Control of Yellow Scale on Oranges, 1988

Abstract

Abstract A randomized complete block experiment with 11 single-tree replicates and 8 treatments was conducted in a commercial field of mature Valencia orange trees near Porterville, Calif. The experiment was focused on yellow scale, but California red scale, Aonidiella aurantii (Maskell), is also very common in the Porterville area and is very difficult to distinguish from yellow scale in the field. TJierefore, all sampling refers to “scale insects,” which probably includes both species. However, several criteria indicated that yellow scale was the predominant species: 1) pheromone traps, 2) the relative lack of scale on twigs (yellow scale infests only leaves and fruit while red scale infests twigs, leaves, and fruit), 3) a preliminary sample from this field in 1987 of scale that were slide mounted for positive identification, and 4) the general appearance of the scale when sampling was conducted in this study (although individual scales are difficult to positively identify as yellow or red scale in the field because their range of body color overlaps between the two species, the “average color” differs between the two species so that populations of scale can be judged to be predominantly red or yellow scale). The 88 trees in the experiment were assigned to 11 blocks based on pretreatment infestation levels of scale insects so that the 8 trees within each block had similar pretreatment infestations of scale. Each of the 8 trees in each block then was assigned randomly to a treatment so that each treatment was represented once in every block. Pretreatment infestation of scale was estimated by sampling 50 leaves/tree and recording the number of leaves that had any scale that appeared to be alive. Posttreatment infestation of scale was estimated by recording the presence or absence of scale on 20 fruit/ tree. In the first posttreatment evaluation, an attempt was made to include only live scale. Because of the difficulty in distinguishing live from dead scale and because infested fruit are downgraded in the packinghouse regardless of whether scale are live or dead, both live and dead scale were included in determining whether a fruit was infested in the second posttreatment evaluation, which was only a few d before harvest.