The phytophagous—plant feeding—insects interact with their host plants through various signaling information, such as smell and taste, as well as visual and tactile information. Scientists have conducted extensive research on the role of host plants’ odors on pest orientation. In the article “Olfactory Response of the Spotted Asparagus Beetle, Crioceris duodecimpunctata (L.) to Host Plant Volatiles” by Pistillo et al. (2021), authors investigated the olfactory responses of a phytophagous insect called spotted asparagus beetle (Crioceris duodecimpunctata). As a major pest of asparagus cultivations, C. duodecimpuntata are found in most areas of native asparagus production and they are expanding their range across Canada and the United States.
“Crioceris quatuordecimpunctata up” by Siga, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
C. duodecimpuntata deposit their eggs on thin asparagus branches. The larvae gain the nutrients for growth from asparagus berries, and fall into the soil to pupate after their maturation. In the spring, they emerge from their shelters in asparagus stems and feed on the spears. Their life cycles cause problems for the commercial and agricultural aspects of the asparagus. For example, they chew on the surface layer of the plants, affecting the plants’ aesthetic as a commercial product and negatively influence the process of photosynthesis. What’s more, the larvae’s feeding behavior reduces seed production.
Life Cycle of C. quatuordecimpunctata
By understanding how C. duodecimpuntata are attracted by their host plant and what chemical signaling compounds play a role in this attraction, this research could contribute to improving the timing of control measurements and enhancing the direct control methods with lower impacts. To investigate the general question, Pistillo et al. proposed three experiments: 1) the male and female responses towards intact and damaged cladophylls; 2) the antennal sensitivity and selectivity towards a range of asparagus volatile organic compounds (VOCs); 3) the male and female responses towards a most actively responded compound called (Z)-3-hexen-1-ol.
In the experiment to investigate C. duodecimpuntata response towards different conditions of cladophylls, males and females presented different behaviors. Males showed a clear preference for the odor of intact cladophylls over clean air, and they spent more time on the intact cladophylls. Males did not have a preference between mechanically damaged cladophylls and clean air, but they spent more time with the damaged cladophylls, which released a greater amount of volatile. Meanwhile, females did not show any significant attraction toward either side when presented with intact plant versus clean air, or with damaged plant versus clean air.
Researchers used a method called Electroantennographic (EAG) assay to analyze the antennal selectivity and sensitivity. In all compounds tested, a variety of them induced the highest EAG responses including (Z)-3-hexen-1-ol, 1-octen-3-ol, 3-heptanone, (E)-2-hexenal, heptanal, nonanal, hexanal, (±)-linalool, as so on. The dose of compounds positively influences the amplitude of EAG responses. Interestingly, the mean response of males towards (Z)-3-hexen-1-ol was significantly higher than the mean response of females.
In the last part of this experiment, researchers took a closer look at the C. duodecimpuntata behavioral responses towards (Z)-3-hexen-1-ol. The 1 μg dose was not sufficient to elicit any responses, but males showed significant interest to the 10 μg dose; and when presented with 50 μg dose, males would stay for a longer time in the mineral-oil filled control arm instead of the 50 μg dose arm. However, neither 10 μg dose nor 50 μg dose elicit any significant behavioral response in females.
The sexual dimorphism in the beetles’ responses for intact and damaged cladophyll
One major finding in the article is the high antennal sensitivity of males and females towards a variety of volatile compounds, in which (Z)-3-hexen-1-ol was the compound with the greatest reaction. The other major finding is that the odor information of the host plant asparagus could elicit sexually-dimorphic behavioral responses. By calling this “sexually-dimorphic,” we are indicating that the C. duodecimpuntata presented different behaviors between the sexes within the same species. Because the research conducted statistical analysis based on the sexes of the beetles, and showed that in some cases males showed first-choice preferences and longer time spent towards the cladophylls, we could conclude that males are more likely to be attracted by certain odor signals.
The presence of sexual dimorphism could help us gain more insights into the mating behavior of C. duodecimpuntata, and how they exploit the ecology during mating. Similar sexual dimorphism of host plant volatile preference could be found in other species. For example, male Colorado potato beetles prefer to orient towards specific blends of volatiles, but the females showed little influence by the volatiles. Such host finding behavior are also seen in the asparagus fly as well, in which the males with great volatile sensitivity initiate the colonization, and begin to release sex pheromone very soon after the colonization. The combination of volatile and pheromone attracts virgin females.
These major findings could contribute in multiple aspects. To begin with, we identified the compounds that C. duodecimpuntata are the most sensitive to. By studying the physiology of the host plant, we could design the control methods based on when these chemical signals are the most likely to release, and come up with potential methods to block these attractive signals from releasing to a further distance. What’s more, we could potentially take advantage of the male beetles’ reliance on certain chemicals, and implement methods to prevent their mating behaviors from happening on the host plant.
“Crioceris quatuordecimpunctata couple” by Siga, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Once we have discovered that (Z)-3-hexen-1-ol plays an important role in the asparagus’ attraction of C. duodecimpuntata, especially in males, we could start to plan out our next steps of exploration. What is the male pheromone that cooperates with asparagus volatiles? What is this blend of this male-produced aggregation pheromone that attracts females? With the knowledge of volatile-beetle coordination, what approaches could we utilize to prevent pests from mating and reproducing on their plant hosts?
Reference:
Pistillo, O. M., D’Isita, I., & Germinara, G. S. (2021). Olfactory Response of the Spotted Asparagus Beetle, Crioceris duodecimpunctata (L.) to Host Plant Volatiles. Journal of Chemical Ecology, 48(1), 41–50. https://doi.org/10.1007/s10886-021-01323-5
