Start low – It’s a numbers game

If you have pest populations in weeds and dirty fallows in spring, you run the risk of early infestation of crops, and potentially larger crop losses.

 Research has shown that the risk of population outbreaks in SLW is primarily driven by two factors; the size of the initial population in spring and the summer temperatures.  Higher starting numbers give the whitefly the edge over their natural enemies and reduce the number of generation it takes to reach outbreak levels. 

 If you start with more pests in the spring, even with careful management and favourable climate (for population suppression) you are going to reach threshold quicker.  For SLW this could be the difference between no control (Zone 1), suppression control (zone 2) or an IGR (Zone 3). 

What risks do the pests pose?

Cotton aphid

There are high aphid populations on volunteer cotton in some cotton growing areas.  In last season’s disease survey, bunchy top was commonly observed on volunteer cotton plants surviving over from the previous season.  The earlier CBT is transmitted to cotton, the greater the potential yield loss. Combined with industry concerns about aphid resistance to neonictinoids, the removal of overwintering hosts where aphids are present should be seen as a priority. 

See the earlier blog for another discussion of the importance of crop hygiene in minimising the risk of bunchy top (http://thebeatsheet.com.au/cotton/farm-hygiene-i…est-management)

Mealybug

In addition to Burdekin and Central Highlands Solenopsis Mealybug have now been confirmed in cotton growing areas of Dawson, Balonne, and Darling Downs. 

In February 2010, a survey was conducted in the Emerald region to determine whether there were any specific conditions associated with the severity of the mealybug outbreak in this region. Results of the survey showed that where mealybug infestations were severe, weeds and ratoon cotton had been present in fallows.  The higher the incidence of weeds and ratoon cotton in-crop the greater the severity of mealybug infestation.

Weedy field perimeters and poor channel hygiene was also associated with mealybug infestations.

 Will mealybug be a problem next season?

In June and July, entomologists from DEEDI conducted repeat surveys of five fields, in the Emerald region. These fields had a history of severe mealybug infestation last season. Mealybug were still present in both surveys despite crop destruction after harvest.  Mealybug were found on volunteer and ratoon cotton. Fields with a high incidence of volunteers consistently had higher mealybug numbers.  Other problem weeds noted as stand outs – in terms of being abundant across fields and having a relatively high frequency of mealybug incidence include pigweed, bladder ketmia and bellvine. Mealybug were found on the plants and also as far as 5cm below the surface – perhaps overwintering here.

Establishing a host free period for mealybug, to minimise carryover, is going to be a challenge this season.  Research by DEEDI entomologists has demonstrated that mealybugs can survive up to 60 days without food, and during that period can continue to produce offspring. 

There are a lot of things we don’t know about mealybug or the potential for another outbreak next season. However, the clear association between in-crop weediness and volunteers and mealybug hotspots makes it clear that farm hygiene should contribute to minimizing the likelihood of a high starting population. 

Managing ratoons and volunteers

Managing volunteers/ratoon cotton is always challenging.  In field, cultivation and herbicides can be effective in controlling volunteer cotton, however the following points should be considered:

• The effectiveness of registered herbicides is generally limited to volunteers no more than 4-6 leaf.  The size of the volunteers needs to be assessed before a herbicide selection is made.
• Larger plants will be more difficult to control in a single pass.
• Using the recommended water volumes for application is imperative for effective control.
• Other weeds within the field should be taken into consideration when making a herbicide selection.
• It is important to read all labels to confirm the correct application timings and rates.  Label directions must be followed.

Control of volunteers growing outside fields (along channels, roads and fences) is even more challenging as herbicide control is not always possible. 

Manual chipping is sometimes the only effective option, particularly where volunteers are well established.  While this is an intensive strategy, it is a good investment when taking into account the potential costs and losses caused by pests such as SLW, aphids and now mealybugs.

Article by Susan Maas, Melina Miles and Kate Charleston

This post is an update on cereal aphid management following a number of enquiries from growers and agronomists over the last week or so.

Which species in crops?
There are two species of aphid you are most likely to encounter in winter cereals (oats, wheat and barley). They are the oat aphid (Rhopalosiphum padi) and the corn aphid (R. maidis). The oat aphid is found around the base of the tillers, and the corn aphid in the whorl and under leaves higher on the plant. Both aphid species are greenish to black with rusty red-purple areas on the rear end around the ‘tail’.

(Line drawings from DPI Victoria’s Insectopedia)
See the Northern Region Ute guide for more detailed descriptions and pictures – or see August 23 posting on the Beatsheet Blog

How much damage can aphids cause?
The key question about aphids is “will the population of aphids in my crop cause damage to the crop, and yield loss?”

Direct aphid damage, as a result of feeding, is difficult to detect. In moisture stressed crops you may see yellowing with high aphid populations. Otherwise, there are generally no early signs of how much impact the aphids are having on the crop.

West Australian research showed yield losses of up to 10%, and reduction in seed size, with aphid infestations (this was without any impact of barley yellow dwarf virus).

Overseas research (Canada, US) suggests that significant yield loss occurs when aphids are present from flowering through to milky grain. The data also suggests that yield loss does not occur when infestations are present earlier or later than this period.

Making control decisions

Corn aphids may disappear by themselves
Corn aphids, the species that lives in the whorl, generally disappears when the crop comes into head. This is because their preferred site is no longer there, and they tend not to survive as well on leaves. If you have the corn aphid in crops, consider delaying a control decision until the crop starts to head.

Control thresholds
Qld and WA threshold recommendations are comparable, with the WA recommendations based on the most recent research that has been undertaken in Australia.

Recommendation are to check crops regularly from late tillering, and consider control if the aphid population exceeds 15 aphids/tiller on 50% of tillers.

http://www.agric.wa.gov.au/content/pw/ph/dis/cer/bydv_aphidfeeding.htm

Other considerations when making a decision about cereal aphids

1. Natural enemies (lady beetles, hoverflies, parasitic wasps) can have a big impact on aphid populations, reducing them to very low levels in many instances.
2. Dimethoate and synthetic pyrethroids (registered for cereal aphid control) are highly disruptive to natural enemies. The application of these insecticides early (e.g. during the vegetative and early tillering stages) may result in a later reinfestation of the crop because small numbers of surviving aphids are no longer controlled by natural enemies. The impact of these products on natural enemies can persist for some days.
3. Pirimicarb (Pirimor®) is a soft option for cereal aphid control, compare its price with that of dimethoate when making a decision – it may be competitive. Pirimicarb has some systemic activity.
4. Oat aphids, at the base of the plant, can be difficult to contact in a dense crop. Dimethoate will kill aphids by contact, but its systemic activity is by translocation upwards, so its efficacy against oat aphid is unclear.
5. The systemic activity of both pirimicarb and dimethoate may be reduced in moisture stressed crops.
6. Rain will reduce aphid populations by knocking/washing individuals of plants, and the aphids tend not to get back on the plants. Often ground predators, like carabid beetles, ants etc will eat aphids on the ground. Check populations of aphids again if you get more than 20 mm rain.

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.