Friendly fighter conquers foe

Microplitis demolitor is just one of many friendly fighters that battle to contain numbers of one of our most important pests, the corn earworm, Helicoverpa armigera.

Corn earworm on grain sorghum is making its presence felt and many crops are being sprayed with Helicoverpa nucleopolyhedrovirus (NPV) to control above-threshold infestations of caterpillars.

The current high value of grain sorghum (over $250 per tonne) means that it is economic to control caterpillars at lower numbers (density) than growers may have sprayed previously when grain value was lower.

Low caterpillar numbers is a perfect situation for Microplitis to chip in a helping hand. It is not uncommon to find 30-40% of small caterpillars on grain sorghum parasitised by Microplitis. In many cases, this level of parasitism may be sufficient to sway a decision to not spray.

What is Microplitis?
Microplitis is a small native wasp that lays it eggs in (parasitises) small helicoverpa caterpillars. The life cycle from egg to adult takes about 12 days. This is made up of 7 days from egg laying to forming a pupa, and then 5 days for pupal development.

Adult Microplitis are small black-brown wasps. They are often seen flying slowly above the crop canopy in search of caterpillars (hosts). A female wasp will parasitise as many as 70 helicoverpa caterpillars. The parasite develops inside the host caterpillar. When fully developed, the Microplitis larva chews a hole in the side of the caterpillar and spins a fawn-coloured cocoon around itself and pupates. The caterpillar that was parasitised may still be alive, but it will soon die.

Clues to identify Microplitis activity include

• Adult wasps foraging on sorghum heads
  
• Split test of caterpillars to reveal internal parasites
  
• Distinctive fawn cocoons next to dead or dying caterpillars
  

Identifying parasitised caterpillars
In the field, parasitised caterpillars can be identified by performing a simple split test. Parasitised caterpillars will only grow to about 15 mm in length, so caterpillars smaller than this are potentially Microplitis hosts. Hold a caterpillar across a forefinger with one thumb and forefinger on the rear end of the caterpillar, and with the other thumb on the head. Gently stretch the caterpillar until the skin ruptures. A Microplitis larva developing within the caterpillar looks like a white maggot up to 4 mm long.

Interactions between Microplitis and NPV
Caterpillars infected with NPV within 3 days of parasitisation by Microplitis will die from NPV. The immature Microplitis will also die because of the death of its host.

When NPV is applied to control corn earworm, it is not unusual for some parasitised caterpillars to survive the treatment. Caterpillars parasitised more than 3 days prior to the NPV treatment will produce healthy Microplitis. Parasitised caterpillars feed less and may not ingest NPV.

In shaking sorghum heads to make post-treatment assessments, parasitised larvae may be dislodged free of the pupal cocoon attached to them. Careful inspection of these caterpillars may reveal a hole in the side of some of these caterpillars, indicating prior parasitisation. These larvae will eventually die.

Microplitis is an important natural enemy of the corn earworm and they need to be considered when making decisions about when to manage corn earworm.

For more information on Microplitis, follow the link to the brochure ‘Microplitis demolitor and ascovirus: Important natural enemies of helicoverpa’ http://www2.dpi.qld.gov.au/fieldcrops/17576.html

Corn earworm chews into sorghum profits

Sorghum growers across the Darling Downs can expect to see an influx of the corn earworm, Helicoverpa armigera, in their flowering sorghum crops over the next few weeks. Growers are well equipped to deal with the problem in an environmentally friendly way.

Moths are active and wanting to lay eggs on susceptible crops, and sorghum crops putting up heads are highly attractive - just what this insect pest loves. The majority of eggs are laid in a narrow window, between emergence of the head from the boot leaf and the commencement of flowering (yellow anthers). This results in highly synchronous development of larvae in a crop – larvae of uniform age in the crop.

Yellow anthers
during flowering
(too early to spray).

Eggs and newly hatched
larvae on sorghum.
Photo: D. Ironside

It is important that growers check their crops because in many cases feeding by corn earworm is likely to cause economic loss. One larva is estimated to consume 2.4 g of sorghum. Larvae up to 13 mm in length feed mostly on anthers and do not affect yield.

The table below provides examples of crop loss for different larval densities.
Table: The value of crop loss caused by corn earworm larvae in grain sorghum, for a range of larval densities and grain prices and based on 10 heads/metre of row on 1 metre row spacing.
*Based on estimated consumption of 2.4 g per larva.

The current high value of grain sorghum (over $300 per tonne) means that it is economic to control larvae at lower numbers (density) than growers may have sprayed previously when grain value was lower.

