The Cicada-Killing Wasp Project
(From Jon Hastings) The most important questions that I have (at least for now) are:
- do female wasps hunt opportunistically, or are they selective?
- is the large difference in mean prey size between locations due to differences in prey availability or due to the the large difference in mean size of the female wasps?
- related question: is the prey of individual female wasps more determined by their size or by their location?
- Is there evidence that individual female wasps specialize on prey by either species or by sex?
- Is there evidence that males are not as meaty even if they're the same size as females? -- Look at wing length versus dry mass.
- This could have implications for wing-length versus mass, if we've combined males and females to do the model.
What we know about Cicadas and Wasps
- Ground-nesting and colonial
- Males are territorial and the larger males have mating advantages.
- Nests are built and provisioned by the females
- 1-2 eggs are placed in cell: 1 for males and 2 for females
- Mass provisioners
- Univoltine (i.e. produces one generation per year)
- Mean female wasp mass (967 mg) larger than mean male mass (424 mg)
- At most locations only 1-2 species of cicadas of similar size available for hunting.
- Exception: Newberry and St. John, Florida (less than 100 km apart from each other)
- Newberry has really small wasps and St. John has really large wasps.
What would we like to know about Cicadas and Wasps
- Do some wasps place more than 1-2 cicadas in their nest? If so, how many and why?
Prior to our focal studies of the Florida wasps this summer, there had been no reports of S. speciosus provisioning nests with more than three cicadas, and the cells found with 3 cicadas have been very few in number. The “1 for boys, two for girls rule” has become part of the dogma. However, our focal studies in both Florida locations suggest that when females provision with the 2 small cicada species (Neocicada and Diceroprocta), they place from 4-8 cicadas in each cell. Prior to our Florida work, all published studies of this wasp species reported that cicadas of the genus Tibicen were used as prey; all species of cicadas in this genus are fairly large. Next summer we plan to excavate a few nests in which female wasps provisioned with the small cicadas, which will allow us to directly address this issue.
- What causes some wasp to only place 1-2 and other wasps to place more?
We have no idea. However, in other species of provisioning wasps, such as in some wasps that provision nests with spiders, females appear to have some mechanism of assessing the mass of their provisions. They tend to provide each nest cell with about the same mass of spiders; when the prey are small, they put many prey in each cell. One idea is that the nest cells are fairly uniform in size; perhaps the females place prey in each cell until it is filled to a certain programmed level.
- Does time have an effect on which species of cicada the wasp will bring back to the nest.
It appears that hunt time varies among the prey species. I do have hunt time data, but probably not enough to make a comparison. If we group the two study sites together, it does seem as if it takes longer for females to return to the nest with one of the larger prey species. I will send you the data later today. Does time of day have an effect? It doesn’t seem to, as we followed several females throughout the day, and their hunting preferences did not change. We noticed no change in the cicada singing throughout the day—all different species seem to be active throughout the entire day time.
- Knowing that there was a difference between the wasp populations in Newberry and St. John, what exactly is the contributing factor? Could it be rural vs. urban areas?
That is my guess. The Newberry aggregation was in manicured lawns of million $ homes. We talked to neighbors and to an exterminator—people in the community hate these wasps because they dig up their lawns. In fact many have taken steps to exterminate them. In St. John’s the aggregation is on a horse farm. The owner is a nature lover who is unhappy that the wasps are near her horse barn, but she believes that the wasps have their place in nature, so she is willing to live with them. The entire community consists of horse farms.
- Is the RWL of cicadas brought to the nest normally distributed within each species.
I believe so. I’ll do the analysis and get back to you.
- How do know that we are looking at the same cicada population in the two Florida locations?
We don’t. However, we do know that the same species are present in both locations, because we have collected specimens of all of the same species at both locations. We have video/audio of the various species singing, so we can recognize the songs of the different cicada species. Based on song, we know that all of the same species of cicadas are active at the two sites. In fact, we are quite convinced that the very small Neocicada are the most abundant at both locations. We base this on song and on casual shed (exuvium) counts. I realize this is very indirect evidence, but it is difficult to census animals that live up in the canopies of tall trees. Another clue: the same diversity of tree species are found at the two locations.
A note from Jon regarding Cicada Mass to Wasp Mass Conversion
From: Jon Hastings <HASTINGS@nku.edu> Date: Tue, Mar 3, 2009 at 12:02 PM Subject: RE: Cicada killer question To: Grayson Rodriguez <email@example.com>
This is unpublished work that is ongoing. It is being carried out by my colleague, Chuck Holliday of Lafayette College. He induced female cicada killers to build nest burrows in PVC pipe that he had inserted into the ground within a colony. He ended up with about 50 individual nest cells, each with a provision of cicadas (most had one cicada, many had two cicadas, and one or two had three cicadas) and a larval wasp. He is keeping these in an incubator in his lab that is tracking the temperature and humidity of a true nest within the wasp colony. All of the larval wasps finished feeding months ago; now they are dormant within a cocoon. Periodically Chuck has "harvested" a dormant wasp--he weighs it and compares the wasp's weight with the estimated weight of the provision it received. Chuck estimates the provision mass by measuring the wing length of the eaten cicadas (the larval wasps don't touch the wings), and using the standard curve for wing length vs. mass for the given species of prey. No doubt the dormant wasps will lose more mass as the year goes on, but they are losing mass at a very slow rate. As of now, at the current rate of weight loss, the conversion rate appears to be just over 25%.
Our field data from locations in which only one or two prey species are hunted leads to a similar estimate. For example, in Joliet, IL, almost exclusively one cicada species is hunted. The mean mass of all cicadas we have captureded and we have retrieved from provisioning females is about 1.56 grams. The average female wasp is about .76 grams. Recall that at most locations, female offspring are given two cicadas by their mothers. This results in a mass conversion rate of about 25%. The average male in Joliet is about 1/2 the mass of the average female, which provides more support for 25% estimate of mass conversion.
Hope this helps.
- Seasonal occurrence of cicadas in Alachua County, Florida Jon says: "I just found this information on a University of Florida website. It lists the species of cicadas found in Alachua County, which is where our Newberry site is located. The 4 species used as prey by our wasps at both sites last year were Neocicada hieroglyphica, Diceroprocta olympusa, Tibicen chloromera, and Tibicen resonans."