There is much we do not yet know about when and how mares might be exposed to caterpillars and be at risk of abortion.
It appears that mares may inadvertently be exposed to caterpillars while grazing around trees where nests are present, and by disturbing old nest material after caterpillars have pupated. It is well known that horses will not eat live caterpillars however, their large, prehensile lips can sift caterpillars from their food.
Current knowledge suggests that mares are most likely to be at risk of inadvertently eating caterpillars or caterpillar exoskeleton when nests and caterpillars are larger (from about March to June) and possibly for some period beyond that due to exposure to old nest material. It is possible that mares might not be at much risk from very small nests since caterpillars are tiny and move mainly from their nest up to the leaves of the same tree and back again.
The risk period for mares is likely to be from February to September with a peak risk period from April to June. This fits with field reports of EAFL cases being first diagnosed in April (and occasionally March), peaking in June-August and then occasionally continuing into September. In February-March, exposures seem likely to be due to inadvertent ingestion or contact with caterpillars or nest material at the base of trees. As nests develop, elevated nests may drop material onto the ground underneath the tree, providing potential for mares to contact material. When caterpillars move down from trees to pupate around April-May they may travel across adjacent pastures and increase exposure risks to mares. Finally, even when caterpillars have gone to ground for pupation, there may be residual nest material and shed exoskeleton that can drop from trees, be wind-dispersed or shifted by rain/flood or baled into hay.
Assessing risk for mare breeders
The first part of this process is to consider historical information and observations for a particular property. Is there any history of pregnancy loss on the property, or have there been any cases of EAFL diagnosed in the past?
Note that mare abortions due to EAFL occur in March, more commonly in April and peak between June and August. Caterpillars disappear into the soil to pupate in April to July which means that when mares are aborting and EAFL is being diagnosed, there may be no caterpillars about.
Also, note that the interval from exposure to abortion is likely to range from several days to 4 months, for more information see Todhunter KH, et al. 2009. This means that you should look at paddocks where mares were located, up to 4 months before they aborted, rather than focussing on the paddock where the mare was when she aborted.
It is also important to note whether anyone has seen evidence of processionary caterpillars on the property or in the district (nests, processionary chains of caterpillars).
Finally, it is useful to know if potential host trees may be present on the property. The main candidates are multiple species of Acacia (wattle trees) and Eucalypts.
Because we don’t know what trees the caterpillars may be found on, the property inspection will be a critical part of the risk assessment. The priority areas of the property to inspect are all paddocks (and adjacent land within 200-400 metres of fence lines) where pregnant mares will be housed between February and September. The caterpillars are capable of travelling up to 200 metres and the 400-metre distance is to ensure a safety buffer for risk management.
Initial inspections should occur between October and December. This is when egg masses have been laid and caterpillars are very small. Ground nesting caterpillar egg masses will appear very prominently as small white, fluffy golf ball sized masses at the base of Acacia spp. Canopy caterpillar egg masses may be hard to spot, they are golden coloured and up in the tree canopy.
Inspections should then continue until February. At present, we believe that the main risk to pregnant mares probably begins around February (may vary with location), when caterpillars start to move from tree to tree and when nests get larger and there is more opportunity for material to drop or be moved by wind and rain.
The interval between November to February, provides a window when nest material and caterpillars can be removed from priority areas to minimise risk to mares.
An important part of this inspection process is to map the trees where nests are viewed and identify them for future records. It may be necessary to wait until trees flower to collect a sample of branch/leaf/flower/fruit to allow accurate identification. Knowing which trees are the preferred host will be really important in the longer term as a guide to future planting and management of the ecosystem.
Removal of nest material and caterpillars
Once processionary caterpillar egg masses or nests are identified, they should be removed and disposed of, preferably by deep burial.
There is no single or right way to go about this. Spraying small egg masses with water or mist may be useful to dampen the egg mass and minimise the risk of inadvertent dispersal as you try and pick it up.
