggots develop inside the buds, causing them to be abnormally swollen but failing to open (see picture above: damaged flower buds left and centre, healthy flower bud on right.) An infested bud can contain several hundred larvae, which may be the progeny of more than one midge. When fully fed, the larvae are 2-3mm (about 1/8in) long and they drop down into the soil where they overwinter as pupae.

Cultivars less susceptible to damage

The midge has one generation a year, with damage to the flower buds occurring between mid-May to early July. Cultivars that flower after that period escape damage. From observations made on the extensive Hemerocallis collection at RHS Garden Wisley, a list has been compiled of late-flowering cultivars that partly or wholly miss the egg-laying period of the gall midge. These cultivars are listed below.

This is not an exhaustive list of late-flowering cultivars and there are likely to be others that are not grown at Wisley.

Cultivars with AGM after the name have received the RHS Award of Garden Merit

Use the online RHS Plant Finder to find suppliers

List 1: Late flowering day lilies less susceptible to hemerocallis gall midge

Distribution

The gall midge occurs commonly in Europe but was not found in Britain until 1989, when samples of galled flower buds were brought from a private garden at Weybridge, Surrey, to the RHS Members’ Advisory Service at Wisley Garden. Since then the spread of this pest has been tracked through enquiries made to the RHS advisory service and other sources. It is now widely distributed in England, particularly in southeast England and East Anglia.

Hemerocallis gall midge distribution at December 2007 (RHS data, map produced using DMAP©)

Click here to submit a record of Hemerocallis gall midge

Control

None of the pesticides available to amateur gardeners carries a label recommendation for use against gall midges. The larval stage is concealed within the buds and therefore difficult to reach with a pesticide. The adult stage is more vulnerable but is active over a period of approximately six weeks during which egg laying can take place. Galled buds should be picked off and destroyed before the larvae are able to complete their feeding. This will reduce damage in the following year but the effectiveness of this depends on how thoroughly the galled bud-picking is done and whether nearby gardens also have infested plants.

Back to Introduction to research