Black Locust Gall Midge (Obolodiplosis robiniae) on the Isle of Wight and some interesting parasitoids
Although I was aware of this gall midge I hadn’t previously looked for it. In June I noticed several False-acacia or Black Locust (Robinia pseudoacacia) trees in flower on Shanklin Big Mead so I had a look for the galls. The large trees were not easy to examine but fortunately there were also lots of self-seeded saplings close by. Checking those saplings the galls were immediately obvious so several were collected for examination.
The gall midge causes thickening and rolled leaf margins in which the larvae develop over a period of about 21 days. There are three or four overlapping generations during the year with the larvae pupating in the gall during the summer and the final generation pupating in the leaf litter and soil to emerge in spring.
There are five previous county records of the galls (black dots on map) but none had been bred through to the adult midge. Those records come from 2011 and 2013.
Galls were collected from Shanklin Big Mead (SZ 57902 80845) on 15 June, 20 June and 28 July, then from Arthurs Hill, Shanklin (SZ 58501 82099) on 05 August (orange dots on map). Despite there being so few previous records of the galls, the ease with which I located them suggests the midge will be present wherever the host tree grows. Black Locust are widely planted across the Island as ornamental trees.
Black Locust leaves with Obolodiplosis robiniae galls and an individual gall. Typically the galls were found to contain one or two larvae but some held as many as five.
I decided to breed some of the larvae through. Galled leaves were placed in a plastic container, actually an old take-away container, with some damp tissue to prevent desiccation. The container was opened daily to examine the contents and to prevent the formation of mould.
The first instar larvae are approximately 1mm in length and are white. They darken as they develop, becoming yellow. The third and final instar is approximately 4mm in length and the orange pupae the same size. Most of the larvae collected were third instar so they quickly developed into pupae from which the adult midges emerged after only a few days. The galls collected on 15 and 20 June produced a total of twenty-one adult midges, the various stages shown below.
The majority of galls collected from Shanklin Big Mead on 28 July progressed in a similar fashion but on this occasion three of the larvae were affected by parasitoids resulting in the formation of pupal clusters.
On 01 August a single ichneumonid wasp emerged from the first cluster of four pupae. It was not what I was expecting and apparently a hyperparasitoid. Gavin Broad, from the Natural History Museum, believes it is likely a male of the genus Scambus and stated that some species in that genus are known to act as pseudo-hyperparasitoids. He described it as an interesting host record.
There were two more pupal clusters from which adult wasps emerged on 06 August. The primary parasitoid of the gall midge is the wasp Platygaster robiniae, which was first described by Peter Buhl and Carlo Duso1 in 2008. These appear similar but would require expert examination for formal identification. Of the nine wasps that emerged, five have been retained. If they did prove to be Platygaster robiniae that would represent a significant record. David Notton, also of the Natural History Museum, says there are none in the NHMUK Collection so there are no reference specimens for comparison and it is unclear whether this taxon has been recorded in the UK before.
The galls collected from Arthurs Hill have all progressed normally and have produced twenty adult midges, three male and seventeen female. It would be interesting to collect further samples, particularly with a view to identifying the parasitoids. Whatever species the wasps are, it has been a fascinating exercise.
I found an interesting paper by Peter Toth, Martina Vanova & Jozef Lukas titled “Impact of natural enemies on Obolodiplosis robiniae invasion”2. The suggestion was “the midge was heavily influenced by a specialized parasitoid Platygaster robiniae, which was a predominant parasitoid and accounted for 98% of all specimens reared”. 98% makes it quite likely that these are Platygaster robiniae but it would be nice to know for sure.
I collected more of the midge galls from Shanklin Big Mead on 20 August. Interestingly not a single midge emerged but I did get another 68 of the parasitoid wasp. Something else I have read in the academic papers is that the parasitoid numbers increase during the year, which is consistent with that last batch I bred through.
- Annals of the Entomological Society of America, Volume 101, Issue 2, 1 March 2008, Pages 297–300, https://doi.org/10.1603/0013-8746(2008)101[297:PRNSHP]2.0.CO;2
- Impact of natural enemies on Obolodiplosis robiniae invasion. Tóth, P., Váňová, M. & Lukáš, J. Biologia (2011) 66: 870. https://doi.org/10.2478/s11756-011-0089-z
15 August 2018