What was it all about?

Running from 2015 to 2018, this project looked at strategies for reducing the development of stable flies in crop residues left after vegetable harvesting. The standout outcome from the work is the confirmation that burial and compaction of post-harvest residues is the most effective option for controlling the pest.

Post-harvest residues of most leafy vegetables rot within days after harvest and provide an ideal environment for stable flies to lay eggs and develop into adulthood, before emerging and affecting both humans and livestock nearby. Burying crop residues offers a simple and unique physical solution to prevent flies from accessing the material they need for egg-laying and development, without the need for pesticides.

The project team note that this approach to management involves two steps – burial of crop residues followed by compaction of the sandy soil above. They recommend the following:

• Burial of the post-harvest residues using either:
  • A mouldboard plough, which inverts the soil profile down to 45cm deep
  • A stone burier or contravator rotary hoe, which places the heavier vegetable residue down to a depth of 20-25cm below a layer of clean, sieved soil
  • A deep, slow rotary hoe where the residue is buried approximately 30cm
• Compaction of the sandy soil using a fixed landroller at or above 5 tonnes/m2.

The soil above the buried residues should be kept moist to facilitate compaction. Based on the research, this method will prevent at least 95 per cent, and as much as 100 per cent, of adult stable flies from being able to dig their way to the soil surface and emerge.

The burial process does not need to be done immediately as the compaction process prevents adult stable fly emergence. The project team recommends that the process is taken within one week after harvest is complete. The research showed that stable flies begin laying eggs onto broccoli, lettuce, silverbeet and celery residues within one to two days after harvest in summer production months.

The project engaged with major vegetable producers in affected areas on the uptake of machinery to bury and compact post-harvest residues. The research showed that smaller growers can employ the newly approved measure of rotary hoeing their residues five times in five successive days to minimise stable fly. This is a viable alternative for growers who may not be able to use a landroller for economic or other reasons.

Local agricultural machinery companies currently sell or can fabricate landrollers for use by vegetable producers. These range from 2.1m-6m wide for use on fixed-sprinkler line properties, or from 6m-12m, 18m and 24m wide rollers that could be used on open, centre-pivot irrigated vegetable operations.

As well as controlling stable flies, there are many other production benefits of burying crop residues for growers, including:

• Retention of all the organic matter from the residues
• Better soil moisture retention
• Less soil wind erosion
• Reduced reliance on pesticides
• Breaking up the soil hard pan that can develop due to the use of farm machinery, and preparing the soil for the next crop
• Less time, labour and machinery costs associated with only needing two operations to deal with post-harvest residues.

To arrive at residue burial as the ideal management option, the project team also looked at other ways to control stable fly on-farm:

• The parasitic was species Spalangia endius was investigated as a biological control option. The trial releases of the wasp did not result in the successful parasitism of any fly pupae and therefore would not be expected to have any measurable impact on stable fly populations.
  
  
• Beneficial fungi were also investigated for their potential to disrupt the stable fly lifecycle, by acting as a parasite of the pest while in the soil. However, the trials showed that applications of fungi did not have a significant impact on the development of stable flies in crop residue when applied either post-planting, two weeks prior to harvest, or both.

• A new version of cattle walk-through fly traps, for the non-chemical removal of flies from affected livestock, was also considered. However during the trial period, the numbers of stable fly never reached high enough levels to warrant putting the cattle through the cattle walk-through trap. This meant that the project was unable to fully assess the effectiveness of the traps against stable fly outbreaks.