Research Article |
Corresponding author: Qingfeng Tang ( tangqf55@163.com ) Academic editor: Mark Shaw
© 2016 Qingfeng Tang.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Tang Q (2016) Olfactory responses of Theocolax elegans (Hymenoptera, Pteromalidae) females to volatile signals derived from host habitats. Journal of Hymenoptera Research 49: 95-109. https://doi.org/10.3897/JHR.49.7697
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The responses of female Theocolax elegans (Hymenoptera: Pteromalidae) to volatile signals derived from its host habitats were investigated in a static four-chamber olfactometer. Our results demonstrated that T. elegans females, irrespective of experience, were apparently attracted by the odors released from the faeces of Sitophilus zeamais larvae and adults, which has never been investigated in previous researches. Moreover, we compared the responses of female parasitoids to odors released from grains of rice damaged by S. zeamais larvae, S. zeamais males, S. zeamais females, and mechanically. Artificially damaged grains do not emit large amounts of the volatiles that attract experienced parasitoid females to grains damaged by S. zeamais larvae. Further experiments revealed that experienced T. elegans females were more strongly attracted to rice grains which had been infused with extract from the heads and thoraxes of weevil larvae than to rice grains that had been infused only with sodium phosphate. The behavior of T. elegans females to odors released from pheromone-releasing S. zeamais males on healthy grains and unmated S. zeamais females on healthy grains were observed. The results revealed that S. zeamais aggregation pheromones are not useful signals for T. elegans females, irrespective of experience. Based on these observations, T. elegans females used faeces to detect potential hosts. Our results revealed that head and thorax of S. zeamais larvae induces rice grains to release volatiles attractive to T. elegans females, particularly after experience.
Chemical cues, Multitrophic interaction, Olfactory host finding, Parasitoids, Sitophilus zeamais , Theocolax elegans
Many parasitoid insects are orientated to the chemical cues released by their target or its environment. The successive steps of the orientation process, or host-finding, have been described as host habitat location, host location, host recognition and host acceptance (
The present paper is devoted to a tritrophic system consisting of the parasitoid Theocolax elegans (Westwood) (Pteromalidae), the maize weevil Sitophilus zeamais (Motschulsky) (Curculionidae), and grains of rice Oryza sativa L. (Poaceae). The beetle S. zeamais is one of the most destructive insect pests of stored cereals in tropical and sub-tropical regions (
The present study investigates the sources of olfactory cues that T. elegans uses to locate infested rice grains. The potential sources investigated were the weevil larvae, their faeces, aggregation pheromone, and the grain material. We also studied the effect of experience with a host on the attractiveness of host-related stimuli to adult females of T. elegans.
All insect cultures were kept at 26±2 °C, 70±5% relative humidity (r.h.) and a photoperiod of L14: D10. To rear T. elegans, 50 newly emerged adult wasps were placed into Petri dishes (9 cm diameter, 1 cm high) with about 50g of rice grains infested by 3rd-4th instar larvae of S. zeamais and kept there until their death. After a developing time of 19–25 days, emerged parasitoids from the next generation were collected daily from each Petri dish. To rear S. zeamais, 30 adults were allowed to oviposit into 300ml of rice grains with about 14% moisture content in glass jars (8 cm diameter, 10 cm high). To obtain unmated males or females of S. zeamais, adults were separated by dimorphic rostral characteristics within 12 h of emergence (
Parasitoids used in experiments were about 2 d old. To obtain experienced parasitoid females, recently emerged (< 24 hr old) wasps were placed in Petri dishes containing rice grains infested by weevil larvae and adults of S. zeamais. Females were allowed to mate and oviposit for 3 days. Subsequently they were removed and kept in Petri dishes with moistened filter paper until they were used in the experiments on the following day.
In accordance with
The response of female parasitoids towards different odor samples was examined using a static four-chamber olfactometer as described by
Evaluations were performed in a constant temperature and humidity room at 26±2 °C and 70±5% r.h., in darkness under red light to avoid distraction of parasitoids by light but to enable observations. Behavioural data were visually recorded using a stopwatch. To avoid biased results due to possible orientation preferences of the parasitoids, the position of the olfactometer was rotated clockwise by 90° after every insect. Contamination of the walking arena with sample odors or by possible pheromones of the parasitoids was avoided by cleaning the walking arenas and glass plates with ethanol and demineralized water before each insect. To avoid biased results due to possible human contamination of experimental material, disposable gloves were worn when carrying out the experiment. For all experiments, odor samples were renewed after five parasitoids each.
