Olfactory responses of Diaphorina citri Kuwayama (Hem.: Psyllidae) to chemical compounds Gamma-butyrolactone and Methyl salicylate in laboratory conditions

Background and Objective
The Asian citrus psyllid, Diaphorina citri Kuwayama (Hem.: Psyllidae), is one of the most destructive pests of citrus because of its ability to vector the causal bacterial pathogen of citrus greening disease. This is one of the most devastating diseases of citrus worldwide. Feeding on citrus by Asian citrus psyllid also induces release of Methyl salicylate (MeSA), suggesting that it may be a cue revealing location of conspecifics on host plants. MeSA is a ubiquitous aromatic ester that is a well-known plant stress signal and enhances defense against herbivores with repellent properties. Gamma-butyrolactone (GBL) is a primary component of the volatile ingredients in the female sex pheromone of the D. citri. The ability to decrypt plant volatiles is essential for herbivorous insects for their choice of host plants and mating partners and oviposition hosts. D. citri is an oligophagous insect with a host range restricted to plants in the Rutaceae family. That is why olfaction plays an important role in its host finding and selection process. The use of olfactory cues by adult psyllids in host plant selection has been reported in many olfactometer studies. In order to achieve the attractive and repellent compounds for pest management strategies against this pest, we investigated the effect of volatiles emitted from Synthetic GBL and MeSA on olfactory response of D.citri in laboratory bioassays.
Materials and Methods
This study evaluated the olfactory responses of D. citri to chemical stimuli from seven concentrations of GBL and MeSA including 0.001, 0.01, 0.1, 1.0, 10, 100 and 1000 µg/ ml with individual release of 30 females and 30 males. The GBL and MeSA (>98% purity) were purchased from Sigma Aldrich (USA) and diluted in dichloromethane. A glass Y-tube olfactometer was used in laboratory conditions (27±2°C, 60–80% RH, under a fluorescent 1600 lux light source, 14:10 (L:D) photocycle period and constant airflow of 300 ml/min).
Results
The results revealed that a significant proportion of male D. citri were attracted to GBL at the 1.0, 10, 100 and 1000 µg/ ml concentrations. But female D. citri showed no response to odors emanating from this compound. The results also indicated that a significant proportion of male and female D. citri were attracted to MeSA at the 0.01 µg concentration. However MeSA at the concentration 1000 µg/ ml was repellent for psyllids. Other concentrations (0.001, 0.1, 1.0, 10 and 100 µg/ ml) did not attract or repel the male and female D. citri.
Discussion
In this study, results showed that GBL at high concentrations is attractive only for male insects. These results indicate the role of GBL as a sex pheromone in attracting male insects. Feeding by herbivores induces the production of MeSA, so that this compound is often found among the volatile compounds of plants damaged by herbivores. MeSA plays a role in plant defense against herbivores by either repelling herbivores or by attracting biological control agents. The results of this study show that, probably, release of MeSA by host plants at low concentrations (0.01 μg), can be a cue of the presence of conspecific for D. citri in the early stages of host finding. With increasing infestation of host plants and increasing MeSA concentration, this compound repels the insects and that this method is considered a defensive tool for the plant. It is also possible that increasing the concentration of MeSA for the insect has this cue that if attracted, it will encounter a high population of conspecifics and the insect would not be attracted to high concentrations in order to avoid potential competition for food sources. Evaluation of chemical compounds that are attractive or repellent to D. citri, can contribute to the improvement in integrated management projects against this pest.