Monitoring saddle gall midge (Haplodiplosis marginata) larvae and adult emergence

Abstract

The aim of this eight month study was to record development of saddle gall midge larvae and pupae, and the timing of adult emergence. This work was undertaken to determine whether monitoring of soil stages of this pest can provide a useful indication of the risk and timing of adult emergence.

The study had the following objectives:

1. Record numbers of saddle gall midge larvae and pupae by soil sampling at regular intervals.
2. Monitor adult emergence using yellow sticky traps checked at regular intervals.
3. Determine if soil monitoring of larvae and pupae provides a useful early warning of adult emergence.
4. Monitor soil temperature and soil moisture levels at regular intervals.

The work was done at two sites in Buckinghamshire which had previously been affected by the pest. It was funded by HGCA, with additional funding from Dow AgroSciences as part of their Pestwatch campaign.

Saddle gall midge larvae were recorded in every soil sample taken throughout the monitoring period at both sites. Numbers declined by 94% at Wendover and 96% at Cadmore End between February and June 2012. Newly developed (neonate) pupae were first recorded on 10 April at both sites and fully formed pupae at Wendover on 8 May. Numbers of pupae remained low throughout the monitoring period. A small number of pupae were also recorded as being parasitised. Saddle gall midge adults were not recorded on sticky yellow traps until 14 May at either site. Numbers of adults on traps never exceeded 0.5/trap/day. There does not appear to be a simple trigger to initiate pupation and it is likely that further data will need to be collected before any firm conclusions can be drawn.

Soil sampling was an effective method of monitoring saddle gall midge development. It should, therefore, be possible to use soil sampling to give an indication of the likely timing of adult midge emergence. It was interesting that the number of midge developmental stages in the soil declined so significantly during the monitoring period. This could be due to parasitism, predation by other insects or birds, or weather conditions. The biggest drop in numbers of larvae was at the end of April, which coincided with some of the wettest weather. It is also possible that larvae moved back down through the soil profile in response to these adverse conditions. Potential future areas for saddle gall midge research are discussed.