Effects of an aphid pest on an invasive plant in the Peruvian Andes

An exotic aphid pest (Aphis cytisorum) has infested an invasive plant (Spartium junceum) in the Peruvian Andes since 2005. In turn, the aphids facilitated the spread of sooty mold fungus (Capnodium sp.) that covers the surface of ephemeral leaves and stems. We evaluated the infestation of aphids and fungus on S. junceum and on another Andean plant species in 2007 and 2015, measuring the effects of infestation on reproductive output of S. junceum in 2008. Individuals of S. junceum were heavily infested by aphids and fungus and close to half of populations were dead in the early years of infestation, although mortality had decreased over time. Infested plants produced larger fruits and seeds, but approximately a quarter of the production of uninfested plants. Viability and germination of seeds produced by infested plants were similar to those produced by uninfested plants. Aphis cytisorum was monophagous on S. junceum, although there is a risk that it can eventually infest other Andean plant species, because this aphid is polyphagous elsewhere. The strong impact of aphid/fungus infestation on survival and reproduction of S. junceum suggest that initially these natural enemies could have worked as a biological control. However, the population of S. junceum persists and shows evidence of repopulation, indicating the ineffectiveness of biocontrol. Our results support the importance of a long-term monitoring to determine the effectiveness of biocontrol agents and provide critical information for future related studies, considering that plant pests and disease infestations will likely increase under changing climate.