The economic cost of midge and heliothis damage

In recent seasons, particularly last summer, midge were again a problem and according to Pacific Seeds Summer Grains Agronomist, Trevor Philp are set to remain so.

“Despite the advent of midge tolerant varieties significantly reducing the threat of midge damage and helping with integrated pest management in grain sorghum, midge and heliothis still pose a significant threat to late planted grain sorghum crops,” he said.

Mr Philp said there is a high degree of confusion surrounding midge tested ratings, what they mean and at what threshold spraying for midge or heliothis is financially advantageous. He has written the article below to help clarify the issue.

A variety with a six midge rating has six times the resistance to midge compared to a variety with a rating of one. At today’s high sorghum prices, damage from midge could be very costly, example; a crop with five flowering heads/meter squared has 10 midge/head, the potential dollars lost for a variety with a midge rating of six would be $121/ha versus $364/ha for a variety with a two rating.

The spread of planting in Central Queensland and the soft humid conditions has provided an ideal breeding ground for midge, and significant damage has already been reported.

Growers and consultants need to rigorously check crops from start of flowering through to the end of flowering, this usually last around five to seven days but uneven crops can flower for several weeks.

Table 1: Visual sorghum midge damage and subsequent yield loss in sprayed and unsprayed crops in midge susceptible and resistant hybrids.

* means in columns (columns and rows for midge damage and yield) not followed by the same letter differ from each other at p<0.05.

‘ Sum of daily ovipositing females per panicle x flowering panicles in 10m in each whole plot.

‘’ Average number of midge ovipositing per panicle per day on 10 panicles form whole plots and six late flowering panicles within whole plots respectively

! Visual % seed set lost to midge damage on 10 panicles in each plot

SOURCE; QLD DPI&F, DAVID MURRY & ADAM HARDY

Generally sorghum midge is sprayed with a synthetic pyreithroid; these chemicals are very effective and are cheap to use. However they are highly disruptive to the survival of beneficial insects. The use of disruptive chemistry to control midge increase the risk of flaring other pest such as Heliothis and Aphids.

Heliothis generally lay their eggs just prior to flowering and develop at a very rapid rate especially when temperatures are high. New research by Dr Bernie Franzman from the QDPI & F has identified that damage from Heliothis larvae is 60% higher than previously thought; new data suggest that larvae eat a total of 2.4g of sorghum not 1.5g.

Checking for Heliothis should start at early flowering, I like to start checking when the anthers have started to turn orange 50mm from the top. Larvae are still very small at this stage and can be hard to see. The easiest way to sample is to beat a number of flowering heads into a bucket, and then count the larvae.

The most cost effective control for Heliothis is nucleopolyhedrovirus (NPV); trade names are Vivus Gold and Gemstar. This product has no impact on beneficial insects and very safe for the operator to use. This product takes around five days to kill larvae, so it needs to be applied to larvae when they are small. Significant damage occurred in crops last year when NPV was applied late on large larvae in high numbers. Overall control on these crops was good but high levels of damage occurred before the larvae died. Recommended size for the control of Larvae with NPV is 5mm and up to 13mm.

Link to midge calculator