How Dangerous is the Oak Processionary Moth for Horses?

Not only humans, but also horses suffer from allergic reactions caused by the oak processionary moth. If your four-legged friend shows a swollen mouth, inflamed eyes or pustules on the head and neck, this does not have to be due to annoying flies and horseflies. The dangerous oak processionary moth can also be behind it.

In our guide, you can find out what you should know about the oak processionary moth as an owner of a horse.

What exactly is the oak processionary moth?

The oak processionary moth is an insect and belongs to the butterflies, more precisely to the moths. It is native to Germany and is found here especially in the northeast and southwest, but also in parts of North Rhine-Westphalia.

The oak processionary moth, like all butterflies, goes through the developmental stages of egg – caterpillar or larva – pupa – fully developed moth. In recent years, it has become increasingly known because it has reproduced en masse. The forestry damage it has caused has become more visible. Its caterpillars eat almost the entire leaf. They cause light and clear-cutting. For humans and animals – including horses – the caterpillars pose a health hazard because the stinging hairs of the caterpillars can cause severe clinical manifestations. Adult moths are harmless. The massive reproduction of the butterfly represents a natural phenomenon that has been self-regulating in the past without human intervention.

Where do oak processionary moths overwinter?

Oak processionary moth moths hatch in August after a pupal dormancy period of about three to six weeks. Shortly after leaving their parental colony to start a new one, females lay about 150 eggs in rows in the upper part of the oak crown. The moths live only a few days. Embryos develop in the laid eggs during the following fall months and remain in the eggs through the winter until caterpillars hatch between early April and early May.

Where do oak processionary moths nest?

The females of the oak processionary moth lay their eggs while still in summer on thin branches in the upper part of the oak crown that receive a lot of sunlight. The next spring, the caterpillars hatch between April and May. The larvae go through six stages, each persisting for about ten days. They spend the day and the molting period between each stage in nests. The younger caterpillar stages produce smaller webs in the upper crown. On branch forks of the trunk or on thick branches of the lower crown are the nests up to the size of a soccer, which caterpillars form from the fifth larval stage. The insects also pupate in these webs.
Why are oak processionary moths called that?

Three characteristics of the oak processionary moth have given it its name:

“oak-“: It particularly colonizes trees of the genus Quercus (oak). That is why oak forests are predominantly infested. However, it also occurs in oak-hornbeam forests and oak-pine forests, and occasionally on other deciduous tree species.

“-procession-“: Oak processionary moth caterpillars live in family groups. They feed predominantly at night and migrate together along the tree trunk or branches. In doing so, they perform a kind of procession (procession; Latin procedere: to advance, to proceed).

“-spinner”: Younger caterpillar stages form small leaf webs in the area of the upper crown. Older caterpillars spin nests that can be as large as a soccer.

Danger: Is the oak processionary moth dangerous for horses?

Yes, the oak processionary moth is not only dangerous for humans, but also for horses. From the third larval stage, the caterpillars form stinging hairs. The thin hairs are hollow inside and contain the poison thaumetopoein, which the caterpillars use to protect themselves against predators. Because the hairs break easily, the toxin is released quickly. When the hairs touch the skin, allergic reactions occur. The stinging hairs have barbs, making them easy to attach to skin and mucous membranes. Irritation occurs more quickly and the venom can penetrate more easily. In addition, inhalation of the hairs can lead to respiratory problems. In rare individual cases, allergic shock occurs.

With each molt, the caterpillar gets more stinging hairs, which are also longer and longer. The caterpillars lose the hairs especially during the feeding season in May and June. This is when the caterpillars are most active.

PROBLEM: Molting and pupation take place in the webs. The caterpillars’ stinging hairs remain toxic for a long period of time, so the nests of molting remains are a hazard for years. The wind carries the hairs into the surrounding area. Thus, the horse does not have to come into direct contact with the caterpillars to be harmed. For example, they remain attached to the ground in the horse pasture, where the horse’s mouth often comes into contact with them.

Symptoms: What signs of disease does oak processionary moth cause in horses?

The horse shows symptoms of allergy when it comes into contact with the oak processionary moth. Symptoms occur depending on where the hairs touch the animal and the severity of the individual animal’s reaction. Common symptoms are on the mouth and in the oral cavity. Here the animals get in contact with the stinging hairs when eating the pasture grass. But also the eyes are quickly contaminated by the fine hairs. Or the animals inhale the hairs and react with respiratory problems.

