The Role of Snails and Ants in Transmitting the Avocado Stem Canker Pathogen, Phytophthora citricola

The sugary exudate appearing on bark lesions of Persea americana Miller and Persea indica plants after infection with Phytophthora citricola contained viable oospores and hyphal fragments in the field and in the greenhouse. This sugary exudate was a source of inoculum and dispersal of the pathogen within and between avocado plants. Spraying water onto lesions moved inoculum from the sugary exudate to wounds below. Water from sprinkler irrigation washed propagules into the soil around the plants. Viable propagules of Phytophthora citricola were identified in the feces of snails (Helix aspersa) that had fed on infected bark tissues. When these snails were moved to healthy plants, they made wounds on succulent tissue, and the infectious feces induced cankers. Ants (Iridomyrmex humilis) were attracted to the sugary exudate and also transmitted infectious propagules to wounds on avocado stems and to the soil. Control strategy for the avocado stem canker disease should consider control of vectors.

Studies on egg predation by ants (Hymenoptera: Formicidae) especially on the eucalyptus borer Phoracantha semipunctata (Coleoptera: Cerambycidae) in Portugal

Iridomyrmex humilis (Mayr) and some other ants, through preference, efficient foraging and recruitment, quickly removed artificially exposed batches of insect eggs including those of the serious pest of eucalyptus, Phoracantha semipunctata (Fabricius). This supports evidence of the importance of some ants as predators of the many insects that lay freely exposed eggs. However, P. semipunctata normally lays eggs in peeling bark crevices where the approximately 60% laid within gaps less than ca. 0.65 mm wide could not be reached even by small ants such as I. humilis and Pheidole pallidula (Nyland-er). In tree trunks, which can become heavily infested after felling, egg predation by ants seems unimportant compared with the later large mortality of competing larvae, so, egg predation cannot account for the observed striking positive correlation between presence of I. humilis and lack of Phoracantha semipunctata damage to standing trees. Egg predation and other effects of I. humilis may be more important on living trees which are attacked initially by relatively few P. semipunctata. This needs more detailed investigation as do the overall egg predatory roles of ant species in biological control.