Dear Gardener,

Thank you for contacting us. While you did not identify your location, I suspect you are dealing with sawfly larvae. In North America there is one which is actually called a loosestrife sawfly. Here is an article about various sawfly species, and control options: http://www.extension.umn.edu/garden/insects/find/sawflies/ 

Please note that Bacillus thuringiensis (BT) is not effective on sawfly. Insecticidal soap is an option, however the spray must make direct contact with the insect to have effect.

I suggest you try to continue hand-picking the pests. You can drown them in a container filled with soapy water. As the indicated in the above article, if the pests are more than half grown, they will pass into their next instar soon. While chemical spray may work, there is substantial collateral damage to the natural predators. Your garden will be much better off if you sustain beneficial insects and wildlife by avoiding chemicals.