The following symptoms can occur in horses, for example:

Swelling in the head and neck area:
For example, horse mouth, tongue, eye
conjunctivitis
Pustules or wheals on certain areas of the body (urticaria, hives)
Itching
Cough
Shortness of breath (lungs affected)
Swelling of legs
Non-specific colic symptoms

TIP: If fly masks and fly blankets don't help allergic reactions in your horse, always keep the oak processionary moth in mind, too!

Immediate Action: What do I do if my horse shows symptoms after contact with the oak processionary moth?

If your horse has had contact with hairs from the larva of the oak processionary moth, you should take action as soon as possible. These include:

Shower your horse off! Then brush him carefully! Do not press too hard so that you do not push the hairs into the skin. Cool the affected areas! Offer your horse water to avoid circulation problems! Contact your veterinarian and describe the symptoms exactly! => The veterinarian will give you further instructions if necessary and will decide if he has to come immediately to administer medication. Put on gloves when you take care of your horse! You will quickly come into contact with dangerous hair. Clean bridles, saddles or other accessories thoroughly if they may have had contact with the hair!

Treatment: How will the veterinarian treat my horse after contact with the oak processionary moth?

The veterinarian will treat your horse immediately with medication if it shows severe symptoms. He may use the following therapeutic agents:

Antiallergics – to reduce allergic symptoms. Anti-inflammatory drugs (cortisone preparations) Painkillers Draining medications Sedatives (for highly agitated animals)

Control: What are control measures against the oak processionary moth in the horse pasture?

IMPORTANT: Only specialists are allowed to dispose of oak processionary moth nests.

Never attempt to remove the nests yourself! This is far too dangerous for your health because of the high concentration of toxic hairs. This is where professionals have to take action, who protect themselves in the best possible way with protective equipment. They have professional working equipment at their disposal with which they can safely remove the nests and caterpillars on the tree trunk and in the tree crown.

The current practical guide for cities and municipalities in the state of North Rhine-Westphalia describes in detail how a current infestation with the oak processionary moth should basically be dealt with:

The affected area is assessed according to the degree of risk. Areas with many pedestrians and high bicycle traffic or areas with kindergartens, retirement homes, etc. have the highest priority for control. Less prioritized are infested trees that are located far away from buildings, paths or roads. Various authorities are involved in the fight against the oak processionary moth – among others the green space administration, the health department, the public order department and the lower nature conservation authority.

If you discover an affected tree in your horse’s pasture or on stable property, notify the farm owner and urge professionals to remove the nests as soon as possible.

If an infested tree is located in public space, for example near the pasture but beyond the farm property, you inform a competent authority. They will decide on the control measures. It also collects data on incoming reports, which helps it get an overview of how widespread the oak processionary moth is in the region. In this way, it is possible to better assess the risk situation.
What measures are available to control the oak processionary moth?

There are several ways to repel the oak processionary moth.

Natural enemies: For example, birds, predatory beetle species, and parasitoids can reduce the population of the oak processionary moth. A parasitoid organism (usually insects, for example ichneumon flies) eventually eats its host. Appropriate measures to encourage counterparts, such as planting species-rich hedgerows, are encouraged.

Traps for the caterpillars: Mechanical barriers on the trunk can be used to trap caterpillars on the trunk. However, they cannot be used in the upper tree sections.

Removal of nests: One avoids contaminating the environment with caterpillars and stinging hairs if one removes all the gossamer nests. Specialists in safe protective clothing collect and vacuum caterpillars and nests. For vacuuming, the experts use certified safety vacuums. They can also bind the stinging hairs in the nests with certain degradable paste. After that, they collect the webs.

Flaming the webs: Flaming of the webs was commonly done in the past. It is now refrained from, as it blows the poisonous stinging hairs into the air. Under certain circumstances they spread over large areas. Also caterpillars cannot be caught safely, so that the effect of flaming is not sufficient. Use of pesticides and biocides: Professionals ideally apply insecticides at the same time as caterpillars hatch. Otherwise, exposure to the stinging hairs is not sufficiently mitigated. In individual cases and under special conditions, the plant protection product may also be applied from the air.

Precautionary measures: How do I protect my horse from the oak processionary moth?

To prevent your horse from coming into contact with the dangerous hairs of the oak processionary moth in the first place, you can take precautions. This includes the following measures:

Your horse should not graze near infested trees.

When riding out, give affected forest areas a wide berth. You should urgently refrain from gathering hay from meadows that are near infested trees.

You should control hay that your horse gets to eat. If you are not sure whether the hay is contaminated, it is better to dispose of it in case of doubt. Are there any oak trees on or near the farm or pasture? If so, check them regularly. Immediately report any infestation to the farm owner or the responsible office.