Mirids in Mungbeans

Mirids can cause significant crop losses in mungbeans with yield reductions of up to 25-50% common where high mirid populations (eg 10/m2) are left uncontrolled. Mirids can reduce yields by 60 kg/ha for every mirid/m2 of crop.

Mirids can be present in mungbeans at any stage from seedlings to podding. Budding, flowering and early-podding crops are at greatest risk. Low populations of green mirids are often present in vegetative crops but there is no evidence they cause ‘tipping’ of vegetative terminals or have any impact on yield. Mirid populations usually increase with the onset of budding and peak during late podfilll. Generally around 80% of mirids present in flowering/podding crops are nymphs.

Damage caused by mirids
Mirids are sucking insects and feed by piercing the plant tissue and releasing a chemical that destroys cells in the feeding zone. This causes plant tissue to discolour and die. Mirids prefer to feed on flowers, buds and young pods, causing these to abort (shed).

Severe mirid damage in mungbean results in fewer harvestable pods being set, however similar symptoms can also be caused by thrips, high temperatures and moisture stress. Mirids may also attack more mature pods, damaging the seeds inside without causing shedding. If pod set is greatly reduced and mirid populations are relatively low (e.g. 0.5 - 1.0 /m2) - loss may be due to other factors.

Mungbeans always set many more buds and flowers than they can convert to harvestable pods. Typically (in dryland crops) only 33% of buds/flowers are converted to harvestable pods. This % may be even lower in moisture stressed crops.

Recent Helicoverpa threshold trials show that mungbeans can compensate for considerable early damage (at budding/flowering) of up to 80% bud loss with no yield loss. However this only occurs when there is adequate moisture and no subsequent pest damage and there will be a delay in crop maturity. . The loss of replacement buds through continuous pest pressure will lead to reduced crop yields.

Medium and large mirid nymphs (instars 3-5) are as damaging as adults. Although small mirid nymphs (instars 1-2) are less damaging, they soon develop into larger more damaging pests (within 3-5 days respectively at 300C).

Identification of mirids
Three mirid species attack mungbeans and include green, brown and (less frequently) crop mirids. All these species are equally damaging.

For more information on the identification of these mirid species please refer to the Integrated Pest Management website by clicking onto the link provided below:

http://www.dpi.qld.gov.au/cps/rde/dpi/hs.xsl/26_5085_ENA_HTML.htm

Where do the mirids come from?
Mirids move into summer crops when alternative host plants dry off and they seek a fresh food source. Lucerne and sunflowers can be important sources of mirids in mungbean growing areas.

Influxes of mirid adults often coincide with northwest winds in spring. There is also evidence of long distance migration, possibly from inland areas, associated with weather fronts. This may be the cause of some of the widespread and repeated influxes of mirids sometimes observed in grain and cotton growing regions early in the summer.

Monitoring for mirids
- Mirids are very mobile and in-crop populations can increase rapidly from budding onwards.
- Crops should be inspected weekly during the vegetative stage (to pick the start of budding) and twice weekly from budding onwards until post flowering.
- In row crops, the preferred method is beat sheeting, as this method is the most effective for helicoverpa and pod-sucking bugs.
- Sample 5 one-metre lengths of row (not consecutive) within a 20 m radius, from at least 6 sites throughout a crop.
- Avoid sampling during very windy weather as mirids are easily blown off the sheet.

Action level
Mirid thresholds for budding/flowering mungbeans are 0.3-0.6 per m2 for aerial and ground rig respectively. While mirid thresholds are very low, this is more a reflection of the cheapness of the preferred mirid pesticide – dimethoate ($4/ha + application). The above thresholds are based on the cost of dimethoate. Were indoxacarb to be used, the threshold for an aerial sprayed crop would be 1 mirid/m2.

Note that these thresholds are the break even point (cost of control = value of likely damage) and that action only needs to be taken if populations exceed these levels. However in practice, mirid populations usually increase rapidly from budding onwards and spraying at-threshold populations would be justified. Because of the crop’s ability to compensate for early damage, spraying low mirid populations (i.e. at threshold) at very early budding can be delayed slightly with no risk to yield or harvest maturity. Taking this approach reduces the need for a subsequent mirid spray.

Cultural control
A crop that has a short flowering period reduces the risk of mirid damage. Flowering periods can be shortened by planting on a full moisture profile and by watering crops just before budding. Consider planting crops in at least 50 cm rows (as opposed to broadcast planting) to facilitate easier pest sampling.

Chemical control
· Dimethoate at 500 mL/ha (all summer pulses) or
· Indoxacarb at 400 mL/ha (mungbeans and soybeans only).

