ABSTRACT.1. A rainfall simulator is described that was used to investigate the effects of rainfall on the cochineal insectDactylopius opuntiae(Cockerell). This species occurs in discrete colonies that are protected by a copious covering of opaque‘woolly’wax, and is the most important biological control agent of the prickly pear cactus,Opuntia ficus‐indica(L.) Miller, in South Africa.2. We record the composition of cochineal colonies (stage and number of individuals) in relation to colony size to allow accurate predictions of colony composition from size measurements of the intact, wax‐covered colonies before their exposure to rainfall.3. Even short exposures (15 min) to simulated rainfall (delivered at a rate of about 50 mm/h) resulted in wax erosion, and the effect was most marked in small, recently established colonies. Coverings of compacted wax protected the mature colonies from erosion.4. The number of colonies that were washed off the plant was a function of colony size and the duration of exposure to simulated rainfall. Most of the small, recently established colonies were washed from the plants after 120 min. All of the large, mature colonies, albeit badly mutilated and containing far fewer individuals, remained on the plants even after 240 min exposure.5. Survival of individuals, in colonies of all sizes, was less than about 40% after short bouts of rainfall (30 min). First‐ and second‐instar individuals accounted for most of the observed mortalities, and mortality of all stages increased with increasing rainfall duration.6. These observations provide a partial explanation for the poor performance ofD. opuntiaeas a biological control agent of cacti in higher‐r