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Forecasts of locust development by geographic location
- Forecasts of the development of Australian plague locusts
As with all insects, the growth of juvenile locusts occurs in discrete stages called instars with a moult between stages. Juvenile locusts, no matter what their instar, are called nymphs or hoppers. In Australian plague locusts there are typically five instars. When juvenile locusts mature, further development is required before they are able to lay eggs but these developments are not externally visible. The development stage of adults sampled in the field are categorised as: fledgling adult; adult with fat; adult with eggs or laying adult . Whether adult locusts have deposited fat or not is an important indicator of their propensity for migration and egg production.
Eggs of the Australian plague locust have three mechanisms that can delay the date on which they will hatch.
- quiescence stage 1 (Q1)
- diapause
- quiescence stage 2 (Q2)
Eggs can undergo one or more of these stages depending on environmental conditions, leading to different pathways of Australian plague locust development.
Q1 occurs when embryos are 25 per cent fully developed if eggs are laid into dry soil, see Embryonic stages of the Australian plague locust. Q1 will be terminated if enough rain falls on eggbeds to allow eggs to absorb water from the soil.
Decreasing day length experienced by the adult locust and temperatures less than 25°C at the time of egg maturation determine whether their eggs will enter diapause. Diapause occurs when embryos are 35 per cent fully developed and may last for 7 to 11 weeks. Egg diapause uccurs in autumn in southern regions and is usually completed by mid June. After this time eggs will resume development. At soil temperature below 16°C development almost ceases, so that eggs laid in southern NSW, SA or Victoria in autumn may not hatch until October. The rate of egg development increases with temperature so that complete development can occur in as little as 14 days at 35°C.
If soil moisture is low at the end of diapause (eggs 45 per cent fully developed) the eggs can enter a secondary quiescence (Q2) and only resume development after rainfall. Quiescence and diapause enable the Australian plague locust to avoid unfavourable conditions for nymph survival.
The development stage of locusts seen in different locations is used to initialise the development model. This information is obtained by APLC field officers during surveys, but can also come from sufficiently detailed Locust Reports sent in by the public. Nymphal development in locations where the stage of development is not known can also be modelled, using likely prior egg-laying activity associated with rainfall. When sufficient rain falls to either terminate Q2 in eggs from the previous locust season, or to terminate Q1 in eggs of adults active earlier in the current season, or initiate laying in adults, the date can be used as the start-date for running the model.
Forecast dates produced by the model are an integral part of the APLC Decision Support System. These dates enable us to allocate staff and resources to locust survey and to align survey with likely locust development in the field.
Forecasts of the development of spur-throated and migratory locusts
The development of spur-throated and migratory locusts is also influenced by temperature, but the lifecycles and species requirements differ. The spur-throated locust have only one generation per year, and generally spend their winter in the tropics as immature adults. Egg development by young adults takes place at the start of the wet season. The hoppers of this locust pass through more instars before reaching adulthood and the number of instars varies depending upon conditions and the gender of the individual; typically, male hoppers have 6 to 7 instars whereas female hoppers have 7 to 8 instars. The migratory locust has similar rates of egg and nymph development to the Australian plague locust, but does not have strong quiescence in dry soil, and the Australian subspecies does not display a low temperature egg-diapause.