Parasites also have parasites.
Along Ash Creek near the Lower Campground (Lassen County CA) I noticed a wild rose (did not attempt to identify the scraggly, dried up plant) covered with round, green galls.
Galls are tumor-like growths that form on plants because of chemical or mechanical stimulation by invading organisms, often as an insect oviposits an egg. This gall was originally induced by the rose stem gall wasp (Diplolepis nodulosa), a cynipid wasp. These wasps are tiny and do not sting or bite. The gall that forms when a rose stem gall wasp oviposits is a long, slightly swollen area located near the lower end of the affected stem. These swellings are often difficult to locate. The D. nodulosa gall is monothalamous meaning it has only one larval chamber.
Enter another cynipid wasp, Periclistus pirata, an inquiline. Inquilines are insects that specialize in eating the plant tissue of a gall induced by another insect and sometimes inquilines also kill and eat the insects in the original gall. When P. pirata oviposits in the D. nodulosa gall the inconspicuous, single chambered gall modifies into a large swelling with many larval chambers. The larva in the host gall are killed when P. pirata deposits its eggs.
Interestingly both D. nodulosa and P. pirata overwinter as prepupae and both emerge in the spring. The timing of the spring emergence is such that the D. nodulosa emergence from uninfected galls occurs about two weeks before P. pirata emerges from parasitized galls. This timing allows the rose stem gall wasp to infect wild (and hybrid) roses and the host gall to form before the inquiline cynipid wasp is ready to parasitize.
These photographs show the larger, green P. pirata galls. The larvae are visible in the multiple chambers.