| Factor | Time or conditions | Effect |
| Minimum time before worm eggs can become infective larvae. | 4–10 days | Short graze periods (less than 4 days) prevent ‘auto-infection’ (sheep or goats becoming infected by larvae hatched from the worm eggs they recently deposited onto the pasture). |
| Conditions required for significant numbers of worm eggs to hatch and become infective larvae. | 4–10 days of:
Brown stomach worm
Temperature: daily maximum >8°C1
Moisture in this time: >10–15 mm rainfall2
Black scour worm
Temperature: daily maximum >15°C for T. colubriformis or >12°C for T. vitrinus
Moisture in this time: >10–15 mm rainfall3
Barber’s pole worm
Temperature: daily maximum >18°C1
Moisture in this time: >10–15 mm rainfall3
| Unsuitable conditions prevent eggs hatching and developing into infective larvae.
Note: The eggs of the brown stomach worm are much more tolerant of cold and dry conditions, and in general, grazing management has less effect on its control.
Footnotes: 1Some hatching of worm eggs of all species can occur below these daily maximum levels, but this is usually at a small and insignificant rate.
2Brown stomach worm eggs can develop at low rates without rainfall even in a relatively dry faecal pellet.
3Development to infective larvae may occur without rainfall if soil moisture profile is high. |
| Maximum time worm eggs can live awaiting suitable hatching conditions. | Brown stomach worm: 21 days
Some brown stomach worm eggs may survive for longer periods. Once hatched, infective larvae can remain in the faecal pellet until conditions are more suitable.
Black scour worm: 16 days
Once hatched, infective larvae can remain in the faecal pellet until conditions are more suitable.
Barber’s pole worm: 5 days | Prolonged periods without the right conditions (temperature/moisture) for egg development will result in the eggs dying. This lowers the worm-risk of paddocks. |
The time for about 90% of the barber’s pole worm infective larvae (L3s) to die (making paddocks low worm-risk).
Note: Larvae of brown stomach worm and black scour worm can survive longer because they can remain in the faecal pellet for extended periods. | Cold: Maximum temperature < 15ºC
Time for 90% to die: 4 months
Warm: Maximum temperature – about 22ºC
Time for 90% to die: 3 months
Hot: Maximum temperature – about 35ºC
Time for 90% to die: 1.5 months
Very Hot: Maximum temperature > 40ºC
Time for 90% to die: 1–2 weeks | L3 larvae do not feed. While waiting to be eaten by sheep or goats, they wriggle randomly in drops of moisture, more so in warmer conditions. Increased activity in warm weather depletes their energy reserves faster, hastening death.
In extremely hot, dry and windy conditions the larvae dry out and die. |
| Minimum time for infective larvae eaten by sheep or goats to mature and lay eggs (the ‘pre-patent period’). | Sheep: minimum of 18 days for most sheep roundworms.
Goats: minimum of 14 (typically 21) days for barber’s pole worm and 21 days for scour worms. | Worm larvae eaten by sheep or goats soon after an effective drench will take at least 18 days (in sheep) or 14–21 days (in goats) before they can lay eggs. During this period after administering an effective drench sheep or goats are not re-infecting the pasture. |
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