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How are the results of release monitored?
The establishment and dispersion of T.n. can be monitored by random sampling in maize stores, or by using pheromone traps. Since experience has shown that two years elapse before clear results are obtained, it is not worthwhile starting such surveys before that date They should, however, be continued for a sufficiently long period. It takes at least three to four years for the beneficial organism to disperse countrywide, and for the reduction in losses to become measurable
The pheromone traps should be placed at least I km from the point of release The predator disperses most rapidly in the direction of prevailing winds, covering around 10 to 50 km in a year
Estimates of losses during that period provide valuable pointers to the economic benefits of release A clear comparison of the situations before and after release is most important In cases where little information is available on losses before release, regions where T.n. is not yet established can also be used for comparison Monitoring should be completed by surveying the affected farmers on the changes they have observed.
In the ideal post-release scenario, a natural equilibrium emerges between the predator and its prey. T.n. keeps P.t. populations at a low level, but does not wipe them out completely The major thrust of T.n. 's impact is the reduction of P. t populations in the field (in forests or bush land). Studies in infested regions have demonstrated that, before T. n release, P. t occurred in every second to every fourth maize store If, three to four years after release, P. t only occurs in every sixth or seventh seven store, the programme can be considered a success Since P.t. is not completely suppressed by T.n., it continues to cause losses, albeit on a considerably reduced scale. Occasionally there may even be further spontaneous local outbreaks of P.t..
Stored maize is susceptible to infestation by numerous pest organisms, including the maize weevil (Sitophilus zeamais). Populations of these stored-food pests are not affected by T.n.. Consequently, losses are not reduced to zero, but continue to occur, albeit on a scale comparable to that prior to the accidental introduction of P.t..