THIS PROJECT FOCUSES ON THE ECOLOGICAL INTERACTIONS BETWEEN TWO GALL-FORMING INSECT HERBIVORES AND THE RUSSIAN KNAPWEED, A WIDESPREAD AND SERIOUS INVASIVE WEED OF RANGELAND AND FARMLAND THROUGOUT THE WESTERN UNITED STATES. RELEASE OF MULTIPLE NATURAL ENEMIES IS COMMON IN MANY BIOCONTROL PROGRAMS (PROVIDED NON-TARGET EFFECTS ARE MINIMIZED), YET LITTLE IS KNOWN ABOUT HOW TWO OR MORE NATURAL ENEMIES INTERACT WITH ONE ANOTHER IN MOST SYSTEMS. WE MAKE THE CASE THAT UNDERSTANDING HOW NATURAL ENEMIES INTERACT WITH ONE ANOTHER (E.G., COMPETITIVE, FACILITATIVE, OR NEUTRAL INTERACTIONS) IS ESSENTIAL TO UNDERSTANDING WHETHER THE STRATEGY OF RELEASING MULTIPLE NATURAL ENEMIES ENHANCES BIOCONTROL EFFORTS OR IS COUNTERPRODUCTIVE. FURTHERMORE, THE STRENGTH AND DIRECTION OF THESE INTERACTIONS CAN CHANGE WITH ABIOTIC CONDITIONS. WE EXAMINE HOW INTERACTIONS BETWEEN THESE TWO GALL-FORMING INSECTS CHANGE ACROSS A GRADIENT OF SOIL MOSITURE CONDITIONS, AN ABIOTIC CONDITION PARTICULARLY RELEVANT IN THE WESTERN US. OUR SPECIFIC OBJECTIVES ARE TO 1) COMPARE ADULT PREFERENCE AND OFFSPRING PERFORMANCE OF MIDGES AND WASPS WHEN DEVELOPING ALONE OR TOGETHER, 2) COMPARE THE EFFECTS OF MIDGES AND WASPS, ALONE OR TOGETHER, ON THE PERFORMANCE OF RUSSIAN KNAPWEED, AND 3) CONDUCT RELEASES OF MIDGES AND/OR WASPS AT RUSSIAN KNAPWEED FIELD SITES THAT VARY IN THEIR SOIL MOISTURE ACROSS COLORADO TO COMPARE THE LONG-TERM EFFECTS IN SUPPRESSING KNAPWEED POPULATION GROWTH.