San Jose scale - overseas biological control

There were early attempts at biological control of San Jose scale in the U.S.A. using Chilocorus ladybirds and fungi. These were not successful and a number of parasitoid wasps were accidentally or deliberately introduced. These became important in control, such as Encarsia perniciosi introduced in the 1940s. However, a different wasp species is now considered most important in California. Populations of San Jose scale in California are normally below economic damage levels. Regulation of the populations at these levels is considered to be primarily due to the parasitoid wasp Aphytis aonidiae (Mercet). Average parasitism of 20% by this wasp has been found sufficient to keep San Jose scale at low density. However, like other Aphytis species, the adult wasps kill scale insects both by feeding on them and by parasitising them. The feeding by the adult wasps is considered important in population regulation. For instance, single A. aonidiae wasps in the laboratory were recorded killing an average 174 scale insects in various stages. Other related Aphytis wasps are also often the dominant parasitoids in San Jose scale populations overseas.

E. perniciosi has made a major contribution to control of San Jose scale overseas. The first generation of this wasp emerges as adults before the scale produces crawlers and this is thought to increase its effectiveness compared to other parasitoids. Apart from being introduced to many countries, E. perniciosi was mass reared and released in Germany from 1954 to 1974 in an inundative control programme. About 27 million wasps were released and San Jose scale populations reduced by 90%. Parasitism from the wasp increased from 20 to 75% in the first three years and averaged 34-43% over the entire period. Overall scale mortality ranged from 70-90%. There has been a substantial reduction of San Jose populations in Europe following this programme.

Overseas studies comparing the efficiency of Aphytis (external parasite) and Encarsia (internal parasite) wasps find that Aphytis are highly dependent on climate and less on the host scale whereas the reverse applies to Encarsia. Aphytis spp. are often found to be the most efficient of the scale natural enemies because (i) their life cycle is shorter than their host, (ii) they cause high mortality by both parasitism and predation, (iii) they are rather host-specific, (iv) they have few hyperparasites, and (v) they have excellent searching ability.