Calculate your costs before tackling mung bean grub problems

Continued from last week, looking at the control methods for helicoverpa grubs in mung bean crops.

USING insecticides as the singular control method only in the reproductive stage makes me nervous, especially when crop expectations and prices are quite solid. So my preferred spot for application is in the vegetative stage, when it appears the helicoverpa consume adequate lethal quantities of these handy biologicals.

Used with conventional companion products is fine, but I suggest not using them on their own in our R stage of growth.

The big decision, of course, is when you hit the go button for spraying.

In other words, the threshold level where damage caused is losing you money and I refer to the plants' reproductive time here.

Our researchers have done a great job isolating and explaining compensatory effects and cost benefit ratios and this knowledge is great to have compared to 12 or 15 years ago, when we were doing an awful lot of extrapolating on damaging grub numbers in our mung bean crops.

I also suspect that the advent or wholesale usage of Transgenic cotton in these big cotton acreage years is acting like a big sink hole or vacuum cleaner for moths laying eggs, with considerably fewer numbers emerging from this fibre crop. As did our recent early sorghum crops with their short flurry of heliothis activity in many areas, which was controlled by the NPV product.

So let's examine our thresholds for mungs with less extrapolation than previous years. Most of our thresholds are based on 35kg/ha of damage/ loss per larvae in a square metre.

In a hectare of mungs at pod fill time, if you have two heliothis grubs in every square metre of the 10,000 square metres in your hectare, you would have 20,000 grubs.

This level of larval activity can damage or consume a total of 70kg in this hectare of mung bean, which this year at $700 per tonne could have a lost value of $50.

So when you calculate the cost of spraying these eating machines at $40 per ha applied, you can get fairly close to creating an acceptable threshold with your running costs and insecticide costs and then factor in the expected price for your crop.

Then you need to take into account crop size (as in dry matter), plus the soil moisture levels propelling a high yielding situation with obvious increased crop compensation from early heliothis losses.

So there is a fair degree of assessment in deciding to spray your mung bean crop and it always helps to be able to electronically access The Beat Sheet Blog by the DPI.

Topics:  biosecurity mung beans paul mcintosh

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