Plants attacked by insects and mites release blends of herbivore-induced plant volatiles (HIPVs), which act as chemical cues that attract predators and/or repel herbivores alongside alerting neighbouring plants. The chemical profiles of avocado (Persea americana Mill. Cultivar Hass) shoots infested with the foliar mite Oligonychus yothersi (Acari: Tetranychidae) were analysed using a gas chromatograph coupled to a mass spectrometer (GC-MS); the different collections of volatiles were performed under semi-field conditions in summer. Four infestation treatments were applied to the avocado shoots (uninfested plant or control; low, medium and high mite-infestation levels). The chemical profiles collected in situ changed qualitatively and quantitatively after infestation, increasing the emissions of (Z)-β-ocimene, (E)-β-ocimene, β-linalool, α-farnesene, farnesyl cyanide and methyl salicylate (MeSA) as opposed to those of the uninfested shoots. The amount of HIPVs emitted by the avocado shoots with the highest O. yothersi population density (250 ± 10 mites per leaf) were greater than those of the shoots infested with the lower population density (60 ± 10 mites per leaf); to the emissions of (E)-β-ocimene, sesquiterpenes and MeSA at higher amounts even after seventeen days of continuous infestation. Likewise, the emissions of HIPVs increased with each infestation level; also fluctuating between collecting dates, which indicates that environmental conditions maximized or constrained those emissions. In addition, the damaged foliar area increased with higher O. yothersi population densities after seventeen days of infestation. These results indicate that higher mite-population densities and continuous O. yothersi-feeding increases the amounts of HIPVs emitted by O. yothersiinfested shoots in summer.
ARTICLE HISTORY
The avocado red mite, Oligonychus yothersi (McGregor) (Acari: Tetranychidae), is a polyphagous pest that causes damage to a wide range of plants. In Chile, is commonly found in avocado (Persea americana Mill.) orchards, and the plant damage is expressed as a reduction of photosynthetic activity in leaves, causing defoliation of trees under severe attack conditions. Oligota pygmaea (Solier) (Staphylinidae) and Parastethorus histrio (Chazeau) (Coccinellidae) microcoleopteran predators are found on 'Hass' avocado orchards, infested by O. yothersi. This study investigated the role of volatile compounds released by avocado in response to O. yothersi infestation in the tritrophic interaction avocado-O. yothersi-O. pygmaea-P. histrio. Plant volatiles were collected from live plants by headspace technique on Porapak Q traps and analyzed by gas chromatography coupled to mass spectrometry (GC-MS). In the chemical analysis, several chemical groups were identified including terpenes, aromatic compounds, and green leaf volatiles. Eight chemical compounds were detected only in the infested plants volatile profile, and differences were observed in the emission of 10 compounds between infested and uninfested plants (P < 0.05). The behavioral responses of the insects to plant volatiles were evaluated in a choice-test using a Y-tube as olfactometer. Uninfested plant volatiles were significantly attractive to O. yothersi, while infested plants volatile extract repelled the mites. Two compounds, R-limonene and α-pinene, elicit an attractant behavioral response from O. yothersi. Oligota pygmaea and P. histrio females registered a strong attraction to volatiles from infested plants, and the compounds (Z)-β-ocimene and methyl salicylate detected only in infested plant also were attractive to predators.
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