Scale insects often cover aloes with a white, crusty layer.  A number of insect pests feed on aloe leaves.  The main ones are the eight-back bug, a small type of black stink bug which can damage plants severely during the winter months, and the yellow-brown stink bug which is active in late summer.  While these two types of stink bug may crate unsightly marks on the plants, unless they are present in large numbers they will not actually kill them.

 

A much more serious pest is the snout beetle - a brownish-black weevil which feeds on the leaves and the crown of aloes, and can cause the whole plant to decay.  The best way of controlling insect pests is to spray the plants with the correct pesticide as soon as insects become troublesome.