Control of pine processionary using bats

The control of the procession by means of bats is fascinating us in Environmental Ideas. Maybe it's because we work side by side professionals Forestry with their own branches of the biodiversity. After diving through the existing literature, we found that studies on the control of forest pests by bats are scarce. In Spain we have found a single experiment of this type carried out in the Balearic Islands (although surely there will be more that we don't know).

We are still looking and our group of WhatsApp of chiroptologists (yes, we have a group of WhatsApp from bat experts, even if it seems crazy) sent us this interesting article by Yohan Charbonnier and collaborators which we translate as: “The numerical and functional responses of forest bats to a large (processional) insect plague in pine forests”

Birds have always been considered to be the most efficient vertebrate pest controllers. However, recent comparative studies carried out in tropical forests show that the number of insects preyed on by bats is much more significant than that preyed on by birds.

Bats, better pest controllers than birds

We know that bats can ingest half their weight in insects per day. We also know that many herbivorous insects are more active at night. Both premises in the shaker give us an effective tool for controlling forest pests. The control of processional and forest pests carried out by bats can therefore be of great interest given the growing concern about the increase in these pests due to climate change.

The article presents an innovative experiment that is carried out using pheromones to control the availability of processional moths (Thaumetopoea pityocampa) without changing local environmental conditions. The work provides three hypotheses about the predation of processionaries by bats:

1. The fact that the processional butterfly hunt takes place and its abundance will be directly related in time and space to the availability of the prey species (the processional)
2. Bats will increase their feeding activity when and where prey is most abundant.
3. The abundance of prey insects will decrease in the next generation, after the most intense feeding episode by bats, due to the consumption of adult processional butterflies and the consequent reduction in their subsequent reproduction.

What did the experiment consist of

The experiment was carried out in southwestern France on plantations of Pinus pinaster during the month of July (period of maximum emergency of the processional butterfly). The experiment was carried out on the basis that synthetic sex pheromone traps are a good way of estimating processional populations.

After two nights of monitoring bat populations, the ultrasound detectors were replaced by pheromone traps in the same study areas within the pine forest. These traps were active for six consecutive nights. Finally, the number of male moths trapped at each point was counted.

In February 2013, the density of processionaria in the larval state was estimated to quantify the predation of moths by bats. A ratio was calculated: number of larval colonies divided by the number of males captured at each of the study points, serving as an indicator of the reproductive success of butterflies (considering that the sex ratio in the processional is 1:1, number of male processionary animals captured = number of couples)

Results. Temporary patterns of bat and moth activity.

1. The maximum activity of bats along the pine forests corresponded to the period of maximum mating activity of the pine processional moths, which occur 4 hours after sunset.
2. Total bat activity and nocturnal feeding buzzes are highly correlated, and both variables show the same temporal patterns throughout the night.
3. The rate of activity of all bat species describe weather patterns identical to the activity of Thaumetopoea pityocampa.

The response of bats to an abundance of moths.

For data analysis, only data from the first 4 hours of the night, when bat activity was at its peak, were used. Total bat activity increased significantly with the increase in the abundance of moths at the forest edge. This activity also increased with the local abundance of moths in several bat species: Eptesicus serotinus— Nyctalus leisleri and Pipistrellus pipistrellus

For a specific area and day, no relationship was observed between the presence of the pheromone lure and the total activity of bats or in the activities of Kuhlii pipistrelle, Eptesicus serotinus— Nyctalus leisleri and Pipistrellus pipistrellus.

The concentrations of moths resulting from pheromone attraction also had no significant effect on the richness of bat species. If there was a significant increase in bat feeding activity near the lure and larger bat species emitted more intensely.

More moths, more activity

At the beginning of summer, the processionary (Thaumetopoea pityocampa) is the most abundant moth species in the pine forests studied, also showing a Circadian rhythm closely coinciding with bat activity. This temporal synchronization is also spatial because of the preference for forest edges of bats and adult moths of T. pityocampa.

This study shows that bat activity increased with the availability of prey (moth abundance). This finding supports the initial hypothesis of a positive response of bats to the abundance of Thaumetopoea pityocampa.

There are previous studies showing that insectivorous bats are able to adjust their predation activity to the availability of prey. This behavior can favor resource-based feeding that is unpredictable over time and space, such as peaks in the local presence of Thaumetopoea pityocampa.

In addition, as the period of emergence of T. pityocampa coincides with the period of higher energy requirements for females, bats may also have a demographic response to the moth.

Changing environmental conditions artificially increasing the availability of a prey species in its habitat can have several effects in addition to the local attraction of males and the corresponding increase in flight activity in intensity and time. And this is that, the more moths of Thaumetopoea pityocampa If they are present in an area, more bats can feed on them.

The study was unable to determine the exact response curve, since feeding activity (number of bat detections) was used instead of actual attempts to capture prey by each individual. It has also been considered that bats can travel long distances, use different feeding tactics and feed on various prey, so it is more likely that the bat's functional response is mainly associated with generalist predators, since they are capable of changing prey depending on availability.

The bats that eat the most processional, and the best

Therefore, bat populations are abundant when the availability of T. pityocampa decreases and they respond quickly when the defoliator builds their populations. The larger bat species showed a significant functional response, for example. Eptesicus serotinus— Nyctalus leisleri and Kuhlii pipistrelle showed significantly more attempts to capture prey in the presence of greater availability of moths.

Pipistrellus pipistrellus did not offer more echolocation sounds in the presence of pheromones and, therefore, with greater availability of moths. It seems that it is too small a species to feed on T. pityocampa and, according to the theory of optimal feeding of bats, it would spend too much time managing this type of prey.

Several other bat species specialize in moths, such as Barbastella barbastellus or Plecotus spp, but they showed no response to higher moth densities in this study. These species can fly with greater precision to collect moths that rest on the foliage in dense vegetation inside old pine trees and mixed forests, which explains the lack of response to the experimentally generated flying moth along the edges of the forest. Another explanation could be the low level of T. pityocampa populations during the experiment. Generalist species are known to have a greater impact on low-density prey populations, while specialized predators are more effective with a high prey density.

The bat reduces the population of processionaries

The study shows that bats' response to moth density offers lower offspring per adult moth. Therefore, bats are very likely to feed on T. pityocampa moths and therefore the reproductive success of this lepidopteran is reduced.

This prediction is reinforced by the low rates of predation and parasitism of eggs in the study area, which is the only intermediate life phase between moths and larval colonies.

However, these results are preliminary and would require further studies, especially in-depth studies related to the diet of bats in pine forests, such as studies of DNA analysis of excrement to confirm this assumption.

It is increasingly recognized that bats can contribute substantially to the regulation of pests in agroecosystems, however, the reasons for this effectiveness are not fully understood. In short, bats are potential biological controllers that could help regulate T. pityocampa populations.

Cover photo: http://www.aintanerd.com/