Iris Sawfly Rhadinoceraea micans
The larva of the Iris Sawfly may be as long as 50 mm and does look like the caterpillar of a White. It can be found on the Yellow Iris exclusively, including the ones in your garden along the pond. The eggs are deposited in May and the larvae eat a lot in just a few months time. By the end of July most of them seem gone. Actually they dug a hole in the groud, made a cocoon in there and remain there till next spring. Especially when appearing in large numbers the Iris Sawfly can do a lot of damage to your plants. The bigger larvae will eat the flowerbuds as well, worsening the damage.