Develop a method for gently removing nests from affected trees and placing them in a container for transport (bucket with a lid). Options may include manual removal with gloved hands, enveloping the nest in a plastic bag and using a small spatula or other implement to separate the nest from the tree and pick it up.
Elevated nests are often adherent to branches and it is easiest to prune the branch with the attached nest and place it in a bag or bucket. In some cases, this will require extended pruning equipment and some farms have used cherry pickers to reach nests that are higher in the canopy of large Acacias and Eucalypts.
Some individuals have reported using portable or hand-held propane gas torches to incinerate egg masses in situ. The very high heat of these torches may make it feasible to quickly incinerate the material. This is a potentially risky approach to the problem because of the hazards of accidental injury or fire.
When transporting material to a burial site, ensure that it is transported safely and in a sealed container or bag to avoid dispersing material as it is moved.
Use of agricultural chemicals such as pesticides is generally controlled by state legislation in each state. The responsibility for pesticide use rests with the user. It may be illegal to use a pesticide unless it is registered for that use or is covered by a specific permit for off-label use. Please check with appropriate authorities (Environmental Protection Authority or EPA in NSW) before using any insecticide.
Cleaning and storage of equipment
- All equipment used in handling and transporting nest material is potentially contaminated with nest material.
- Gloves or other tools that come into direct contact with nest material should be stored carefully and separately to avoid spreading material.
- Clothing may also be contaminated and use of disposable outer coveralls offers a simple option for managing this by bagging and disposing coveralls after each removal session
Health and safety
- Wear personal protective equipment (PPE): boots, jeans, long-sleeved shirt or disposable coveralls, gloves, eye protection and mask to cover nose and mouth.
- Do not perform removal activities in windy weather since this will increase risk of dispersal of nest material.
- Individuals with asthma, respiratory conditions, or who are prone to allergies should avoid all activities that may have risk of exposure to nest material.
- Record the paddock/GPS location of trees that have been identified as having nest material. Have host trees correctly identified by supplying dried reproductive material to the state Herbarium.
- Record what disposal and management methods work best for you and what doesn’t work and please share this information. Our ability to effectively manage risk into the future is dependent on learning what works best and sharing that information.
Management of trees and plantings
Identifying non-host species will allow these species to be planted that have low risk. We currently do not have this information. This is why we need your observations on what trees are affected and what trees are not affected. Sending this information into shared sites such as this one will ensure that it is communicated across the industry.
Not all Acacias and Eucalypts are caterpillar hosts. Elevated nests may occur in other tree types including casuarinas, kurrajong and possibly others. More information is needed to characterise which tree species might be at higher or lower risk of being infested.
Removing trees that are known to be infested from paddocks where mares are currently grazed, may need to be carefully managed. Disturbing soil around these areas may inadvertently dig up old nest material from previous years and lead to increased risk of exposure. It is suggested that any such activity be planned for a time when pregnant mares are not grazed in the paddock, and that disposal of removed trees be managed in a way that avoids inadvertent risk of dispersal of material.
Be aware also that some trees may be protected in some regions. Make enquiries before you embark on any drastic action.
Could there be risk associated with ploughing paddocks?
Processionary caterpillars pupate in the soil. It seems possible that if a paddock is ploughed up, the process could bring pupating caterpillars to the surface and expose other nest material. This is an unknown area at this stage.
The most likely risk to mares is probably from this year’s caterpillars and nest material. It is not clear how long it takes for nest material to break down but initial observations suggest that most material has largely degraded within a year (by the time the next years nests are developing). However, there are reports in the literature of people being affected by caterpillar setae that may have been in the environment for more than a year. The long-term risk of setae in the environment is unknown and more research is needed to answer these questions.
Todhunter KH, Perkins NR, Wylie RM, Chicken C, Blishen AJ, Racklyeft DJ, Muscatello G, Wilson MC, Adams PL, Gilkerson JR, Bryden WL and Begg AP. Equine Amnionitis and fetal loss: The case definition for an unrecognised cause of abortion in mares. Australian Veterinary Journal 87:35-38, 2009.