This stem gall was spotted at 9 Wells nature reserve this week. Home to the larvae of a small wasp this gall is known as Diastrophus rubi and is host specific to the Dewberry or Bramble but has been recorded on Raspberry canes. Galls are very interesting natural phenomenon as they form on plants in response to secreted hormones from mainly insects but they can also be triggered by bacteria and fungus. They are not a sign of disease but consist of healthy plant tissue which is full of nutrients for the growing larvae within. This tissue is also easily digestible for the young larvae. This particular gall would have begun life in the spring as a green elongated swelling punctuated by red pimples indicating the chamber of a larvae. These galls can contain up to two hundred chambers. The gall changes colour from green to yellow to purplish then to light brown with the larvae overwintering inside before emerging the following spring. Once the galls become woody they can stay on the stem for several years.
Autumn and winter are a good time to spot them due to the lack of foliage.
The elongated swellings are green at first punctated with red pimples that indicates the inner chamber of a larva. In summer the galls go from yellow to purplish to light brown with the larva overwintering inside before emerging the following spring. One gall may contain up to 200 rounded chambers. Once used the galls become woody and can remain on stems for several years