Cassava is extensively grown by small farmers worldwide, and is often intercropped with shorter-duration crops. When these intercrops are harvested, the conditions of the cropping system change drastically. If the direct effects of intercropping are critical to the reduction of cassava pests, these effects would be reduced after harvest of the intercrop. It would be more beneficial to have the effects last into the postharvest period because cassava has no critical period for yield formation. Damage to the cassava crop is proportional to the duration of the insect attack.
Maize and cowpeas were harvested 17 weeks after planting (WAP). Maize and cowpea plant biomass was cut and left as mulch. A regional variety of cassava was used for the intercropping trials, and was harvested 45 WAP. Cassava variety CMC 40 was used in monocropped plots, and was harvested 35 WAP. Trials were weeded manually until harvest of the intercrops, then paraquat was used until 22 WAP, after which the trials were not weeded.
The cassava cycle was broken into four periods: establishment (4 to 6 WAP), preharvest (8 to16 WAP), postharvest (18 to 35 WAP), and mature (39 to 45 WAP). Harvest refers to the intercrop. Whitefly egg densities were analyzed for the entire trial, and for the different cassava periods.
Sampling whitefly egg populations. Whitefly populations were determined at leaf and plant levels. Egg populations per plant were estimated, and analyses were conducted separately for the two whitefly species.
T. variabilis egg densities were significantly lower in cassava/cowpea systems than in cassava/maize or monoculture (average 48 percent lower than monoculture), but egg densities in cassava/maize systems did not differ from that in a monoculture.
Egg populations per plant. Combined egg populations per plant for both the pre- and post-harvest periods averaged 1.07 million for A. socialis and 0.28 million for T. variabilis. Egg populations were greatest at the end of harvest for A. socialis , and at 16 WAP for T. variabilis.
In both intercroping systems, egg populations of A. socialis per plant were much lower than in the monoculture (69 percent lower in cassava/cowpea and 54 percent lower in cassava/maize). Greater reductions occurred after intercrop harvest.
Cassava/cowpea had 71 percent fewer T. variabilis eggs than monoculture, with the greatest reduction occurring after intercrop harvest. Cassava/maize systems showed lower egg populations only after harvest.
Intercrops may indirectly affect herbivore levels through changes in host plant quality (size, leaf area, and nutrient content of the leaves). Differences in host plant size might affect the microclimate of the whitefly habitat through lower leaf production rates and thinner canopies. In all plots, greater numbers of whiteflies tended to occur on larger plants, thus it appears that host plant size and perhaps architecture did have a major influence on whitefly populations.