The large narcissus fly (Merodon equestris) is the most important pest insect of narcissus crops in the UK. Traditionally, Cornwall and the Isles of Scilly have been thought of as those areas most affected by the pest, due to a warmer climate and closer rotations. However, increasingly warm summers have seen populations on the increase in eastern counties, and the pest is now a significant problem on a number of farms. Last summer (2004) appears to have been particularly favourable for narcissus fly survival and there are reports that infestation levels are higher in Lincolnshire and Cornwall than they have been for some time. Infestation levels in certain crops have been as high as 30% and many crops have 5-10% bulbs infested. This has considerable implications for the quality and value of the UK bulb crop.

Adult narcissus flies emerge during May and June. After a period during which they feed and mate, the flies lay their eggs in the soil, close to the base of narcissus leaves, at a time when the leaves on many crops have already senesced. The eggs hatch after several days and the newly emerged maggots crawl down the outside of the plant and enter the bulbs through the basal plate. The maggots feed and grow inside the bulbs. Although more than one maggot may enter a bulb, only one survives. The maggots are usually fully-grown by early winter and overwinter in this stage within narcissus bulbs. In March or April, the maggots leave the bulbs and burrow through the soil to form pupae near the soil surface, from which the next generation of flies emerge. Infested bulbs rarely produce flowers and many rot and die.

In the UK, narcissus crops remain in the soil for two or sometimes more seasons, which means that they are exposed to two or more periods of infestation by the narcissus fly. The use of chlorpyrifos for large narcissus fly control was investigated in HDC Project BOF 24 and resulted in chlorpyrifos receiving off-label approval for use as a bulb dip. This treatment provides high levels of control during the first growing season. However, there is no effective insecticide treatment for control of narcissus fly during the second and third growing seasons, although early lifting can help to reduce attack. The overall aim of current HDC-funded research at Warwick HRI (BOF 53 & BOF 55) is to determine which, if any, of the currently approved insecticides, or insecticides likely to achieve approval within the next 2-3 years, are effective against either large narcissus fly adults or maggots. For further information about these projects, please contact the HDC.

References

Collier, R.H. & Finch, S. (1992). The effects of temperature on development of the large narcissus fly (Merodon equestris). Annals of Applied Biology, 120, 383-390.