Monitoring and Forecasting Locust Populations

The Australian Plague Locust Commission monitors the population status of the three pest locust species listed under its charter using a range of information sources, including surveys undertaken by APLC officers, light trap information and reports from the public and state agriculture authorities.

The information on current locust distributions is used with meteorological and environmental information to produce short-term forecasts of likely development and possible population changes, which are summarised in the Locust Bulletin. The information below provides more detail on these activities.

Monitoring 

Surveys
Reports
Light Traps

Forecasting

Forecasting locust development
Mapping suitable habitat and vegetation conditions
Decision support system

Surveys

The APLC constantly monitors the locust situation in eastern Australia by conducting regular ground surveys in the major breeding areas. Field officers located at bases in Narromine, Broken Hill (NSW) and Longreach (Queensland) conduct surveys according to seasonal timing and habitat conditions.

The standard method of survey is to drive a vehicle through areas of suitable habitat, making regular stops (about every 10 km) to conduct foot transects. At each stop the number, development stage and species of locusts encountered is recorded, as well as the type and condition of ground vegetation. Information is recorded using GPS-connected palmtop computers and is regularly transerred to a Geographic Information System (GIS) server at APLC Canberra HQ.

Field officers also contact local landholders and officers from relevant state agencies to exchange information on the current locust situation. All this information is relayed to the APLC headquarters in Canberra by HF radio.

Fixed wing aircraft or helicopters are sometimes used to survey large areas that require rapid assessment. This typically occurs when an upsurge is expected or detected, or when conditions make ground survey impossible. 

The image below is a summary of survey records for the Australian plague locust collected during April 2010. Adult density is shown by pink symbols, with red indicating swarms, and nymphs are shown by green symbols. Red circles in Victoria show reports of any locust activity.

Reports

The APLC relies on landholders and the general public to report any locust activity in their area. Reports are subsequently checked by APLC, state agency or LHPA officers to verify the species of locusts and to determine whether control is required. See our Reporting Locusts page for details on how to report locust activity to us.

Light traps

Locusts have the ability to migrate hundreds of kilometres in a single night and may do so on successive nights.  The APLC operates a network of eight light traps across eastern Australia to detect nocturnal activity or long distance migration of locusts. Night flying locusts in the vicinity of the trap are attracted to light. Sudden increases in locust numbers caught often indicate regional migratory activity, including possible immigration.

A light trap designed to monitor locust migration.
Adult locusts flying during the night are attracted to the 250W
mercury vapour lamp and fall into the base filled with water where
they are identified and counted the following morning

Light trap locations

Light traps are operated throughout the locust season from August until the end of May. The number of each of several locust species caught is recorded daily.

Forecasts of the locust development

Forecasts of likely locust developments are made for the current and next generation.  The forecast period is for the following six weeks or from autumn to spring. Forecasts attempt to address the location, timing and scale of developments and possible migratory redistribution. Regional risk statements take into account both the probability and consequences for agriculture of possible outcomes. There is currently no reliable method for estimating inter-annual locust population levels in a given region.


Regional forecasts and seasonal outlook are based on

• current population distribution and behaviour
• recent rainfall and habitat condition
• seasonal rainfall expectation 
• historical frequency of infestations 
• longterm meteorological indices
• habitat stratification 

Australian plague locusts

The rate at which individual locusts pass through the different stages of their development is determined primarily by temperature. Rainfall influences the timing of certain developmental milestones in eggs indirectly through soil moisture and the development rate of nymphs and sexual maturation of adults through food supply. Previous basic research on the developmental rates of eggs and nymphs over a range of constant temperatures have been used to develop lifestage specific models to forecast the timing of locust developmental stages based on the temperatures they are likely to experience in the field. Nymph and adult locusts are also able to their regulate body temperature by basking in the sun which hasten development. The model for the egg stages includes diapause and quiescence, which delay the time taken for eggs to hatch.

The growth of juvenile locusts, called nymphs or hoppers, occurs in discrete stages called instars. In Australian plague locusts there are typically five instars. Adults also pass through stages of maturation and egg development before laying.

The known development stage and population density of locusts in different locations is used to initialise the development model.  Nymphal development in locations where population development is not known are also be modelled, using likely prior egg-laying activity associated with rainfall. These dates are used to allocate staff and resources to locust survey and to align survey with the timing of critical lifestages likely in the field.

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 spends the winter in 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 egg quiescence in dry soil, and the Australian subspecies does not display a low temperature egg diapause.

