Oviposition and Egg Development:
Adult locusts feeding on green vegetation can mature eggs and begin laying within 5-7 days. The eggs are laid in the soil. Females drill holes in the ground and lay pods of 30-60 eggs. After laying, the pod is sealed with a frothy plug to protect them.
In the field females usually lay from one to three egg pods. If ample green food plants are available, females can lay their egg first pod 5-7 days after maturing and any subsequent pods can be laid in five day intervals during summer or 10-14 days during autumn.
In very dry conditions oocyte development in females can be delayed for many weeks. After migration, laying can occur even if the vegetation is drying off, but the first laying does not usually occur for ten days or more.
During summer, egg laying usually occurs in areas with some grass cover and egg pods are aligned laid vertically in the soil at a depth of 6-8 cm. In the autumn eggs are often laid in bare soil (frequently on claypans or hard ground along tracks and fences) and may be laid obliquely to the ground surface at a depth of 3-5 cm.
Female locusts often make test drill holes in the soil without laying any eggs. The extent of egg pod density can therefore be over-estimated if the drill holes are taken to indicate the presence of eggs. Gregarious egg laying often occurs in high density populations.
Locusts crowd together and many females may be seen with their abdomens in the ground at the same time. Egg pods in these egg beds can reach densities over 100/m2 in places. Egg development is strongly influenced by temperature and moisture conditions.
Eggs can either develop directly or development can be arrested by quiescence or diapause. Direct development occurs only in warm moist conditions. The rate of egg development increases with temperature so that complete development can occur in as little as 14 days with a daily maximum of 35o C, while at 25°C it can take a month. Egg development ceases below about 15°C.
There are several mechanisms which can delay the hatching of eggs of the Australian plague locust.
- 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. Quiescence occurs where growth is arrested directly by either cold or dry conditions. Cold induced quiescence occurs in winter in the southern areas.
Dry induced quiescence can occur at any time of the year. Q1 occurs when embryos are 25% fully developed if eggs are laid into dry soil. Q1 will be terminated if enough rain falls on eggbeds to allow eggs to absorb water from the soil. Most eggs can only survive for about six to eight weeks in dry soils when temperatures are very high (about 40°).
However, most eggs survive in the winter when the soil is dry. In dry conditions eggs enter quiescence at either 25% development if the soil was dry at laying, or at 45% if the soil became dry while the eggs were in diapause.
Diapause is the state where growth is arrested by an indirect effect of the environment. Decreasing light and temperature in the autumn increase the temperature threshold for development from about 15°C to 25-35°C. Eggs enter diapause if temperatures are below this higher threshold (after early March in southern NSW, late March in central NSW, mid April in southern Queensland).
Diapause ends after about 2 months, the embryos resuming development when conditions are warm and moist. Diapause occurs when embryos are 35% fully developed and may last for 7 to 11 weeks. Egg diapause occurs in autumn in southern regions and is usually completed by mid June.
After this time eggs will resume development, but low soil temperature in southern NSW, SA or Victoria delay hatching until October. If soil moisture is low at the end of diapause (eggs 45% 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.
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