The economic threshold (i.e. the number of larvae per head where the cost of control is equal to the value of the grain saved) can be calculated using the formula:

No. larvae/head = (C x R) ÷ (V x N x 2.4)

where
C = cost of control ($/ha)
R = row spacing (cm)
V = value of crop ($/tonne)
N = number of heads/metre of row
2.4 = weight of sorghum (grams) lost per larva.

How to sample sorghum heads for corn earworm
Count the number of larvae dislodged from 30 heads to arrive at a control decision. Obtain 5 consecutive heads at the brown anther stage from at least 6 locations in a field, each location preferably more than 50 m apart. Use the palms of your hands to spin each of the heads into the bucket. Pour the contents of the bucket onto a beat sheet or tray and count the number of larvae in each size class
very small (VS=less than 3 mm in length)
small (S=3–7 mm)
small-medium (SM=7-13 mm)
medium-large (ML=13-21 mm)
large (L=greater than 21 mm).

Control
Effective larval control can be achieved with the use of commercially available nucleopolyhedrovirus or NPV sprays, sold as either Vivus Max® (succeeding Vivus Gold®) or Gemstar®.

NPV is dynamite against corn earworm larvae in sorghum and has the bonus of being a natural disease of the pest, so that spraying only kills the pest and not other beneficial insects and spiders in the crop.

Gemstar® and Vivus Gold® have both been registered for use on sorghum at 375 mL/ha. Lower rates (250-300 mL/ha) have been used successfully by many growers.

Please be aware that Vivus Max® now replaces Vivus Gold®. It is a more concentrated product (2.5 x) and has a registered rate of 150 mL/ha in sorghum (equivalent to 375 mL/ha of Vivus Gold®).

Research into the use of NPV sprays has shown several key points that growers and consultants should remember when using NPV.

First, checking is easy and important – it not only tells you whether you have the pest in enough numbers to justify spraying, but it also gives you information on when to time an NPV spray, since it works best when targeted against the very youngest larvae.

At the end of flowering (heads with brown anthers to base), most larvae will be first or second instar (less than 7 mm in length), and ideal to target with NPV. The best spray timing is when 50% of heads in the field have brown anthers to their base. A further delay of 3 days will help conserve the important larval parasite, Microplitis demolitor.

In crops where there is a large spread of flowering, it is better to spray before 50% of the heads are at the brown anther stage. In these cases, experience has shown secondary infection by NPV can kill a high proportion of the caterpillars that hatch after the NPV application.

NPV should not be used against larvae greater than 13 mm in length.

Good coverage over the plant and especially the sorghum head is critical, since a larva has to actually feed on an NPV particle to become infected with the virus. Sprays should be put on at the time of day that is best suited to getting good coverage, and this will often be in the morning.

Ultra low volume (ULV) sprays of NPV applied by a plane are highly effective in sorghum and allow for large areas to be treated in a relatively short time – this is good news when the pressure is on to treat large areas.

For ULV application, NPV is combined with approved spray oils such as D-C-Tron, Canopy or Biopest oil, to make a minimum spray volume of 3 L/ha.

Corn earworm larvae killed by NPV.

Further information on the use of NPV can be found in the brochure 'Using NPV to manage helicoverpa in field crops' by following this link

http://www2.dpi.qld.gov.au/fieldcrops/17677.html

Rutherglen bugs are everywhere!

Rutherglen bug (Nysius vinitor)(RGB) is one of the insect species that arrives in crops in spring in large numbers, usually in association with storm activity. You may also have seen them on your windows and screens (and around the lights) at home in recent days. It is likely that the bugs are moving around in the environment, perhaps even transported from some distance away on storm fronts. You will probably find RGB in all crops and weeds at the moment, not just the winter cereals.

Because RGB are moving so much in the environment, and probably migrating in on storm fronts, we are likely to see ongoing infestations of crops for some time. This means that any decision to control RGB needs to be done with full knowledge that the treated crop may be infested again in a short period of time.

(Photos: RGB adult (top) and RGB late instar nymph (bottom). Keith Power)

Rutherglen bugs in wheat and barley
At this stage in the season, RGB is not going to have any impact on yield in winter cereals. Grain that is hard will not be damaged by this sucking bug.

The main issue with RGB around harvest time is contamination of harvested grain. When RGB are in very large numbers they can cause a number of issues at harvest:

• Live bugs in the sample can result in rejection of a load at the delivery point
• Large numbers of bugs (and bits of bugs) in the grain can elevate grain moisture. This problem is probably worst when RGB are breeding in the crop, and there are large numbers of nymphs - this is unlikely to eventuate in wheat.
• There are no insecticides registered for RGB in winter cereals. If you are controlling armyworm, or helicoverpa (except with NPV) then some of these options will control RGB to some extent. But be very aware of the WHP of any insecticide used this late in the season.

