1. Diet for mass rearing of insects that feed on hard, woody plant tissue comprising a homogeneous mixture of a first portion which is subjected to sterilization in an autoclave comprising water and agar in amounts to form a gel containing the other diet components and a nutrition providing effective amount of salt mix, and of a second portion which is not subjected to sterilization in an autoclave comprising a larval development effective formulation compatible amount of ground host plant part fed on by larvae of the insects, a nutrition providing effective amount of vitamin mix and a pathogen growth suppressing amount of antimicrobial agent(s).2. The diet of claim 1, wherein the first portion comprises by weight of the diet of 0.01 to 5% salt mix and the second portion comprises by weight of the diet of 0.1 to 40% ground host plant part, and 0.01 to 5% vitamin mix and for the antimicrobial agent(s) 0.01 to 5% sorbic acid and 0.01 to 5% methyl paraben.3. The diet of claim 2, wherein the first portion also comprises by weight of the diet from 1 to 25% protein nutrients and from 0.5 to 15% of a sugar.4. The diet of claim 3, wherein the protein nutrients consist by weight of the diet of 0.5 to 10% yeast extract and from 0.5 to 15% casein.5. The diet of claim 4, wherein the first portion comprises by weight of the diet from 5 to 70% water and from 0.5 to 10% agar.6. The diet of claim 5, wherein the first portion comprises by weight of the diet from 55 to 70% water, 1 to 5% agar, 1 to 5% yeast extract, 1 to 5% sucrose, 0.1 to 2% salt mix, and 1 to 5% casein, and wherein the second portion comprises by weight of the diet from 20 to 30% ground host plant part, 0.1 to 2% vitamin mix, 0.05 to 1% sorbic acid, and 0.05 to 1% methyl paraben.7. The diet of claim 6, where the weevils are root feeding weevils and the ground host plant part is ground host plant root.8. The diet of claim 7, here the weevils are Hylobius transversovittatus, and the ground host plant root is ground purple loosestrife root.
9. The diet of claim 7, where the weevils are Cyphocleonos achates Fahraeus, and the ground host plant root is selected from the group consisting of ground Centaurea maculosa root and ground Centaurea diffusa root.10. Method of mass rearing of insects that feed on hard, woody plant tissue comprising the steps of:(a) placing first instar insect larvae on an artificial diet comprising ground host plant part fed on by the larvae;(b) incubating the larvae on the diet in darkness at a temperature of 15 to 30° C. and a relative humidity of 50 to 90%; and(c) retrieving adults as they emerge.11. Method of mass rearing of the root-boring weevil Hylobius transversovittatus for use as a biological control agent of Lythrum salicaria, comprising the steps of:(a) placing first instar Hylobius transversovittatus larvae on an artificial diet as claimed in claim 8;(b) incubating the larvae on the diet in darkness at a temperature of 15 to 30° C. and a relative humidity of 50 to 90%; and(c) retrieving adults as they emerge.12. Method of mass rearing of the root-boring weevil Cyphocleonus achates Fahraeus for use as a biological control agent of Centaurea maculosa L., comprising the steps of(a) placing first instar Cyphocleonus achates Fahraeus larvae on an artificial diet as claimed in claim 9;(b) incubating the larval on the diet in darkness at a temperature of 15 to 30° C. and a relative humidity of 50 to 90%; and(c) retrieving adults as they emerge.
