EVALUATING THE ECONOMIC DAMAGE THRESHOLD FOR BONT TICK (AMBLYOMMA-HEBRAEUM) CONTROL IN ZIMBABWE

Controlling ticks and tick-borne diseases by frequent applications of acaricides (e.g., dipping) is costly, and can leave treated livestock vulnerable to epizootics of tick-borne diseases should the system of a pplying acaricides break down. The concept of only applying acaricides on an infrequent (strategic) basis often relies on the target tick po pulation displaying a seasonal cycle. However, as adult bont tick (Amb lyomma hebraeum) infestations in Zimbabwe's lowveld do not have a stri ctly seasonal pattern of occurrence, it is recommended that tick contr ol only be applied when bont tick infestations are equal to, or greate r than, their economic damage threshold. The economic damage threshold is the minimum average weekly standard female tick burden sufficient to cause damage equal in dollar value to the costs of applying tick co ntrol. Assuming that each standard female tick represents a 10 gram we ight loss, the economic damage threshold (standard female ticks/week) is equivalent to the ratio of the producer price of beef (liveweight e quivalent): per head cost of dipping (Eqn (3)). To illustrate the appl ication of the threshold methodology, it was assumed that the producer price of beef was Z$1.63/kg (U.S.$0.33/kg) and that tick control cost Z$0.29/hd/dip (U.S.$0.06/hd/dip). This gave a threshold of 18 standar d female ticks/ head/ week. Using tick counts obtained from 20 Brahman cattle held at Mbizi in southeastern Zimbabwe, it was shown that for the 1988 calendar year there were only 32 weeks when the economic dama ge threshold was met or exceeded. This is substantially less that the dippings per year that have been, until very recently, legally require d in Zimbabwe. Sensitivity analysis showed that a 10% rise in the cost of dipping reduced to 23 (a 28% decrease) the number of weeks when ti ck burdens exceeded the economic damage threshold. By further assuming that an acaricide application and residual effects will cause a 3-5 w eek interval before the next application may be required, the number o f weeks when the tick burden was equal to or greater than the threshol d of 18 standard females/ week fell to just 9-12 weeks. Three factors may cause an alteration in the economic damage threshold: i) tick burd ens may cause damage to the udders, ii) secondary infestations (e.g., screw-worm) may cause economic damage; and iii) nutritional stress of the cattle may reduce the actual average per tick weight loss. Until f urther data becomes available, it is recommended that the economic dam age threshold methodology be used as described here, and that farmers closely observe their herds during the suggested weekly tick sampling for udder damage and secondary infestations.