Treating a crop now for RGB is no guarantee that there will not be a reinfestation before harvest

What can you do if you have large numbers of RGB at harvest?

The best approach is to try and limit the number of RGB that end up in the harvested grain. Some of the suggestions for doing this include:

• Harvesting at night
• Fitting screens to the header
• Leave the tarp off the load for as long as possible to allow RGB to escape post harvest

If you are storing grain, RGB will not cause damage to grain in storage, and there is no need to treat grain to kill them. Large numbers of crushed RGB in harvested grain have been identified as tainting the grain with the oily exudates from their scent glands. It is unclear over what period this tainting persists.

What about RGB in other crops?

Seedling and vegetative crops
RGB is primarily a seed-feeding species, and have the capacity to damage crops during grain filling – but we know very little about how much damage in any crop other than sunflower.

In very large numbers, RGB can damage seedling crops purely by weight of numbers feeding on seedlings. In more advanced vegetative crops they will not cause any impact as long as the crop has adequate moisture and is growing actively. Be alert to RGB in sunflower at budding and flowering, and in sorghum from flowering through to soft grain; these infestations may warrant treatment.

RGB in sunflower
Whilst RGB numbers may be high in vegetative sunflower now, it is important to weigh up the decision to spray with the following:
Actively growing plants, with adequate moisture, will not be greatly impacted on by RGB feeding
Reinfestation is a real possibility, and if treating during the vegetative stage, it is likely that insect control in the crop will run to 3 sprays (vegetative, budding and flowering) if RGB pressure remains high and helicoverpa infest the crop as well.

In sunflower there are two critical periods during which RGB can cause significant crop damage:
Budding: bugs congregrate on the upper stem and bud. Bug feeding on the stem behind the head may cause the stem to wither and the bud droop.
Flowering: eggs are laid in the head and nymphs emerge in about a week and start feeding on developing seeds. Adult numbers are often minor in comparison with the size of the population once nymphs start to emerge.
Feeding on developing seeds causes yield loss, and a loss of oil content and quality of grain.

Thresholds for sunflower:

Budding: 10 bugs per head
Flowering: 20-40 bugs per head
If it is necessary to treat at flowering, do so before the heads turn down, otherwise it is difficult to get good contact with the bugs in the flower.

Control considerations in sunflower

Synthetic pyrethroids (SP) are the most effective option for controlling RGB

• If RGB are in large numbers at budding and flowering, but there are a few helicoverpa present, consider an SP/NPV mix. Steward™ (for use in sunflower under permit to control helicoverpa and RGB) will provide, at best, suppression of RGB and will not provide adequate control of a large population.
• The impact of insecticides on bees is an important issue in sunflower, particularly if there are hives nearby. Spraying later in the day, when bees are less active, will reduce the impact on them.

No concern for tell-tale holes

Corn earworm larvae on vegetative sorghum crops produce characteristic holes in the leaves after feeding in the throat of the plant. These tell-tale signs are of no great concern as this type of feeding will not affect crop yield.



Caption: BEB alias Austin McLennan showing a characteristic holey sorghum leaf.


The recent presence of high numbers of corn earworm, Helicoverpa armigera, on winter cereals and chickpea could herald the beginning of a busy time ahead for grain sorghum in southern Queensland.

As larvae on cereal crops mature, they climb down the plant and burrow into the soil to pupate. Moths emerge from these pupae 2 to 3 weeks later, and start the next generation by laying eggs on suitable host plants.

Vegetative sorghum is attractive to egglaying moths, and larvae hatch from newly laid eggs in 3 to 4 days. Survival of larvae on vegetative crops may not be high, but vegetative sorghum can be an important intermediate host that bridges the gap between winter and summer.

Armyworm larvae may also be present in vegetative sorghum. Armyworm larvae cause sorghum plants to look ‘ragged’, but again this leaf feeding does not result in any yield loss in advanced and actively growing seedling crops.

Control of larvae on vegetative sorghum is generally not recommended as the damage is cosmetic and unlikely to affect yield.

While corn earworm larvae are advertising their presence in southern Queensland grain sorghum crops, of greatest importance are larval infestations after flowering and during grain fill.

Egglaying by corn earworm moths and larval management on sorghum heads will be the subject of a future posting.