Diabrotica flight in time and space as monitored with a high capacity trap placed in Zea mays fields of Illinois, USA.

Monitoring of insects by trapping is one of the prime tools of field entomologists. The leaf beetle Diabrotica virgifera virgifera LeConte (Coleoptera:Chrysomelidae) is no exception. Since its numbers (and consequently its mass) in a field population can be enormous, tools must be adapted to hold this many insects for later counting, sexing, biomass determinations, and additional investigations to follow. Since counting the high numbers during the flight peak may not be feasible at all, weighing and extrapolating to numbers by the correlation factor 1g = 160 to 170 beetles of mixed female and male sex was the method of choice. Around the perimeter of a hybrid maize (Z. mays) field of 0.6 ha, 16 high capacity traps were established at elevations ranging from 0 to 2 m above ground level. Optimal trapping is possible between 0.5 to 2m which field entomologists intuitively knew from experience and tradition. Below and above that level, the number of beetles is not zero but significantly below the optimum. High capacity traps can be left in the field with one loading of lure for four to seven days. Lures were the D. v. virgifera beetle sex pheromone 8-methyl-decane-2-ol propanoate (0.1 mg, collecting mainly male beetles) and the plant kairomone 4-methoxy-trans-cinnamaldehyde (10 mg, collecting both male and female beetles). The specific kairomone action (being much weaker than the pheromone) can be increased by simultaneously offering a feeding arrestant powder (e.g. prepared from ground Cucurbita texana or C. okeechobeensis) which keeps the beetles attracted by the kairomone lure close to the trapping site. There they can be immobilized and knocked down by a tiny source of carbaryl or dichlorvos fixed inside or at the surface of a plastic pellet or sheet. The high capacity traps are commercially available and can be reused for several flight seasons.