Are aphids sucking away cereal profits?

Aphid control decisions tend to be more problematic in moisture stressed winter cereal crops, since in a well supplied crop the level of moisture extracted from the crop by aphids is of little concern. However, in dry times every drop seems precious.

Had we not received the recent rain over the last few days throughout southern Queensland and northern NSW grain growing areas, we could have expected enquiries about aphids in stressed winter cereal crops. But even so, the crops aren’t finished yet, and so it’s timely to be reminded of what the different aphid pests are in winter cereals and the principles for managing them.

Aphid species
Four different species of aphid commonly attack barley and wheat in Queensland. They all prefer barley more than wheat. Aphids suck sap from the plants. Under heavy infestations plants may turn yellow, be stunted and appear generally unthrifty.

Oat or Wheat Aphid (Rhopalosiphum padi)
The oat aphid is brown to muddy green with rusty red patches at the base of the tubes at the rear end of the body. It normally occupies the base and lower portions of the plant. This is generally the most common aphid attacking winter cereals.

Corn Aphid (Rhopalosiphum maidis)
The corn aphid is green to dark olive-green with a purplish area at the base of the tubes at the rear end of the body. It normally lives on the tops of the plants particularly within the rolled up terminal leaf.

Rose-grain Aphid (Metopolophium dirhodum)
The rose-grain aphid is pale green with a darker green stripe along the middle of its back. It normally occupies the undersides of the leaves. It colonizes the lower leaves and moves up the plant as leaves senesce.

Rice Root Aphid (Rhopalosiphum rufiabdominalis)
The rice root aphid is a honey-brown colour with a rusty red area at the base of the tubes at the rear end of the body. It normally occupies the roots of the plants under the soil surface.

Making decisions
The decision as to whether controlling aphids on winter cereals will provide an economic return is often complex, and is not just dependent on the size of the aphid population.

Several other factors could influence the control decision. Aphids are more readily controlled in seedling and pre-tillering crops which are less bulky than post tillering crops. Aphids tend to disappear as crops come into head.

Prolonged infestation of moisture stressed crops can exacerbate the effect of moisture stress. Yield potential, value of grain and cost of control are important considerations, but anecdotal evidence suggests that direct feeding by very large numbers of aphids is needed to impact on yield.

Natural enemies (ladybird beetles, ladybird larvae, hover fly larvae, lacewing larvae or parasitic wasps) can exert effective control on small to moderate aphid infestations.

Photo: Larva of hover fly in an aphid colony.

A general recommendation is to check for aphids by choosing six widely-spaced positions in the crop and at each position examine five consecutive plants in a row. If 27 out of 30 plants are covered with aphids and if there are less than two natural enemies per plant on each of the infested plants, then consider treatment. Delay any planned chemical control if rain is forecast and check again after rain.

Dimethoate at 500 mL/ha of 400 g/L product is the recommended chemical control. It has a withholding period of 28 days for harvest and one day for grazing.

Dimethoate has a contact action but is also a systemic insecticide taken up through the leaf and then translocated through the upper portion of the plant. Aphids are subsequently controlled when they feed on the plant.

The rice root aphid feeds below ground and can’t be effectively controlled by spraying.

Dimethoate may be tank-mixed with certain broadleaf herbicides. Check the label before use. Also check water quality as high pH can affect performance of dimethoate.

Dimethoate will kill natural enemies, increasing the possibility of subsequent aphid infestations later in the season.

St George growers meet to discuss area-wide SLW management

On Friday August 10, St George growers and agronomists met to discuss strategies to manage the Silverleaf whitefly population in the irrigation area.
Over the past two seasons, a number of cotton fields in St George have been treated for SLW. Last year more fields were treated earlier, and required a second treatment. It is clear that SLW is now part of the insect pest complex in St George, and it is important that St George growers take the same area-wide approach to managing SLW as has the Emerald irrigation area.
Richard Sequeira (DPI&F entomologist, Emerald) discussed the role of cotton as the main driver of the local SLW population. Tactics discussed that can contribute to minimising SLW outbreaks include narrowing the planting window as much as possible, avoiding the use of broadspectrum insecticides early in the season for other pests (OPs and SPs for mirids and helicoverpa) and the correct timing of SLW control.
Melina Miles (DPI&F entomologist, Toowoomba) discussed the survey data from 06-07 that shows the rate of population growth in St George is very similar to that in Emerald. In contrast, data from the Downs shows that growth is significantly slower and the risk of outbreak on the Downs is low in most seasons. Overall parasitism of SLW was very low in 06-07.
The development and implementation of tactics to manage SLW will be an ongoing process for growers and agronomists in this area. For more details contact Dallas King (Regional Extension Officer, St George 0427 635 621) or Richard Sequeira (Emerald, 49837410).