Mapping suitable habitat and vegetation condition

Satellite images of vegetation greenness are used to identify areas where ground vegetation in locust habitat areas has responded to rainfall and might provide suitable conditions for locust survival, breeding and potential population increase. Current vegetation index imagery is available within two weeks of the satellite observation period and is used to make decisions as to where locust surveys will be carried out.

The condition of vegetation is also assessed directly during survey by recording the presence and amount of green leaf material on a scale from green shooting to completely dry, for several ground vegetation components (perennial grasses, ephemeral grasses and forbs).

The example image below shows the relative greenness of vegetation in broad locust habitat areas for the 14-day period beginning on 20 March 2010.

 

Source: Department of Land Information - Government of Western Australia (Satellite Remote Sensing Services)

NB: the APLC area of operations is to the west and north of the red line.

The image above shows where vegetation is green using the Normalised Difference Vegetation Index (NDVI) obtained from the MODIS instruments on board US National Aeronautics and Space Administration (NASA) earth observation satellites. In arid and semi-arid areas of Australia, open plain landscapes with a sparse covering of grasses and forbs are important habitats for locust breeding, especially when plants produce new growth following rain. Landscapes such as forest and scrub, dunes and rocky hills are largely unsuitable for locust breeding and have been masked out in grey to emphasise the condition of predominantly grassy vegetation in locust habitats. The image is re-scaled to show current NDVI values as a proportion of the historic range in values for the same locations. It has the effect of emphasising growth in response to rainfall in sparsely vegetated landscapes relative to much larger NDVI values for dense vegetation, such as coastal forests or crops.

APLC Decision Support System for monitoring and forecasting locust populations

The APLC uses a GIS based decision support system (DSS) to assist locust forecasting and operations. The data collection components of the DSS include transfer of locust survey data from the field and daily internet collections of weather data. Locust distribution and age information collected by APLC officers on regular vehicle surveys using GPS-connected palmtop computers is emailed to Canberra for transfer to a GIS server.

This enables the integration of those environmental factors which determine habitat suitability and those which influence locust distribution and recruitment, with known distributions to model development across the entire monitoring area. In this way short term predictions of development stage and reproductive success can be made for known and potential populations at different locations.

The primary management decisions - the early intervention strategy, control opportunities, constraints and methodology, the use of spotter and spray aircraft - utilize the experience gained from years of research and operations. Operational campaign decisions - population thresholds to launch or cease campaigns, control agent, technique and identification of targets - require intensive field assessment and are made by officers conducting a control campaign.

The decision support system consists of a set of computerised secondary decision tools that have been added or modified as technology, information sources and operational needs change. They provide access to databases, simulation models, spatial analyses and maps to support both forecasting and operations.

Forecasting support tools

• Daily weather analyses – rainfall, temperature, windfields
• Gridded accumulated rainfall for consecutive days
• Locust distributions- survey data
• Location-specific weather data for development models
• Stage-specific development models using DYMEX 
• Automatic modelling of potential and known populations
• Wind trajectory analysis
• Processed data outputs from Insect Monitoring Radars (IMR) 
• Satellite vegetation greenness index (NDVI) imagery
• Bulletin maps

Operations access tools

• Field survey data collection, transfer and GIS processing
• Display interface to survey and control information
• Historical locust databases
• Landuse limitations – nature conservation, organic production, threatened species 
• Property boundaries – state digital cadastral extracts
• Control information navigation and data management software (OpsManager) 
• Property control maps
• DGPS target logfile conversion and management

Online support tools

• Regional weather forecasts and observations
• GMS weather satellite IR hourly cloud images and loops
• Bureau of Meteorology weather radar, meteograms and weather analysis charts
• 4 – 10 day rainfall forecasts and longterm (SOI) seasonal outlook

The DSS is built using a commercial vendor GIS system and uses scripting to coordinate data ingestion, processing, modelling and visualisation. It depends on regular internet data feeds of reported and modelled weather data from the Bureau of Meteorology via file transfer protocols and locust distribution data from surveys as input to development and movement models. The operation of the DSS for forecasting involves modelling critical lifestage events and likely outcomes for the current and offspring generation from initial distribution estimates based on survey or reports. It is designed to give experienced staff access to all relevant primary or derived information. For operations it provides map overviews and detailed regional views of the current locust situation that can be combined with relevant environmental conditions and infrastructure.