Dimethoate is often applied at lower than label rates (eg 200-250 mL/ha). These rates give excellent mirid control but have far less impact on many beneficials. Trail results have shown that the addition of salt (0.5% NaCl) as an adjuvant, improves the effectiveness of dimethoate at lower rates. The amount of salt used (0.5%) has no phytotoxic effect on summer pulse crops. ‘Hard’ water can markedly lower the effectiveness of dimethoate and should be countered by adding a buffering agent such as LI700.

Indoxacarb is not recommended at high mirid pressure (>2/m2) because it is less effective than dimethoate. Because it is restricted to one (1) spray application per crop, indoxacarb is best reserved for Helicoverpa control during podding. As mentioned previously, the mirid thresholds are considerably higher if indoxacarb is used.

Natural enemies
There are no beneficial species that are recognised to be regulators of mirid populations in mungbeans. However damsel bugs, big-eyed bugs, predatory shield bugs, as well as lynx, night stalker and jumping spiders are known to feed on mirid adults, nymphs and eggs. Naturally occurring fungi (eg Beauvaria) may also infect and kill mirids, but are rarely observed in the field.

Article by Kate Charleston and Hugh Brier

Late season pests of pulses and cotton

Go soft early
Go soft early is a basic integrated pest management (IPM) strategy to avoid using non selective pesticides for as long as possible. This approach encourages a build up of predators and parasites to keep early pests in check and buffer the crop against attack later in the season. This strategy is particularly important in soybeans because of the risk of flaring whitefly. With no registered pesticides against whitefly in soybeans the going soft early approach maximises the chances of whitefly parasitism by the recently released wasp Eretmocerus hayati.

Green Vegetable Bugs (GVB) in soybeans
Intervention with non-selective pesticides may be required during pod-fill to control pod-sucking bugs. The most abundant pod sucking bug in soybeans is the green vegetable bug.

GVB are often found in the crop prior to pod-fill, but at this stage they are not causing economic damage. By delaying insecticide use untill pod-fill reduces the risk of flaring whitefly, because as pod-fill progresses, the leaves become progressively less attractive to this pest.

GVB is primarily a pod feeder with a preference for pods with well-developed seeds. Summer pulses remain at risk until pods are too hard to damage (very close to harvest). Damaging populations are typically highest in late summer crops during late pod-fill (when nymphs have reached or are near adulthood).

While many cultivars can compensate for yield loss caused by moderate bug populations, seed quality is adversely affected, particularly for culinary beans which have very low damage tolerances (a maximum of 2% damaged seed). Bug damaged seeds have increased protein content but a shorter storage life (due to increased rancidity). In soybeans, bug damage also reduces seed oil content.

The only effective registered pesticides for pod-suckers are non-selective and include deltamethrin (Decis) and trichlorfon (Dipteryx). Of these, deltamethrin is the most effective. Note that dimethoate and methomyl are also registered but were found to be ineffective against pod-sucking bugs.

To mitigate the impact of non-selective insecticides, delay spraying podsuckers until early podfill.

Green Vegetable Bugs (GVB) in cotton
Cotton is susceptible to damage from GVB from flowering through until one open boll per metre. Damage symptoms from GVB cannot be distinguished from damage done by mirids which include warty growths and brown staining of lint in developing bolls.

Thresholds for GVB in cotton are 1 adult/metre from flowering through to harvest. For insecticide options please refer to the cotton pest management guide which can be found on the Cotton Catchment Communities CRC website.

http://www.cottoncrc.org.au/content/Industry/Publications

Grass blue butterfly
Large populations of grass blue butterfly have been observed in soybeans this season. The green slug-like caterpillars feed mainly on leaves. They may be confused with the larvae of the hoverfly which is a beneficial insect and often found near aphid colonies on which they feed.

While control is rarely needed, high numbers can damage terminals resulting in plant branching and pods being set closer to the ground. This can indirectly impact on yield as low-set pods are more difficult to harvest. There are no insecticides registered for this pest specifically but anecdotal evidence (from a leading Goondiwindi grower) suggests they are readily controlled with Bt (Dipel).

Pale cotton stainers
Cotton stainers are occasional pests of cotton that feed on developing and mature cotton seed. While previously controlled by broad spectrum insecticides for other pests in cotton, the reduction in chemical use, especially on Bollgard II®, may lead to increased populations which may need to be managed.
For more information about pale cotton stainers click on the link provided below:

http://www.cottoncrc.org.au/content/Industry/Publications/Pests_and_Beneficials/Sucking_Pests.aspx

Article by Kate Charleston and Hugh Brier

Soybean Moth causing major damage in soybeans

Soybean moth Aproaerema simplexella is a very common pest of soybeans but is usually only present in very low numbers. This season high numbers of moths and caterpillars were found especially in the Wide Bay Burnett region with some fields sustaining extensive damage. Similar leaf miners attack many horticultural crops, but are species other than A. simplexella. Weed hosts of soybean moth include emu foot (Psoralia tenax)

Identification
The soybean moth is a small narrow winged moth, up to 6 mm long. The forewings are dark brown/grey, each with a white bar across them, and pale brown hind wings. In soybean crops with large infestations, moths can be seen flying up from the foliage when disturbed.