A better way to make decisions about Helicoverpa control in chickpea

A new guide to calculating economic thresholds available now
Growers and agronomists are now able to calculate how much yield they will lose if they do not control an infestation of helicoverpa in chickpea. They can calculate the point at which it is economic to control an infestation based on the potential yield loss, their costs of control, the expected grain price and their own preference for benefit:cost.

Being able to calculate the economic thresholds based on an individual grower’s situation is a significant advance on the old recommendations which were fixed at 1-4 larvae per square metre, and did not allow for adjustment when there were changes in grain price or costs of control.

The calculation is now possible following several seasons of research by Melina Miles and Richard Lloyd in Toowoomba, and Paul Grundy and Sherree Short in Biloela. The DPI&F entomologists have worked out that one helicoverpa larva, surviving from hatchling to pupa, will consume 2 grams of chickpea grain. This information is the basis on which potential crop loss ($/ha) is calculated.

The research also examined the impact of spray timing on yield and found that no yield loss occurred when larvae were allowed to feed in vegetative and flowering crops. Yield loss only occurred when larvae fed on developing, filling and maturing Many hours observing larval feeding in the field supported this result. Observations found that larvae of all sizes fed readily on leaves, large larvae did the majority of feeding on pods, and that neither small nor large larvae fed on buds and flowers. However, there are seasons when larval pressure in vegetative and flowering crops may necessitate control.

Grain quality was assessed in both threshold and spray timing trials, and no loss in quality (increase in splits or damaged grain) was seen within the range of densities that it was economic to spray.
In summary, the recommendations for helicoverpa management in chickpea are:

• Use a beatsheet to sample helicoverpa in chickpea.
  
• Exclude very small larvae from threshold calculations – they are difficult to assess accurately in the field – they will be counted as smalls at the next count.
  

Adjust the number of small larvae for natural loss from disease, predation etc (30% loss)




• Calculate potential crop loss ($/ha) using the equation.
  
• Unless pressure is extreme, delay any control until crops are setting, filling or maturing pods. It may be necessary to apply control during flowering if targeting small larvae that will be medium-large by pod set.


• Details of the research, and calculations of economic thresholds are in the new brochure – get yourself a copy!
  
  This research was supported by GRDC and DPI&F.

Get a copy of new Helicoverpa Management in Chickpea brochure from the DPI&F Call Centre, phone 13 25 23. Will soon be downloadable from the web.

IPM Researchers Forum 2007

About 50 devoted enthusiasts of Integrated Pest Management (IPM) gathered in Toowoomba on 24-25 July 2007 for the annual Northern Farming Systems IPM Researchers Forum.

Participants in the forum were from industry, research, extension and stakeholders. The aims of the IPM Forum were to exchange the latest information about research and projects and to maintain linkages between IPM projects and people across industries.

The Forum mostly focused on R,D & E being conducted in NSW and Qld, in the broadacre farming system. The broad range of topics covered in presentations included some key pests – green mirids, stink bugs, helicoverpa and silverleaf whitefly.

Of particular interest were presentations by a number of postgraduates from The University of Queensland.

• James Hereward - Dispersal and host plant interactions of green mirids (or ‘Where do mirids come from – are we dealing with locals or immigrants from far inland?’)


• Corinna Lange - The population genetic structure of native budworm, Helicoverpa punctigera, in the Australian landscape (or ‘My trip to pick grubs off wildlowers in the middle of Australia’)


• Jamie Hopkinson - Host acceptance behaviour of an aphid parasitoid (or ‘How I learnt to dissect tiny aphids and find even tinier wasp eggs inside them’).

It was interesting that many IPM Forum participants were noticeably envious of Corinna’s work – but less so of Jamie’s!

Other welcome participants included the Grains Research and Development Corporation Crop Protection Program Team who seized the opportunity to meet with project leaders and discuss research results.

If you have any questions about the 2007 IPM Forum, leave a post below.

Until next time, Big Bug.

Photo: The GRDC Crop Protection Program Team includes (L to R) John Sandow, GRDC Crop Protection Manager, Ralph Burnett, Western Panel member, Allan Mayfield, Southern Panel member, Rohan Rainbow, GRDC Project Manager, and Bill Yates, Northern Panel member. Photo provided by GRDC.