Development of phytosanitary irradiation against Aceria litchii (Trombidiformes: Eriophyidae) on lychee

Abstract

The lychee erinose mite, Aceria litchii (Keifer) (Trombidiformes: Eriophyidae), is the most important pest of lychee (Litchi chinensis Sonn. (Sapindales: Sapindaceae) in parts of China, India, Southeast Asia, South Africa and Brazil. This study sought to develop the basis for phytosanitary irradiation of lychee to provide quarantine security against this pest. New methodology had to be devised for this purpose because the adult, the largest life stage—about 200 μ long—cannot be seen without magnification, and because this species does not survive more than a few d even on detached young lychee leaves, or under other artificial conditions. Initially we adapted a method devised by Azevedo et al. (2013) for keeping the adults alive long enough to evaluate the lethal effects of candidate acaricides for at 48 h post treatment. We collected infested leaves from a lychee orchard and irradiated then with doses increasing by increments of 200 Gy in the range 0–2,000 Gy. Each infested leaf had 30 to 40 adult mites. Each of 3 replicates involved ~816 adult mites and ~2,450 adult mites per treatment. Because of the presence of predators hidden within the erinea, we collected 30 adult mites per replicate immediately after irradiation, and placed them in a 14-cm-diam petri dish with a new young lychee leaf and moist cotton. We covered each petri dish with parafilm® to prevent escape of mites and loss humidity. At 24, 36, and 48 h post irradiation, we counted the numbers of live and dead mites. At 24 h post irradiation mortality occurred only in 1,800 and 2,000 Gy treatments, and it was only 1.7% in both treatments. At 36 h mortality had increased to 11.1 and 24.4% in the 1,600 and 2,000 Gy treatments, respectively. At 48 h statistically significant mortality occurred with all doses in the 200–2,000 Gy range; and it was 73.3% and 100.0% in the 1,800 and 2,000 Gy treatments, respectively. Since 2,000 Gy is unacceptable for phytosanitary irradiation, a dose had to be identified that prevents reproduction, i.e., a F1 generation. To prolong the survival of irradiated mites for at least 13 d, which appears to be the generation time of Aceria spp., we cut infested fragments of leaf blades, examined the under a microscope to remove adults and immature forms of predators mites, placed them in petri dishes, irradiated them with doses increasing by increments of 100 Gy in the range 0–500 Gy, and glued each irradiated leaf fragment onto a newly flushed leaf of a potted lychee tree in a screened greenhouse. Each treatment had 4 replications, and each consisted of ~10 adult mites for a total of ~40 adult mites per treatment in the first trial and 18 adults per replicate for a total of ~72 mites in the second trial. By 72 h post irradiation the mortality rate was considerably greater with all doses in the range 200–500 Gy than at 48 h in the earlier experiment, and the percent mortality with 100 Gy was significantly greater than in the control (0 Gy). Therefore, 72 h post irradiation seems to be the minimum time required for the lethal symptoms of irradiation to develop within the dose-range that is relevant to phytosanitary irradiation of fresh plant materials. In both the first and the second trials, moderate symptoms of erinose developed during 18 d post irradiation in the treatments with 0–300 Gy, but no erinose symptoms developed in the 400 and 500 Gy treatments. All symptomatic leaves displayed the patchy growth of erinea, i.e., abnormal felt-like hairs on the abaxial leaf epidermis. In addition by d 18 some galls had been formed on the upper sides of some of the leaves with erinea. These data show that irradiation of A. litchii with ≥ 400 Gy prevented it from reproducing, which is a critically important criterion of phytosanitary irradiation. Additional experiments are needed in the 300–400 Gy range to find the minimum dose required for phytosanitary irradiation.