There are several crosses between gooseberries and blackcurrants, one of which is the Jostaberry. We had a great specimen, planted winter before last and due to bear its first modest crop this year. However, it has been badly damaged by the most problematic pest of gooseberries, the larvae of the gooseberry sawfly. Their favoured feed is gooseberry foliage, but they will also attack currants, and clearly the hybrid Jostaberry is susceptible.
The adult sawfly lays its eggs in the host bush, which then hatch out into caterpillar like larvae. These are quite distinctive and readily identified, often marked with copious black dots on their bodies. They feed on the leaves and can strip a plant bare in just a few days. If permitted, they head to the soil, pupate, and hatch out later to continue the cycle. There can be several generations in one season. The first generation appears in April to May, but I am fairly sure they were not present on Thursday when I looked at the Jostaberry, but a few days later the bush was looking rather sad, having lost quite a lot of its foliage. It is a robust specimen, so I expect it to recover well, but this year’s crop may fail, and next year’s could also be affected.
I do not use synthetic sprays in our garden. They are not necessary, in my opinion, and can be harmful to desirable wildlife such as bees. I also have some reservations with the so called organic controls. That a spray is derived from some naturally occurring extract does not make it suitable for use in the kitchen garden. I have tried some in the past, but have observed that they can have a detrimental effect on the plants, not to mention desirable insects. With a quick search online, one can find several suggestions for homemade insecticides. These may well work, but I am rather reluctant to experiment with anything unless I am confident of its effects on the plants, other wildlife, especially bees, and any possible impact on the crop.
There is a biological control, a nematode, that can deal with sawfly, but by the time the pest has been observed, I rather think it is too late to order and apply such a control and wait for it to take effect. Observing the rate of destruction it seems quite clear that as soon as the pest is spotted it needs to be dealt with straight away. This leaves manual removal as, perhaps, the best approach. It is a time consuming task, and needs to be approached methodically, to work over the entire bush, removing all of the larvae that can be seen. I feel certain that I will have missed some, and will need to go over the bush once again in a day or two.
Whilst removing the larvae, I also observed a few of some pale caterpillar like pest that I did not recognise, although they might perhaps also be sawfly larvae, along with a small number of aphids clustered around the growing tips of a few branches. The latter is most readily dealt with by either washing with a jet of water or using a spray of water and soft soap. Soft soap is a very useful means of controlling greenfly and blackfly, especially on things such as broad beans and runner beans, which can attract them in great numbers. It is important, though, to be careful with any spraying so that beneficial insects, such as the ladybirds that feed on aphids, are not also killed in the process. I rather doubt whether the soft soap spray will have much impact on any remaining sawfly larvae, but it should deal with the aphids.
The fruit cage does not currently have its mesh fitted, so it is a little disappointing that the birds have not helped to deal with this pest. It is also a little strange, and quite fortunate, that the gooseberries, which are planted next to the Jostaberry, have not yet been affected. The currants, which are planted some 30 feet away, do not show any signs of infestation either. However, with the possibility of several generations of this most troublesome of pests, and perhaps more adults still in the area, I must try to be more vigilant.