What was it all about?

With an increase in redback spider numbers being reported in broccoli, this short 2018 investment reviewed all available information on the problem. It examined factors that may be contributing to the spiders entering broccoli crops, or contaminating broccoli after harvest, and produced the below key findings to help explain the situation:

• As female redback spiders can mature in six to eight weeks at 25°C, spiderlings that enter a broccoli crop at planting can potentially mature by harvest.

• A single egg sac can cause a significant infestation –the average sac produces an average of 110 spiderlings, with female spiders typically lay four to seven egg sacs over summer (and photos of spider contamination on social media appeared to show newly mature female spiders, which is consistent with them having emerged from egg sacs laid mid-summer).

• Long-distance dispersal of redbacks can occur through transfer on equipment and machinery, with spiders most likely to enter crops with this human assistance.

• Redback spiders can survive long periods without food and can survive extremes of heat and cold, so can easily tolerate the times and temperatures in broccoli supply chains.
  
  
• Redback webs have a number of key distinguishing features, that make them easy to tell apart from those of other spiders. Social media photos did not show any webbing within the contaminated broccoli heads, which may indicate the spiders were not living in the broccoli in the field, but actually entered at or soon after harvest.
  
  
• The shift to integrated pest management (IPM) using selective insecticides may have increased survival of both redbacks and their non-target prey organisms. Key prey for the spiders include beetles, millipedes and other ground-dwelling insects that are not usually regarded as pests.

The researchers noted that peaks of redbacks being found in broccoli appeared to correspond with hot, dry summer conditions. “The risk appears to be increased when an autumn crop of broccoli follows a summer crop of cucurbits, such as pumpkin,” they noted. “It is unclear whether this is because significant redback populations remain in the field, or whether redbacks sheltering on equipment and harvest bins enter broccoli after harvest. However, it is noted that the major outbreak of redback spiders in broccoli [in south east Queensland] occurred after a dry summer when pumpkins were in major oversupply, resulting in abandoned crops and extended storage of harvested produce.”

In regard to reducing the risk of redback infestation, the project team recommended thoroughly cleaning equipment and machinery used to grow and harvest broccoli and, if spiders are found in the field, destroying crop residues that they could use as harborages.

ACT NOW

The project produced a handbook for growers to better understand the risk and management of redback spiders in broccoli. Download it here.