Lucerne flea

The problem of early hatching of lucerne flea may be the next generation

 

Where have they been reported?

Mixed populations of juvenile and adult lucerne flea have been observed widely throughout the South West Slopes district of NSW (Lockhart, Walla Walla & Albury) and northern Victoria (Katamatite, Rutherglen & Oxley Flats) suggesting that many will have hatched after the March and then early April rains. (Thanks for the reports*). This is about six weeks earlier than in recent years. In most instances, the occurrences to date have been sporadic and caused only minimal feeding damage.

Lucerne flea habits and behaviour

Lucerne fleas (Sminthurus viridis) hatch from their summer diapause following periods of good soaking autumn-winter rainfall. Egg development is stimulated when the soil remains moist for at least 3 days and the average daily temperatures for the following 11 days remain below 22°C. This is unlike earth mites that have much cooler temperature thresholds. The early hatch this year in some regions of southern NSW and northern Victoria is likely to result in a second generation before winter, placing later-sown crops at risk.

Lucerne fleas typically cause significant damage to emerging crops and pastures. They can also cause considerable damage to older crops in spring if numbers build up under favourable conditions throughout the season. They are generally a problem in regions with loam/clay soils because of their egg-laying requirement. Paddocks are most likely to have problems where they follow a weed-infested crop or pasture in which lucerne flea has not been controlled.

Lucerne fleas have a wide host range and will attack most broad-acre crops, including canola, lucerne, pastures, some pulses and cereals. Grass species (including cereals and pasture grasses) are less preferred hosts, but they can still damage seedlings. Young nymphs feed on the soft tissue on the underside of leaves leaving transparent ‘windows’, whereas adults and older nymphs rasp and chew irregular holes in leaves and can completely defoliate plants.

Our advice

If chemical control is required, do not use synthetic pyrethroids. In paddocks where damage is likely, a border spray may be sufficient to prevent movement of lucerne fleas into the crop from neighbouring paddocks. As lucerne fleas are mostly distributed patchily within crops, spot spraying is often all that is required. Do not blanket spray unless the infestation warrants it.

Control options

If the damage warrants control, treat the infested area with an insecticide about three weeks after lucerne fleas have first emerged in autumn. This will allow for the further hatching of over-summering eggs but will be before they reach maturity and begin to lay winter eggs. Lucerne fleas have a high natural tolerance to synthetic pyrethroids and should not be treated with insecticides from this chemical group.

To avoid populations building up in subsequent years, consider some of the cultural control options. Grazing with livestock in spring to reduce the height of pasture will limit food resources and increase mortality of lucerne fleas. Control broad-leaf weeds (e.g. capeweed) to remove alternative food sources that would otherwise assist in population build-up. In pastures, avoid clover varieties that are susceptible to lucerne flea damage, and avoid planting susceptible crops such as canola and lucerne into paddocks with a history of lucerne flea damage.

Description

Adult lucerne fleas are approximately 3 mm in length and appear yellow-green to the naked eye, although with the aid of a hand lens their globular abdomens are often a mottled pattern of darker pigments. Newly hatched nymphs generally resemble adults but are only about 0.75 mm long and pale yellow in colour. Lucerne fleas can ‘spring-off’ vegetation using a specialised organ (furcula) under the body that allows them to jump when threatened.

 

* Sources of field reports of lucerne flea

Josh Douglas – Researcher, The University of Melbourne

Warwick Nightingale – Agronomist, Delta Ag (NSW Riverina)