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Worms have an Achilles' heel

By Deb Maxwell, ParaBoss Operations Manager

July 2015


The rate at which worms hatch and survive varies throughout the year. Knowing a little about their climatic likes and dislikes lets you control them more easily.

The first thing to appreciate is that worm eggs are far less tolerant of tough conditions than the larvae that emerge from them.

Barber’s pole worm and black scour worm eggs have a fairly short life. Once the eggs are passed in the sheep’s dung, barber’s pole worm eggs need the right conditions within a week for development to start, whereas black scour worm eggs can survive for a few weeks and eggs of brown stomach worm (also called Osties) survive even longer, until the right moisture and temperature conditions are available.

Don’t worry about trying to memorise the following information because Your Program in WormBoss tells you quite simply ‘what to do and when to do it’.


These worms need sufficient moisture in the dung pellet to start the development process. Barber’s pole worm requires the most moisture and Osties the least. Generally, this is provided by 10–15 mm of rainfall over a few days, however, waterlogged ground can be enough. But Osties are tougher and given the right conditions, can develop at low rates without rainfall, even in a relatively dry dung pellet.


Eggs from barber’s pole worms need daily maximum temperatures of 18°C or more to develop to infective stages, whereas black scour worms are more cold tolerant. The more common species of black scour worm, Trichostrongylus colubriformis, needs 15°C days, whereas the more damaging species, Trichostrongylus vitrinus, will actively develop when daily maximums are only about 12°C, which is similar for Osties.


When conditions are right, hatching and development to infective larvae takes about 4–10 days (the shorter times when it is warmer). The infective larvae wriggle in moisture films, ending up on pasture plants where they can be eaten by stock.

While they are microscopic and are very exposed in their location, worm larvae are incredibly tough. The cold weather in Australia, including snow and frost, has little effect on them. It also needs to be pretty hot before they become dessicated (dry up) and die, generally maximum temperatures over 40°C for a couple of days are required.

The infective larvae on the pastures have a limited energy reserve, as they cannot eat due to their protective coating. As they wriggle, they use up their energy and, sooner or later (if not eaten with the pasture), they die.

Exploiting the Achilles’ heel

This knowledge can be used to prepare a low worm-risk paddock for lambing or for weaners. In cool conditions, most of the larvae die within 6 months, but when it’s warm they are more active, their energy is used faster, and most die within 3 months. If no new larvae are added during this time then the paddocks will become low worm-risk. These particularly suit more susceptible classes of sheep, such as weaners and lambing ewes.

Worm infections during periods when eggs are not developing can cause some suspicion about this information. For example, one of the most common questions received at WormBoss workshops in the New England region of NSW is “Why do my sheep still get wormy in winter time if it’s too cold for the barber’s pole and black scour worms to develop?”

Let’s go back to the key messages from the previous paragraphs: eggs are fussy about developing, but larvae live for many months.

So, in winter, in cold areas, no new larvae are emerging, but those that hatched in late summer and autumn—the ones we are waiting on to die to create ‘clean’, low worm-risk paddocks some months later—are still available to be eaten by the sheep during winter.

You don’t need to be an expert to use this information. Your WormBoss Worm Control Program on the WormBoss web site includes simple practices and when to use them to take advantage of the worm’s Achilles’ heel.

WormBoss also has more about worm control with grazing management, as well as the worm life cycle and factors contributing to paddock contamination with worms.