Variation in weather among years may affect biological control of insect pests by influencing how well matched in phenology specialist parasitoids are with their pest hosts. A 10‐year study in western North America (Utah) revealed greater change with warm versus cool springs in the life cycle timing of the cereal leaf beetle (CLB), Oulema melanopus (L.), than of its principal enemy, the parasitoid wasp Tetrastichus julis (Walker). The beetle laid eggs, and larval populations developed in crop fields earlier on a calendar‐day basis, but nonetheless after more degree‐days had accumulated, in warmer than in cooler springs. The phenology of parasitism by wasps, in contrast, varied little among springs in relation to accumulated degree‐days. Consequently, in warmer springs, larval phenology of the CLB was delayed relative to adult parasitoid activity, and parasitism was reduced. Presently, a significant degree of biological control of the CLB results from parasitism by T. julis. By promoting phenological mismatch between host and parasitoid, however, a warming climate could weaken this biological control of the insect pest.

Natural enemies of insect pests of annual crops have been hypothesized either to lag, or alternatively not to lag, behind their prey in dispersing to and colonizing new habitat. We examined parasitoid dispersal and parasitism of the cereal leaf beetle (Oulema melanopus [L.]; Coleoptera: Chrysomelidae) by the host-specific wasp Tetrastichus julis [Walker] (Hymenoptera: Eulophidae) in wheat fields of northern Utah to assess whether a colonization lag occurred. Equally high rates of parasitism of beetle larvae (including second instars early in the year) occurred in 2010 and 2011 in fields that were newly planted to wheat vs. in fields where wheat had been grown also the previous year. A caging experiment demonstrated that parasitism in these newly planted wheat fields did not arise from parasitoid adults that had matured within the fields; instead, upon emerging in other fields, parasitoid females dispersed a minimum of 100-250 m to parasitize beetle larvae early in the spring in the newly planted fields. A transect study in 2012 revealed that T. julis females dispersed rapidly at least 600 m into a newly planted wheat field to parasitize most of the early maturing beetle larvae, which occurred at very low density. Thus, the parasitoid has very strong ability to match its host in dispersal over long distances across a highly disturbed agricultural landscape, and colonization lag appears of little importance in affecting biological control associated with this host-parasitoid interaction.

Scouting at key times in the seasonal development of insect pest populations, as guided by degree-day accumulation, is important for minimizing unwarranted insecticide application. Fields of small grains in northern Utah were censused weekly from 2001 to 2011, to assess infestation by the cereal leaf beetle, Oulema melanopus (L.) (Coleoptera: Chrysomelidae), and develop degree-day guidelines for measuring cereal leaf beetle abundance at peak egg and larval densities in any given year. Even in years of high overall numbers of cereal leaf beetle, relatively few fields were heavily infested (with 20 or more cereal leaf beetle eggs + larvae per 0.09 m2) at either egg or larval peak density during the growing season. In individual fields, the number of immature cereal leaf beetle (eggs + larvae) at peak larval density was positively related to the number of immature cereal leaf beetles present earlier at peak egg density. Although there was large variation among years in when cereal leaf beetle egg and larval numbers peaked during the season as measured by degree-day accumulation from 1 January, much of this variation was accounted for by the warmth of the early spring before significant egg laying occurred. Hence, degree-day estimates that account for early spring warmth can guide growers in scouting grain fields at peak egg densities to identify fields at high risk of subsequent economic damage from cereal leaf beetle larval feeding. The relatively low incidence of fields heavily infested by cereal leaf beetle in northern Utah emphasizes the benefit that growers can gain by scouting early before applying insecticide treatments.