caterpillars on apple tree

http://www.gardenerscorner.co.uk/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=15;t=000768

help please

 

Looks like sawfly caterpillar, you can spray with a organic pesticide like liquid derris, if your tree is small take them off by hand and use those size nines of yours,prevention is better than cure treat fruit trees in winter/spring before the buds break with a winter wash you can no longer use tar oil but there is an alternative using plant based oils this will prevent a lot of the common pests.