Mirids in Mungbeans

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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.

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