Dorsal visual cues in German wasp navigation

anglais. The German wasp Vespula germanica is found in the Palaeartic region of the globe. Outside this area, it is highly invasive species. It has an omnivorous diet and is able to hunt other insects and scavenge for food from both natural and anthropogenic sources. It has been previously shown that this species has efficient and specialized foraging strategies depending on the resource. It is capable of navigating successfully through closed habitats, has a large landmark learning capacity as well as flexible responses to changing environmental variables. They are also able to recognize a shape irrespective of color and contrast with the background. Our study shows that this species is able to detect patterns and their orientation in the dorsal field, ability that is lacking in honeybees. Furthermore, our data suggests that each forager is able to remember landmarks associated with not only the current, but also the previous resource location, something that has already been proved in previous studies.

anglais. German wasps keep an eye on the sky during flight The insect world has many examples of superb fliers that are capable of amazing feats. Among these, wasps are some of the very best. They are an amazing combination between a jet fighter and helicopter, by being able to fly at high speed in all directions, including backward, as well as hover. German wasps (Vespula germanica) are a native species to the Palearctic region. In other parts of the globe they are highly successful pests. Like all wasps, they are skilled hunters, preying on a variety of insects and other invertebrates. Amazingly, the adults are vegetarians with a sweet tooth, preferring foods containing high amounts of sugar such as fruits and honey-dew from aphids. However, their larvae require proteins, which are obtained by the workers not only through hunting, but also by scavenging. The success of this species may be explained by the fact that it is able to take advantage of food sources both natural and anthropogenic spanning a wide range of habitats. They also have efficient foraging strategies depending on the food source: for proteins, they learn the exact location of the food source, whereas for carbohydrates, they learn the general location. A dead animal is not able to move, thus knowing and learning its exact position is very important. On the other hand, carbohydrate sources, such as flowers, occur in patches over a relatively large area. Knowing the exact position of a flower is not necessary. German wasps are central place foragers, meaning that after each trip they return to a “central place” such as the nest. In order to do this, they must be able to find their way back after each trip. This of course requires a very good memory. In order to make the learning process effective, only the endpoints of each journey are memorized in detail. The majority of the route in between remains mainly unknown with the exception of a few conspicuous landmarks such as a tree. By learning only these landmarks and nothing else, the amount of stored information is minimal. In order to navigate between landmarks, foragers use a system called path integration, in which the distance traveled and angles turned are used to continuously update an estimation of their goal’s direction. The direction is obtained from celestial cues such as the sun, stars, moon or polarized light pattern of the sky. The distance comes from measuring the pattern of apparent motion of objects, also known as optic flow. Put in a more simple way, wasps use a system similar to our own GPS, in which you track your own movement in relation to the position of your goal. However, this is achieved without the help of satellites and with a brain smaller than the size of a pinhead (0.041 mm3)! Nevertheless, all of this would be impossible without one very important component: vision. If we humans can still move without this sense, this would be impossible for a wasp. Insect eyes are regionalized, such that different parts may perform different tasks, including tracking the position of a female flying above while at the same time avoiding bumping into objects ahead, or on the ground beneath. Surprisingly, vision in German wasps is still a poorly researched subject. When navigating dense habitats such as forests, the dorsal area of the eyes is of utmost importance. Wasps need to “keep an eye on the sky”, as the canopy pattern can act as a landmark, thus enabling the wasps to navigate more easily. In bees, the dorsal part of the eye is incapable of using color information and detecting pattern orientation. In the German wasp these capacities in the dorsal area of the compound eyes remain unknown. The purpose of my project was to determine if Vespula germanica is able to find a food source by using information coming only from the dorsal visual field. This was studied with the help of a Y-maze (Figure 2) in which the ceiling had black and transparent stripes of different orientations (perpendicular/ parallel to the length of each arm). In order to preserve the optic flow which enables the wasps to fly, but remove any visual cues, the sides of the maze were covered with self-adhesive patterned vinyl window film. With the help of a feeder containing the wasps’ favorite food, sugar solution, foragers were trained to associate a perpendicular orientation of the stripes with the food source. During the experiment, they were subjected to a choice: choose the arm containing the pattern parallel to the main axis, or choose the arm containing the trained pattern. As in any school, any possibilities for cheating were removed. Thus, the patterns in each arm were switched every 10 minutes. In this way any correct choices made simply by associating a particular arm with the food source were eliminated. Like many insects, wasps are also capable of using their sense of smell, and this too was considered cheating. However, wasps do not have a nose like that of humans that can easily closed. This difficult problem had a simple solution: put an identical feeder in each arm of the maze and place a net on the top of the feeder in the “wrong” arm. Thus, even if the wasps can smell the sugar solution, or even leave pheromones on the feeder, they would not be able to get any treat unless they used the information coming from the dorsal area. It was easy to see that this was very frustrating for individuals making the wrong choice as, in the beginning, they were furiously biting the net trying to get through. Amazingly, during the four days of the experiment, the proportion of correct choices made increased to 92%. These results prove that unlike bees, German wasps are able to detect and discriminate pattern orientation in the dorsal area of their eyes. However, unlike bees, German wasps often fly in complex habitats such as a forest in order to find protein rich food for their larvae. Thus, this ability could be beneficial for foraging in a complex forest habitat and might be favored during evolution. Why might this be a useful ability in a forest-navigating insect? Imagine that you are moving through a dense forest from point A to B. How do you find your way? Remember, you have no GPS, no compass and no tree marking is allowed. In a couple of minutes you would probably be completely and utterly lost. Now imagine that at the same time as you are walking, you are able to see the silhouetted canopy. What you would see would be a pattern of light and dark patches similar to the stripes in my experiment. This has one important property: it is unique from one place in the forest to another. Now, if you were able to remember this pattern then you would be able to navigate the forest in no time. Between day 1 and day 2 as well as day 2 and day 3, one and respectively two days with no experiment occurred because of bad weather conditions. What is amazing is that the percentage of correct choices increased continuously from 62% to 92%. Thus, the wasps could not only remember what they learned, but they even got better at it! The findings of this experiment show that German wasps have remarkable visual abilities. However, this is only the beginning of understanding how these amazing insects see the world around us. Future studies are needed in order to see if they can detect pattern orientations other than 90° and whether the color or the pattern of polarized skylight also play a role. Supervisors: Eric Warrant and Marie Dacke Masters Degree Project, 30hp, 2015 Department of Biology, Lund University