Fifty parasitoids were tested for each type of sample. Each individual parasitoid was used only once. At the start of each bioassay, the parasitoids were released individually in the center of the walking arena and their arrestment times in the four sectors above the arena were registered for 600 sec. The time the parasitoids spent walking in the areas directly above the Petri dishes with odor samples was compared to the areas with control Petri dishes and used to assess the arrestant effect of an odor sample. Parasitoids that walked for less than 50% of the total observation time were not included in the statistical analysis.
Three different experiments were conducted using the static four-chamber olfactometer descried above. (1) 100 mg of faeces of S. zeamais larvae (LF) versus three empty Petri dishes (C); (2) 100 mg of faeces of adult S. zeamais males (MF) versus three empty Petri dishes (C); (3) 100 mg of faeces of adult S. zeamais females (FF) versus three empty Petri dishes (C).
Fifty parasitoids were tested for each experiment. Larval faeces from S. zeamais were obtained by sieving grain infested by 3rd-4th instar weevil larvae. Adult faeces from S. zeamais were obtained by sieving grain infested by unmated weevils.
We conducted a series of experiments to test the attraction of T. elegans females to herbivore-induced odors emitted from rice grains. (1) 50 grains infested by weevil larvae from which larvae, faeces, and egg plugs had been removed [infested grain only; (LIGO)] versus 50 healthy grains, which had been artificially damaged [artificially damaged grain; (AG)] and two empty petri dishes (C); (2) 50 grains infested by adult S. zeamais males from which weevils and faeces had been removed [infested grain only; (MIGO)] versus 50 healthy grains, which had been artificially damaged (AG) and two empty petri dishes (C); (3) 50 grains infested by unmated adult S. zeamais females from which weevils and faeces had been removed [infested grain only; (FIGO)] versus 50 healthy grains, which had been artificially damaged (AG) and two empty petri dishes (C).
Fifty parasitoids were tested for each experiment. The infested grain was obtained by dissecting grains infested by 3rd-4th instar weevil larvae, from which the larvae were removed, and removing faeces using a fine brush. Artificially damaged grains were cut with scissors, knives or needles in order to better mimic damage caused by the gnawing larvae or adults.
Extract from the heads and thoraxes of S. zeamais larvae were prepared using the method described by
A hole approximately 2 mm deep was drilled into the base of healthy rice grains with a 1 mm diameter drill. Some of the rice grains (EG) with dug holes were infused with 2μl extract from the heads and thoraxes of S. zeamais, the remaining (SG) with dug holes were infused with 2μl of 0.05 M sodium phosphate (pH 8.0) alone. EG and SG were separately placed in petri dishes in humidifiers containing a saturated sodium chloride solution at 65%–70% r.h. for seven days before being used.
Two different experiments were conducted using the static four-chamber olfactometer descried above. (1) 1000 μl extract from the heads and thoraxes of 3rd-4th instar weevil larvae (E) versus 1000 μl sodium phosphate (S), and two empty petri dishes (C); (2) 50 EG that had been infused with 2 μl extract from the heads and thoraxes of 3rd-4th instar weevil larvae (LEG) versus 50 SG, and two empty petri dishes (C). Fifty parasitoids were tested for each experiment.
Fifty parasitoids were tested for each experiment. One experiment was conducted using the static four-chamber olfactometer to test the attraction of T. elegans females to aggregation pheromone of S. zeamais. (1) 20 pheromone-releasing S. zeamais males on 100 healthy grains (GM) versus 20 unmated S. zeamais females on 100 healthy grains (GF) and two empty petri dishes (C).
The Friedman ANOVA was used to test for differences between the four areas. In case of significant differences the Wilcoxon-Wilcox-test for multiple comparisons was used to determine which sectors are different from each other.
