MILD, wet conditions are ideal conditions for growing grass and other crops, but they are also ideal for many pests to build their numbers.

Despite the fact farmers were warned by the Bureau of Meteorology (BOM) to expect a dry, hot summer, across the western districts we have experienced higher than usual amounts of early summer rain, and both December and January were mild. 

This has made for some excellent summer crops, with maize across the district flying along, and brassica crops standing as high as the fence.

Many graziers have also had the luxury of green pick through January in their pasture paddocks.

However, with the good growth has come large numbers of armyworms and also white cabbage moths and butterflies in plague proportions. February saw large white clouds of cabbage white butterflies swarming, with farmers needing to control their larvae on brassica crops. Vegetable growers have warned the impact the pests are going to have on vegetable supplies and in turn cost on supermarket shelves.

Cabbage moth & cabbage white butterflies 

There are two species currently giving brassica crops across the district a hard time, and they are often confused with each other.

Cabbage moths, also known as diamondback moths, are small brown moths around 10mm in length, whose larvae generally cause the most damage to brassica crops year in, year out.

There is some issue with resistance to a lot of common pesticides within this species, and there are currently only a few options for effectively controlling them. 

Cabbage white butterflies are the swarms of white butterflies that have been plaguing the district, however in a usual year they are less of a problem to growers. Their larvae is less resistant to common pesticides, and they are therefore easier to control. However, given the huge numbers these butterflies are present in this year, it doesn’t take long for serious damage to occur to crops. Some sprays also come with seasonal limits, and especially with vegetable crops, some plants can’t be sprayed at certain stages.

With the large number of brassica summer crops planted this year due to favourable seasonal conditions, farmers will need to be on their toes with pesticide applications to get the most benefit for their livestock. Particularly as drier conditions lead to reduced growth rates and the increased value of the green feed available, spraying becomes a much for viable option for control.

Armyworm

The tell-tale feature of armyworm caterpillars is the three white parallel body stripes, always visible on the collar behind the head and usually continuing down the length of the body. Body colour can vary, but they are generally green, brown or yellow. After hatching, the caterpillars are 1 mm long, and can grow to 40 mm before pupating. Armyworms are the larval or caterpillar stage of night-flying moths.

The diet of armyworms found in southern Australia is generally restricted to grasses (and cereals including maize), with damage to brassica, legume, or other broadleaf species a rare occurrence.

Armyworms almost always fly into pastures and crops following medium to long distance migrations. After eggs hatch, they feed exclusively on leaves, although under conditions of food stress they will feed on the seed stem, resulting in head or panicle loss.

The change in feeding habit is caused by depletion of green leaf material or crowding. In the unusual event of extreme food depletion and crowding, they will ‘march’ out of crops and pastures in search of food, which gives them the name ‘armyworm’.

Whether to spray or not is an individual decision; many will choose to in late spring if there are high numbers of armyworm, particularly if there is good ground moisture and pasture would otherwise continue to grow well. However, by mid-summer a lot of farmers don’t worry, given pasture growth is limited by moisture any way.

Maize growers are however inclined to spray, at both establishment and cob stage of the maize crop. Armyworms can do serious damage to a maize crop, and it is well worth paying the cost of a drone or helicopter to apply the spray if the armyworms are in high enough concentration. They can do a devastating amount of damage to the high value crop in a relatively short amount of time.