Potential distribution and population dynamics of Pulvinariella mesembryanthemi, a promising biocontrol agent of the invasive plant species Carpobrotus edulis and C. aff. acinaciformis

The geographic distribution of the invasive plant Carpobrotus spp. and its potential biocontrol, the scale insect Pulvinariella mesembryanthemi (Vallot), was predicted in NW Spain using the maximum entropy algorithm (MaxEnt). Since both species are mainly present along the coast the potential distribution of the species overlapped. Plant distribution was influenced by soil thermal regime (51%) and continentality index (22%), while insect distribution was determined by the continental index (39%) and the soil thermic regimen (38%). The population dynamics of P. mesembryanthemi was examined through a 2-year field study in four localities invaded by Carpobrotus taking into account climatic factors and biotic interactions. In the studied localities, this scale was bivoltine, reaching its highest densities in the warmer and drier months of summer and sharply decreasing in winter. Only in a specially sheltered locality (Mera) did the insect densities stay high throughout the year. Pulvinariella mesembryanthemi infestation increased necromass proportion of its host plant. However, Chalcidoidea parasites and Coccinellidae predators limited P. mesembryanthemi density, and therefore may restrict its efficiency as a biocontrol agent. Chalcidoidea parasitism affected predominately the largest scales (particularly when P. mesembryanthemi reached high densities), slowing down the development of scales in summer. The excretion of honeydew by these scales attracted ants, and exceptionally, led to mould fouling. Carpobrotus plants were also attacked by fruit predators (facilitating seed dissemination and germination), other sap-sucking insects (i.e. aphids, mealybugs, spittlebugs), and viruses. There were no severe consequences. The utilization of the scale-insect as biological control due to complex interactions between both invaders and a set of generalist enemies and mutualists, as well as its implications, are discussed.