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Biological responses and control of California red scale Aonidiella aurantii (Maskell) (Hemiptera: Diaspididae)

Mohammed, Khalid Omairy (2020) Biological responses and control of California red scale Aonidiella aurantii (Maskell) (Hemiptera: Diaspididae). PhD thesis, Murdoch University.

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In many citrus areas around the world and within citrus-producing regions of Australia, the California red scale (CRS), Aonidiella aurantii (Maskell) (Hemiptera: Diaspididae), is considered the most important pests of citrus. The main biological control agents of Ao. aurantii in this zone are the parasitoid Aphytis melinus DeBach (Hymenoptera: Aphelinidae). In order to improve the biological control of Ao. aurantii several biotic and abiotic factors were studied, that may affect the efficiency of A. melinus in the laboratory and the field.

More concretely, reproductive potential and age-specific fecundity schedules of Ao. aurantii were studied in the laboratory at constant temperatures (20, 23 and 27°C), while the biological parameters of its parasitoid A. melinus were conducted at 27°C. Results revealed that the net reproduction rate (Ro) was considerably higher for Ao. aurantii than A. melinus, which reached 28.14 at 27°C, indicating its high reproductive capacity. Moreover, the net reproduction rate obtained for A. melinus indicates a low substitution potential for each female having Ao. aurantii as a host under laboratory conditions. The intrinsic rate of increase (rm) of A. melinus (0.188 ♀/♀/day) was significantly greater than that of Ao. aurantii (0.080) at 27°C.

Plants produce volatile organic compounds (VOCs) in response to herbivore attack, and these VOCs can be exploited by parasitoids of the herbivore as host location cues. The VOCs from non-infested and Ao. aurantii-infested citrus fruit were investigated using headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS). The data showed that more than 52 different compounds were identified, and different emissions associated attributed to herbivore activity were found for all fruit species (lemon, orange, mandarin and Tahitian lime). However, a single compound was exclusively produced by infested lemon fruit, while two compounds were significantly increased, and two compounds were only present in non-infested lemon. Five compounds were significantly increased in infested mandarins. For orange, five compounds were increased, and five compounds were exclusively presented in infested fruit. For lime fruit, eighteen of these compounds were increased, one was decreased, whereas five compounds were produced exclusively from infested lime fruit. Two putative herbivores-induced plant volatiles, d-limonene and β-ocimene, were significantly increased by Ao. aurantii infestation in all infested fruit, regardless of the citrus species.

Subsequently, the preferences of female parasitoid on infested or healthy fruit in olfactometer bioassays were evaluated. Then in order to understand the magnitude of volatile attractiveness, the innate attractiveness of VOCs to A. melinus females in varying densities were tested in the laboratory. The results of the olfactometer assays that tested the behaviour of A. melinus to the different compounds emitted from infested and non-infested citrus fruit showed no such preference when compared between non-infested and infested oranges, mandarins and lime fruit; whilst, there were significant preferences for lemon fruit infested with Ao. aurantii over non-infested ones. For assessment, the attraction of synthetic Herbivore induced plant volatiles (HIPVs), four different concentrations (5,10, 15 and 20 μl/ml) of d-l-limonene and β-ocimene were investigated. However, mated A. melinus females preferred the reward-associated VOC more than hexane control in the case of d-limonene at the tested dosages of 15 and 20 μl/ml, β-ocimene at tested dosages of 10, 15 and 20 μl/ml.

Finally, this study evaluated the dispersal ability of released A. melinus adults and their effect on the parasitism percentage, using d-limonene and β-ocimene with yellow sticky traps and scoring percentage parasitism on infested fruit. Under field conditions, the natural enemies’ effectiveness in controlling pests is largely correlated with their capability to spread towards infested crops. In this study, d-limonene and β-ocimene were examined for their attractiveness to California red scale parasitoid A. melinus in the field after augmentative releases. Field experiments demonstrated that lures baited with isolates of d-limonene and\or β-ocimene, which significantly attracted some species of natural enemies but had no significant impact on others. The number of A. melinus captured during the whole trial was greater in the traps treated with volatiles than the control. Finally, the overall parasitism rates were not increased by synthetic HIPV lures, but there was evidence that lures may increase parasitism of California red scale when there is a decrease in the amount of volatile organic compounds due to lack of healthy and infested fruit. In conclusion, HIPVs can potentially play important roles in attracting and exploiting natural enemies to reduce pest infestations.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): College of Science, Health, Engineering and Education
Supervisor(s): Ren, Yonglin and Agarwal, Manjree
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