Tropical grass webworm

Scientific name: Herpetogramma licarsisalis
  • Key characteristics
  • Biology
  • Impacts
  • Control
  • Further information

  • Found in frost-free areas of Northland
  • Larvae are the damaging stage and can build up in numbers very fast in hot, humid weather
  • They prefer kikuyu, but will eat other grasses including paspalum, fescue, and perennial ryegrass
  • At high enough densities can chew out several hectares of pasture in less than 48 hours
  • The appearance of hard grazed patches in an otherwise lush kikuyu-dominant pasture indicates tropical grass webworm may be active
  • Evidence of webworm feeding can be seen when kikuyu is parted at the base; caterpillars, silk webbing and bright green droppings will be present.

  • The webworm is widely distributed throughout the tropics and subtropics. It most likely was blown over from Australia by wind
  • The adult webworm is a drab, fawn-coloured moth, about 2.5cm across, sometimes with dark spots scattered over its delta shaped wings. They are attracted by light
  • Female moths lay around 250 eggs in batches on leaf surfaces. A generation, from egg to adult, takes about a month in summer
  • Caterpillars are 5-20 mm long and very active when disturbed. They are translucent and range from pale green to dark brown depending on what they are feeding on
  • The caterpillars live in the pasture, feeding mainly at night. They can completely strip grass of green tissue, often leaving only peeled white fibrous stalks
  • During the day, the caterpillars shelter in silk tunnels in the grass. This silk webbing and abundant green fecal pellets are clear signs of webworm
  • On maturation, the larvae make cocoons of webbing and dead grass. The pupae is tan to dark reddish brown
  • All life stages are killed at temperatures below 5oC.

  • Tropical grass webworm prefer grazed kikuyu in warm and sheltered north-facing paddocks
  • Damage occurs between February and May with the most damaging populations arising from adults emerging in March
  • Large populations of early instar larvae can be present but damage not evident.
  • 80% of what a larva consumes during its lifetime is during the final larval instar
  • Damaging populations of up to 1500/m2 final instar larvae can strip pasture in days
  • High risk factors for an outbreak are:
    • Temperatures remaining above 11oC into late autumn
    • Sufficient rainfall January to April to provide lush kikuyu and humid conditions within the pasture
    • ¬†Mild frost-free winters allowing tropical grass webworm to extend its range and commence development earlier.

  • Generalist natural enemies help suppress tropical grass webworm most years
    • These include the parasitic wasps Meteorus pulchricornis and Lissopimpla excelsa and a Tachinid fly Pales sp.
  • A proportion of at-risk farms could be sown in less vulnerable grass species with black beetle-active endophytes, such as ryegrass and tall fescue
  • An early warning system is in place in Northland but farmers on at risk properties could check overwintering sites (well-drained, north-facing or sheltered pastures) for larvae in late spring. Over 50 larvae/m2 means trouble
  • If high populations are predicted farmers can:
    • Harrow close-grazed pasture to expose larvae to desiccation and sunlight
    • Use insecticides registered for use in pastures against caterpillars.

Publications Dymock JJ, Gibb AR, Suckling DM. 2009. Monitoring and predicting populations of the tropical grass webworm (Herpetogramma licarsisalis) a pest of kikuyu pasture in Northland. Proccedings of the New Zealand Grassland Association 71: 25-30.