While the lucerne flea (Sminthurus viridis) is most infamous during autumn as a winter establishment pest, they do stick around until late spring – they are just less problematic as crops advance and get ahead of, and outgrow, their damage.
But their presence can be seen or felt in spring, especially in pastures and spring-sown susceptible crops (e.g. lucerne and canola) with adequate moisture, demonstrated by windowing in leaves and skeletonised foliage as observed by some growers and advisers recently.
Lucerne flea damage is quite characteristic. They are ‘chewing’ invertebrates so when feeding, they scrape off bits of leaf tissue and eat it, resulting in small holes or a thin clear layer of leaf membrane that appears as transparent ‘windows’ through the leaf.
In severe infestations, this damage can skeletonise the leaf and stunt or kill plant seedlings.
Lucerne flea will work up a plant from ground level – damage is most visible on lower leaves first.
If you are assessing if lucerne flea requires chemical intervention at this time year, consider the following:
Are the affected plants getting away from their damage?
Lucerne flea is usually found patchily in paddocks. Examining how the affected plants compare to growth in the unaffected plants growing under the same soil, moisture, and nutrient conditions can give a clue as to whether intervention is needed.
How big is the population of lucerne flea?
If lucerne flea populations are large, beneficials may not be able to slow the tide of their feeding in seedlings. While there are predators such as spiders, snout mites and carabid that eat lucerne flea, these ground dwelling predators are more likely to have a suppressive effect and make more of a dent in smaller populations. Especially if there is a history of broadsprectrum insecticide use in paddocks and beneficial populations have not had the chance to establish.
Does the whole paddock need to be targeted?
As lucerne flea is generally patchily distributed within crops, spot or border spraying may be sufficient. In addition to saving on chemical costs, this approach limits the impact of insecticides on beneficials.
What chemistry are you considering?
Lucerne flea has a high natural tolerance to synthetic pyrethroids and should not be treated with insecticides from this chemical class.
Ben Dumesny – Premier Ag (South West VIC)
Rob Fox – Agrivision (Wimmera VIC)
Casey Willis – Rodwells (Gippsland VIC)