Citricola Scale Insecticide Trial, 2018

Citricola scale causes loss of tree vigor, yield reduction, and downgrading of fruit in the packing house due to sooty mold growth on honeydew deposited by the insect. A field trial was conducted in a 36-yr-old ‘Washington’ navel orange orchard at the Lindcove Research and Extension Center, Exeter, California, to compare various rates of insecticides for efficacy against citricola scale. Insecticides were applied using 200 PSI and 500 gpa to 10 trees per treatment using a 100 gal high-pressure D30 diaphragm pump sprayer with mechanical agitation. Treatments were assigned to trees on 15 May 2018 in an RCB design based on pretreatment counts of mature female citricola found on five twigs (~40 cm in length) randomly selected from the northeast corner of the tree. All insecticide treatments were applied with 0.5% Summer IAP oil on 31 Jul or 1 Aug, when the population consisted of first-instar nymphs infesting leaves. Posttreatment sampling consisted of counts of first-instar citricola nymphs infesting five leaves collected from the NE corner of each sample tree during Sep and Oct 2018. An additional sample of mature female citricola on five twigs per tree was collected in spring on 22 Apr 2019. Data were analyzed using one-way ANOVA after testing for NS block effect. Differences in the mean numbers of nymphs per leaf or adult female scale per twig were determined using Fisher’s protected least significant difference test (P ≤ 0.05) after log₁₀(x + 1) transformation of numbers.

During the fall of 2018, all treatments applied significantly reduced the number of nymphs below the economic threshold of 0.5 nymphs/leaf (Table 1). In the spring of 2019, all treatments had significantly reduced the number of adult females below the threshold of 1.0 adult/twig when compared to the untreated check.¹

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