In the past, farmers and scientists have proposed that the differences in insect populations between mixed and single crop systems were due to various microclimate effects of the canopy structure. It has been suggested that shading might be unfavorable for pest colonization and even lead to increased mortality and emigration to sunnier sites.
Recently, researchers from the International Centre of Insect Physiology and Ecology conducted experiments with a cowpea/maize mixed crop in Western Kenya to evaluate the effects that various amounts of light reaching the cowpea canopy have on the abundance of the damaging bean flower thrips (Megalurothrips sjostedti). Maize and cowpeas were planted in alternate rows, 38 - 40 cm apart.
During the initial stages of crop development, there was no difference between the mixed and the single crop in respect to the amount of light reaching each crop. However, as the maize plants grew taller, they began to shade the cowpea plants. By 49 days after plant emergence (DAE) the maize canopy had fully closed over the cowpeas. At this time only about 34% of the incident light reached the cowpea canopy. There was no difference between the number of thrips on the intercropped or monocropped cowpeas until day 49 DAE when there was a sharp increase in the number of thrips in the single cropped cowpeas. There was a direct relationship between thrips abundance and the amount of light at the cowpea canopy.
Therefore, shading, particularly from a non-host plant, affords a protective effect for cowpeas from thrips. This protection, while not complete, contributes to the overall pest management system in cowpeas. Crop Protection, June 1988, 7:186-189
Dept. Crop Science
P.O. Box 7062