Sawfly Larvae   On eucalypt or portulaca sawfly larvae browse and drowse all black and clumped in colonies the birds avoid. Scientists surmise the predators abhor the herbivores because larvae of the sawfly taste awfully.   As they forage in the foliage baby birds acquire knowledge quicker than the entomologists. Sawfly grubs are blacker than other grubs on portulaca and they have a knack of spittin. Entomologists have written their apologies and sent them to the ornithologists.   Mike Cooper