Green mirid adult                                                                    Green mirid nymph

Development of plant based thresholds

The aim of this trial is to determine the amount of fruit loss that occurs per mirid/day. and continues work from last season. Results from this trial will be incorporated into existing thresholds and will help growers and consultants to determine a threshold appropriate for specific fields given mirid activity and plant response.

The trials will be conducted at Nandi, Macalister and Jandowae with each site having 3 treatments. These treatments will be 1) unsprayed, 2) sprayed and 3) commercial (farmers management) resulting in different levels of mirid activity and damage.

Impact of mirid management on predators and parasitoids of whitefly
Management of mirids with broad-spectrum insecticides can lead to flaring of whitefly populations.

The word flaring is commonly used in entomology and refers to a population outbreak or explosion.

Anecdotal evidence suggests that broad-spectrum chemicals kill parasitoids and predators that play an important role in keeping whitefly populations in check. In absence of their natural enemies whitefly population grow rapidly in favourable environmental conditions.

The objective of this trial is to gather hard data to test this hypothesis. This trial will be conducted in Macalister with three treatments to create different disruption level scenarios.

The treatments consist of:
1) Spray at threshold using integrated pest management (IPM) control options
2) Spray at threshold with disruptive control options
3) Spray at below threshold with disruptive control options.

Pheromone traps as monitoring and management tools

 This trial will be conducted in Macalister by Dr Khan (pictured)  in collaboration with Dr Alice Del Socorro (University of New England, and CCC CRC).

The objective of the trial is to evaluate mirid pheromone traps as a monitoring and management tool.

Six traps baited with pheromone will be set up in a 4 ha (400 x 100) field.  An adjacent field, 500 m away without pheromone traps, will be used to compare mirid number and management.                                                                                  

For more information about these trials please contact the entomology team via the comments section of this blog posting.

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

Where do mirids come from?

In order to make this decision, we need to understand the population dynamics of mirids, their damage potential and what is currently happening to the crop.

What is happening this year?
Although the past winter was relatively wet, mirid numbers have been very low across the cotton growing areas so far this year. The reason for this phenomenon is not well understood. It could be that alternative hosts are still fresh enough to continue to support mirids, or the severe and prolonged winter may have had a detrimental effect on mirid populations.

Do we need to be worried about square loss at this stage?
If square retention is above 70% and mirid numbers are very low (as is the case for many crops this year), there is no need to be unduly concerned. Squares may be lost for various reasons e.g. insect feeding and physiological reasons.

If mirid numbers are very low and retention is less than 70%, square loss may be due to reasons other than insect feeding and a spray will not help. Square loss at this stage will be compensated fully provided plants do not further suffer from water or nutrient stress or from any other insects.

Figure 1. Compensation for mirid damage at squaring stage

Figure 1 shows plant mapping data in Bollgard® II..

Observation 1 shows the percentage fruit loss for 3 treatments at the squaring stage while observation 2 shows the same 3 treatments at cut out. The treatments consisted of the following:

In treatment 1 (blue) plants were sprayed on a regular basis starting from when 60% of plants had their first flower until cut out. Treatment 2 (red) was not sprayed for mirids at any time during the season. Treatment 3 was sprayed regularly throughout the season to provide total protection against mirids

In observation 1 – the squaring stage – there is a significant difference between treatment 1 (sprayed from squaring) and treatment 3 (sprayed all season) with 18% fruit loss for treatment 1 and 8% loss in treatment 3.
Consequences of unwarranted mirid sprays at this stage
There are several consequences of an insurance spray.
- It is an unnecessary expense.
- Disruption of benefcial communities at this early stage could flare secondary pests such as aphids, mites and whitefly.
- Sprays at this stage, whether high or low rate, will not provide long residual protection. Mirids could move in from alternative hosts at any time, and it is best to save the spray for when it is really warranted.

Article by Dr Moazzem Khan

However if we now look at what happened in these treatment at cut out, we find that there is no significant difference between treatment 1 and 3. What this shows is that in this experiment the plants had the ability to make up for the early season fruit loss.