This post was written by Shawn Steed, ornamental production Extension agent in Hillsborough County. His blog is here:http://ow.ly/5XLUi
I have recently been to a couple of tree producers and have seen the dramatic effects of diaprepes root weevils in their nurseries.
On the right is a picture of a dead holly tree that seemingly had root damage caused by both insects and disease. These medium-size insects can have devastating results to larger grown material. The adults can be seen in ornamentals by the damage they inflict by notching the leaves. Usually this is not bad enough to render a plan unsalable, but it is a good indicator of the insect and of the need to treat it.
The bad stage of this insect infestation is what lies below the ground unseen. The juvenile stage of this insect is a root grub that will girdle roots of plants and trees. If infestations are extremely high plants may show symptoms of lack of vigor and even death. Along with root girdling, plants may become infected with soil borne pathogens usually this is Phytophthora. Roots become susceptible when the juveniles wound the roots and allowed root exudates and carbohydrates to leak and feed pathogenic fungi.