Oak leaf feeders sabotage but don’t kill oaks

Oak leaf trees harbor a lot of insects and Álvaro is on a mission to analyse the impact of global warming on oak leaf feeders, and its natural enemies (small wasps).

- The oak leaf feeders can influence how well the oak grows and produces, because they survive by feeding on the oaks. However, the oak leaf feeders don’t have the power to kill the oak, says Álvaro Gaytán, PhD candidate at the Department of Ecology, Environment and Plant Sciences.

An example of leaf miner (oak leaf feeder): Acrocercops brongniardella miner and the damage it can cause (the white layer) when eating the leaf.

The oak leaf feeders also have natural enemies, small wasps (some of them are like the head of a pin).

A free-living parasite on a blank sheet, living outside its oak leaf feeder host. Photo: Álvaro Gaytán de la Nava.

When the small wasps are larvae they live as parasites in the oak leaf feeders and eventually kill their larvae host. By predating the oak leaf feeders, the small wasps can control them from exploding in numbers.

What Álvaro wants to know how much damage the oak leaf feeders can cause to the oak leaves, and if the damage is affected if the oak leaf feeders are parasitized by the small wasps or not.

- Only 10-12% of the oak leaf feeders that I catch are able to obtain adult small wasps. Therefore, I will need as much as at least 300 caterpillars per species to have a good sample size in my experiment (30 or 35 parasitoids).

This is one of the oak leaf feeders that Álvaro catches, shown in larval stage. The type is: Amphipyra berbera caterpillar. Photo: Álvaro Gaytán de la Nava.

 

The caterpillar are fed with oak leaves in petri dishes in the lab. Photo: Álvaro Gaytán de la Nava.

Light traps attract more males

 
The light trap seen from a distance. In the photo: Álvaro Gaytán de la Nava.

Álvaro is now on a mission to collect these harmful moths whose eggs he will rear to larvae in the lab. Álvaro uses light traps in the woods to catch the oak leaf feeders. The light bulb attracts them, and they are trapped in the lower part of the trap; a box containing pieces of cardboard where moths alight.

The light trap. In the photo: Álvaro Gaytán de la Nava.

- I aim to catch females because each female can lay around 30-40 eggs and this is the most productive way to obtain “parasitoids-free samples” that can be used as control. But most of them that I catch in my traps are unfortunately male… says Álvaro.

Álvaro is catching the moths with light trapping, using a blank sheet and a light source to attract them. In the photo: Álvaro Gaytán de la Nava.

The reason why males are more attracted to the light traps might be that the males are more active and there are trying to find females to fertilize. The physical structures of males like antennae and eyes, are more adapted to catch stimulations. Some females don’t even develop wings (look at the photo below)! (for example in the family Geometridae)

The adult female oak leaf feeder Agriopis marginaria - this species females lack real wings. Photo: Álvaro Gaytán de la Nava.
 

What will happen to the oaks in a future climate change scenario?

Álvaro is also interested to know how these insects will be affected by extreme warm conditions, current conditions and in colder conditions, to predict the fate of the oaks and the food web inhabiting them.

For example, it’s important to know if the small wasps can mitigate the increasing harm that oak leaf feeders cause due to that they grow exponentially in a years’ time, and how temperature influences this.  

Different temperatures can also create a mismatch in how the insects synchronize. For example if an oak leaf appears earlier in the spring, due to increased temperature, the oak leaf feeder might not be developed yet and will not eat the oak leaves.

- Sometimes I find some difficulties added to my work, such as nights of wind, activity of wild animals that see my trap as an appetizing snack or curious or worried people with my fieldwork. In any case, all these anecdotes enrich my experience and help avoid problems in the future, says Álvaro Gaytan.

The adult oak leaf feeder Phalera bucephala, on Álvaros hand. Photo: Álvaro Gaytán de la Nava.