Small elongated eggs are laid on both the top and underside of leaves, generally near leaf veins. Larvae of soybean moth are pale green/grey and grow up to 7 mm (in length).

Other caterpillars that mine and web soybean leaves include soybean leafminer Lithocolletis aglaozona, a much smaller and less abundant species, and legume webspinner Omiodes diemenalis, common in coastal regions, but which is much larger (up to 15 mm) and is bright green. Legume webspinners also web leaves together, rather than mining leaves (feeding inside them).

Similar leaf miners also attack many horticultural crops, but are species other than A. simplexella. Weed hosts of soybean moth include emu foot (Psoralia tenax).

Damage
Larvae initially feed inside leaves (i.e. in leaf mines) and emerge after approximately four days to feed externally, folding and webbing leaves together to form a protective shelter. Infested leaves are often crinkled and pulled together in the middle and this together with webbing of leaf is the most obvious symptom of damage. In low numbers the larvae only cause cosmetic damage. While larvae normally feed on leaves only, extremely high populations will also graze on the surface of pods, after they have denuded the crop of leaves.

Soybean moth infestations are favoured by hot, dry weather, with crops under severe moisture stress most at risk. Large populations can cause extensive damage by stripping all leaves from crops and so reduce photosynthesis and grain production.

Monitoring and thresholds
Monitor crops regularly for the early warning signs of rare plague events. Look for numerous small, pale patches (leaf-mining) on the leaves and large numbers of soybean moths in the crop or around lights at night.

The indicative threshold is based on defoliation, with. 33% pre flowering and 15-20% defoliation during early to mid pod-fill.

Control
In most years control is not required but large infestations in the Bundaberg region will need chemical control to prevent total crop loss. Check thoroughly before spraying, as larvae may have already pupated (as black pupae within the webbing) or reached full size (7 mm) and stopped feeding.

There are no specific registrations for the control of soybean moth. However pesticides effective against Helicoverpa (except Helicoverpa virus and Bt), and targeting that pest in soybeans will most likely also give reasonable control of soybean moth.

DPI&F plans to investigate the effectiveness of a number of pesticides registered in soybeans (against other pests,) as part of GRDC-funded pulse IPM research project DAQ00086. The hope is to identify at least a moderately selective pesticide to preserve soybean moth parasites such as Temclucha sp., a small Ichneumonid wasp (8 mm). This species has been observed in very high numbers in some crops infested with soybean moth.

Article by Kate Charleston and Hugh Brier

Silverleaf whitefly update

There are reports of large infestations of silverleaf whitefly (SLW) from the Narrabri/Moree region. Exponential growth in whitefly numbers coupled with honeydew on leaves indicates that the whitefly are probably SLW and not East Australian native Bemisia or Greenhouse whitefly (GHW).

It is thought that increased host plant availability from a wet winter/spring, warm conditions and a decline in natural enemies due to the use of broad spectrum insecticides has contributed to the high SLW numbers in this more marginal area of occurrence.

DPI&F entomologists will be visiting Narrabri on Thursday, 11 February 2009 and Moree on Friday 13 February 2009 where they will meet with growers and consultants and speak about SLW and the management options that are available.

Cotton fields around St George are at or reaching high densities of SLW. Reports indicate Admiral® has been applied on many fields to suppress SLW populations. Parasitism levels of 50% and 70% were recorded from two fields in the St George area. This should help to keep SLW in check later in the season even if they start to re-infest crops post Admiral® spray.

SLW numbers in Biloela and Theodore are reportedly dropping off. This may be in part due to parasitism levels. Recent testing for insecticide resistance in populations of SLW from Biloela and Theodore show no alarming results for Admiral®. These results were expected due to the minimal use of Admiral® in central Queensland this season.

GHW are in moderate densities in the Norwin region on the Darling Downs. While GHW will produce honeydew it does not normally cause the same problems as SLW which has a wider host range, higher reproductive rate, develops resistance to insecticides rapidly, and is adapted to high temperatures. Where populations are a mix of SLW and GHW, consider treating as if all are SLW.

The report on managing silverleaf whitefly by Richard Sequeira and Tracey Farrell can be accessed through the cotton CRC using the link below: http://www.cottoncrc.org.au/files/5743fee2-f978-4a79-a9d1-9b1800e899cf/Whitefly_Management[1].pdf.
This document outlines sampling, thresholds and management options for SLW. Remember only one spray of Admiral® is allowed per season.

Article by Zara Ludgate