Both naive and experienced parasitoid females spent significantly (Naive, LF, P=0.0009; Naive, MF, P=0.0015; Naive, FF, P=0.0006; Experienced, LF, P=0.0022; Experienced, MF, P=0.00016; Experienced, FF, P=0.0008) more time walking in the sector above the faeces than in the sectors with the control (Figs
Mean walking time (± SD; n = 50) of naive females of Theocolax elegans in a four chamber olfactometer. LF: areas above Petri dishes with faeces of
Mean walking time (± SD; n = 50) of experienced females of Theocolax elegans in a four chamber olfactometer. LF: areas above Petri dishes with faeces of
Both naive and experienced T. elegans females spent significantly more time in treatment odor fields compared to control odor fields in experiments involving either infested rice grains by S. zeamais from which weevil, faeces, and egg plugs had been removed or artificially damaged rice grains (Figs
Mean walking time (± SD; n = 50) of naive females of Theocolax elegans in a four chamber olfactometer. LIGO: areas above Petri dishes with grains infested by weevil larvae from which larvae, faeces, and egg plugs had been removed, MIGO: areas above Petri dishes with grains infested by adult
Mean walking time (± SD; n = 50) of experienced females of Theocolax elegans in a four chamber olfactometer. LIGO: areas above Petri dishes with grains infested by weevil larvae from which larvae, faeces, and egg plugs had been removed, MIGO: areas above Petri dishes with grains infested by adult
Naive T. elegans females showed no statistically significant difference (LIGO, P=0.2512; MIGO, P=0.1693; FIGO, P=0.2178) in choice between rice grains infested by S. zeamais and artificially damaged rice grains (Fig.
Experienced T. elegans females showed no statistically significant difference (P=0.4659) in choice between areas containing extract from the heads and thoraxes of 3rd-4th instar weevil larvae (E) and sodium phosphate alone (S) (Fig.
Mean walking time (± SD; n = 50) of experienced females of Theocolax elegans in a four chamber olfactometer. E: areas above Petri dishes with 1000μl extract from the heads and thoraxes of 3rd–4th instar weevil larvae, S: areas above Petri dishes with 1000 μl of sodium phosphate alone, C: areas above control Petri dishes. Bars with different letters are significantly different at P < 0.05 (Friedman ANOVA followed by Wilcoxon-Wilcox-test for multiple comparisons).
Experienced T. elegans females were more strongly (LEG, P=0.0381) attracted to the rice grains which had been infused with 2μl extract from the heads and thoraxes of weevil larvae over the rice grains which had been infused only with 2μl sodium phosphate (Fig.
Mean walking time (± SD; n = 50) of experienced females of Theocolax elegans in a four chamber olfactometer. LEG: areas above Petri dishes with grains which had been infused 2 μl extract from the heads and thoraxes of 3rd-4th instar weevil larvae, SG: areas above Petri dishes with grains which had been infused 2 μl of sodium phosphate alone, C: areas above control Petri dishes. Bars with different letters are significantly different at P < 0.05 (Friedman ANOVA followed by Wilcoxon-Wilcox-test for multiple comparisons).
Naive and experienced T. elegans females spent significantly more (P < 0.05) time in treatment odor fields compared to control fields of the olfactometer in all experiments (Figs
Mean walking time (± SD; n = 50) of naive females of Theocolax elegans in a four chamber olfactometer. GM: areas above Petri dishes with 20 pheromone-releasing
Mean walking time (± SD; n = 50) of experienced females of Theocolax elegans in a four chamber olfactometer. GM: areas above Petri dishes with 20 pheromone-releasing
The results show clearly that the naive and experienced T. elegans females can be attracted by faeces of host S. zeamais. Results similar to those for S. zeamais faeces are responsible for attracting female Lariophagus distinguendus (
Parasitoids of phytophagous hosts can be attracted directly by infested host plants (
The experimental data provided here demonstrates that the behavior of experienced T. elegans females is not affected simply by extract from the heads and thoraxes of 3rd-4th instar weevil larvae or sodium phosphate. More interestingly, when the extracts from the heads and thoraxes of S. zeamais were applied to the artificially damaged grains, the T. elegans females can be strongly attracted, which apparently indicated that the specific defense chemical cues attracting T. elegans females were released as a result of the presence of S. zeamais extract. It appears that the extract from the heads and thoraxes of S. zeamais larvae induced the wounded grains to release volatile chemicals for attracting T. elegans. Thus it seems likely that specific parasitoid attracting volatiles are also induced by the saliva of the feeding weevil larvae.
It appears that many predatory and parasitoid arthropods are able to intercept the sex pheromone signals of their prey or hosts. For example, Bedard (as cited in
Under conditions whereby specific pest-derived chemical cues are used by natural enemies (
I am very grateful to Dr. Mark R. Shaw for modification and insightful comments that have improved the text. This work was financially supported by the Key Program of Natural Science Foundation of the Higher Education Institutions of Anhui Province, China (Grant No. KJ2014A075), Key Project for University Excellent Young Talents by Anhui Province, China (Grant No. gxyqZD2016035) and National Natural Science Foundation of China (Grant No. 31500313).