Micrometeorology of summer alfalfa in Phoenix, Ariz., and its relation to resident insect pests, particularly the beet armyworm,Spodoptera exigua(Hübner), is discussed. Solar and net radiation, air and soil temperatures, wind speed, relative humidity, evaporation, soil heat flux, and soil moisture data were collected at weekly intervals throughout a typical growing cycle. Measurements were made concurrently within a mature uncut canopy and a freshly cut field to emphasize the impact that periodic canopy removal has on insect populations. Evapotranspiration and shading by a dense interlocking canopy prevented midday temperatures from reaching 35°C (Tamb= 44°C) and kept vapor pressure deficits (VPD) less than 10 mm Hg. Simultaneous measurements in the cut field revealed surface temperatures of 63°C and VPD's at +1 cm of 77 mm Hg. Temperatures and VPD's in the cut field 1 wk after irrigation were more moderate in comparison.The windrow microenvironment provides a temporary refuge from climatic extremes present in the cut field until plant growth, stimulated by irrigation, produces conditions more favorable for insect development. Eggs and larvae are especially limited by the microclimatic extremes and reduced food supply during the pre irrigation period. Pupae collected from burrows beneath windrow vegetation exhibited significantly less mortality than those collected in the open. Adults utilize both windrow microhabitat and cracks within the soil surface to reduce evaporative water loss and prevent body temperatures from surpassing physiological tolerances.Cultural practices which maximize the effect of macroclimatic extremes such as timed cutting, rapid windrow removal, and a short delay in irrigation are indicated when insect populations attain economically important levels.