Distribution and abundance of the oleander scale (Homoptera: Diaspididae) on jojoba

The oleander scale, Aspidiotus nerii Bouche, is a polyphagous, cosmopolitan species almost always present on jojoba, Simmondsia chinensis (Link) Schneider, in its native Sonora desert and on plants introduced to Israel's Negev desert. Monthly samples of oleander scale taken at an experimental field at Omer, in the northern Negev, showed that the overall population of the scale has 2 prominent peaks; one in spring and the other in autumn. There was a marked reduction of the population in the period May-August when jojoba fruits ripen. Data on the abundance of immature stages suggested 3 generations annually. Comparison of scale densities on male and female jojoba plants showed no difference in overall density. However, the sex ratio of 2nd instars heavily favored females on male plants. This difference was significant in 8 of 13 mo, as was a weighted average over the whole period. The distribution of male and female 2nd instars on the upper and lower side of leaves differed significantly in 7 of 12 mo, although not when the whole year was considered. Analysis of the spatial distribution of scales by Taylor's power law showed that the scale distribution is highly aggregated on jojoba. On irrigated and fertilized plants the scale density was very high, whereas on untreated plants the scale was almost absent, suggesting that cultivation is responsible for the large oleander scale populations on jojoba. Despite its high density on jojoba, oleander scale has not emerged as an economic pest in either Israel or the United States. A conjecture is offered to explain this.