Abstract
Problem statement: Evidence of rejection or acceptance by phytophagous pests of citrus plants treated with mineral oils suggests that impa cts of oils may be related to detection by olfactor y and/or contact chemoreceptors of repellent oils or plant volatiles. Alternatively, the oils may mask o r suppress attractant host plant volatiles. To date, no studies have reported that oils elicit an olfact ory response in Diaphorina citri Kuwayama (Hemiptera: Psyllidae). Therefore, we undertook two studies to determine how oil deposits on citrus leaves infl uenced the behaviour of D. citri . Approach: The attractiveness of citrus leaves treated separately with an nC21 horticultural mineral oil and an nC24 agricultural mineral oil to D. citri was evaluated under laboratory conditions. In the first of two studies, the oils were applied as 0.5% aqueous emul sions to mandarin leaves and responses of adults determined after they walked on surfaces saturated with either distilled water or 2% aqueous emulsions of the oils. In the second study, responses of adul t psyllids to host-leaf volatiles, mineral oil vola tiles emanating from leaves dipped in 2% v/v aqueous emulsions of each oil and similarly treated filter paper paired with water-dipped leaves were determined in single, linear-tube olfactometers. Results: In the first study, deposits of both oils significa ntly reduced the proportion of D. citri attracted to citrus leaves. Prior exposure of the insects to oil deposi ts did not influence this response. However, prior contact with the HMO slightly reduced the proportio n of psyllids alighting on the water-dipped leaves. In the second study, oil volatiles emanating from f ilter paper significantly reduced the attractivenes s of water-dipped mandarin leaves. Conclusion: These results indicate that application of the oil s to the mandarin leaves may have: (a) suppressed release of attractant host plant volatiles; (b) masked attractant host plant volatiles; (c) led to the rel ease of repellent volatiles from leaves and/or (d) to adults being repelled by oil volatiles. The results from the study using the olfactometer suggested th at detection of mineral oil volatiles by olfactory rec eptors of adult D. citri contributed to their response to oil deposits on the mandarin leaves.
Highlights
In Asia, the Asiatic citrus psyllid, Diaphorina citri Kuwayama [Hemiptera: Sternorrhyncha: Psyllidae] is the only known vector of huanglongbing, a devastating disease of citrus that is known as Citrus Vein-Phloem Degeneration (CVPD) in Indonesia and more widely as citrus greening (Soelaeman, 1981; Aubert, 1990; da Graca, 1991; Halbert and Manjunath, 2004)
In all choice tests, where adult D. citri were given the option of alighting on mandarin leaves dipped either in water or in 0.5% aqueous emulsions of Horticultural Mineral Oil (HMO) or Agricultural Mineral Oil (AMO) (Fig. 2), significantly larger proportions of psyllids alighted on leaves dipped in water only
Of the different proportions moving towards the odour source was determined by χ2 tests. These results suggest that oil volatiles: repel adult psyllids; interfere with psyllid recognition of attractant leaf volatiles and/or influence the release of attractant or repellent plant volatiles from the water-dipped leaves
Summary
In Asia, the Asiatic citrus psyllid, Diaphorina citri Kuwayama [Hemiptera: Sternorrhyncha: Psyllidae] is the only known vector of huanglongbing, a devastating disease of citrus that is known as Citrus Vein-Phloem Degeneration (CVPD) in Indonesia and more widely as citrus greening (Soelaeman, 1981; Aubert, 1990; da Graca, 1991; Halbert and Manjunath, 2004). We determined the response of adult psyllids with Olfactometer responses of adults to citrus and oil oil-contaminated tarsi to deposits on leaves of an nC21 volatiles: Each single, linear-track olfactometer was Horticultural Mineral Oil (HMO) and an nC24 constructed from a 31.5 mm diameter (internal), 260 We mm long, transparent glass tube, sealed at the proximal used single linear-track olfactometers to determine end with a rubber plug connected by a 2.3 mm diameter responses of adult psyllids to oil-treated leaves and oil- (internal) plastic tube to a 3 W aquarium aerator impregnated filter paper.
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