In with the old and out with the new: are old associations or new associations better for the biological control of Cactaceae?

Abstract Most biological control programmes against invasive alien plants (weeds) utilise agents collected from indigenous populations of the target weed that are thought to have a long coevolution history between the target weed and the agent. These biocontrol agents are referred to as old associations. There are, however, several examples of cactus weeds being successfully controlled by new associations, which are agents collected on other cactus species rather than the target weed itself and are thought to lack any significant coevolutionary history with the target weed. It is therefore unclear whether new or old associations are more effective biocontrol agents for invasive alien Cactaceae. This study aimed to evaluate whether old or new associations of Dactylopius tomentosus Lamarck (Hemiptera: Dactylopiidae) are more effective biocontrol agents for invasive Cylindropuntia (Cactaceae) species. Three lineages of D. tomentosus were tested on seven Cylindropuntia species. The fitness of each lineage was assessed using adult survival, female body mass, and a fitness index, with phylogenetic distance between the primary host plant and closely related species used as a predictor. The results showed a significant negative correlation between fitness and phylogenetic distance across all lineages, with the highest survival, body mass, and fitness index on the primary host and its closest relatives. These findings support the use of old associations as biocontrol agents for Cylindropuntia species, and for invasive alien plants in general.