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

The exotic species of mealy bug is commonly known as the Solenopsis mealybug (Phenacoccus solenopsis).

The Solenopsis mealybug is a native of North America – first collected and described in New Mexico in 1897. In 1990, it was reported as a pest of cotton in Texas. From there it moved into Central and South America, and is now known to occur in Ghana, Nigeria, Israel, Pakistan, India, Indonesia, Thailand and China.

The Solenopsis mealybug is a polyphagous pest which means that it feeds and reproduces on a wide range of plants. In Pakistan it has been recorded on 154 plant species including field crops, vegetables, ornamentals, weeds, and trees.

Other key reasons for the fast spread and difficulty to control this pest include:

• The bugs possess a waxy coating that protects them from insecticides and natural mortality factors
• They have a high reproductive rate
• They have the ability to hide in soil cracks and crevices.
• They are spread through natural carriers such as raw cotton seeds, wind, water, rain, birds, humans, farm equipment and animals.

Identification and lifecycle of the Solenopsis mealybug
The female mealybug is wingless with a 3-4 mm long oval shaped body which is covered with white hydrophobic (water repellent) mealy wax. There are dark bare spots on the thorax and abdomen, which appear as dark longitudinal lines.

The adult male is about 1 mm long, with a grey body and a single pair of transparent wings. Two filaments of white wax project from the end of its abdomen. The adult male has no feeding mouthparts and causes no damage.

Mature females lay eggs in waxy pouches called ovisacs. Each ovisac contains between 150- 600 eggs, the majority of which are female. The eggs hatch after three to nine days into nymphs called ‘crawlers’, which are very mobile.

No parthenogenesis (asexual reproduction) is reported in the literature and therefore it is assumed that the species reproduces through sexual reproduction.

The female crawler undergoes four larval instars before turning into an adult (there is no pupal stage). The total life span of a female mealybug is 30–48 days, which includes 21 days as adult.

Male crawlers undergo three larval instars over 13–17 days before spinning a cottony cocoon in which it passes a pupal stage for 6-8 days. A male adult lives for only 3-5 days. Mealybugs can have 12–15 generations in a year.

The species (as eggs in ovisacs or in other life stages) can survive cold conditions, both on the host plant and in the soil. In warm climates, mealybugs reproduce all year round.

Mode of damage
Mealybugs have sucking mouth parts at all stages of their life cycle, which they use to extract large amounts of plant sap. During the feeding process a significant amount of sap oozes out as honeydew which forms a sticky deposit on the leaves and stem. Honeydew promotes the growth of sooty mould fungi which inhibit photosynthesis.

Symptoms of plants infested during the vegetative phase include:
• Distorted and bushy shoots
• Crinkled and/or twisted and bunchy leaves
• Stunted plants that dry completely in severe cases.

Symptoms of late season infestations during the reproductive crop stage include:
• Fewer, smaller and deformed bolls
• Reduced plant vigour
• Early crop senescence.

Mealybugs can also stain cotton lint and reduce quality hence this pest has the ability to cause damage to both the quality and quantity of cotton.

Methods of spread
Mealybugs are generally disseminated as crawlers. The crawlers can move from an infected to a healthy plant as well as infected field to adjacent healthy fields. While this type of movement is localised, bugs have the means to travel long distances and infest new areas. The waxy coating on the mealybug crawlers facilitates passive transport of the insect by sticking onto equipment, other insects (e.g. bees), birds, animals or people. Small crawlers are also readily transported by wind and rain or in water in irrigation channels. Long-distance movement through the transport of infested plants is also possible.

Ants are also a significant factor in the spread of mealybugs. In return for providing the honey dew that the ants feed on, they spread the mealybugs, protect them from natural enemies and keep their colonies clean.

Clusters of tiny crawlers – ready for dispersal  (image by Zara Ludgate)

Management of mealybugs
There are no insecticides registered for the control of mealybugs in cotton. However there are a number of management options that can reduce infestations and the overall impact of this pest.

• Mealybugs multiply on different hosts and may initially breed on weeds before migrating to cotton crops
• Weeds in and around fields should be removed.
• Do not throw uprooted weeds into water channels.
• The removal of affected plants at the early stage of infestation may reduce mealybug numbers in the rest of the crop.
• Avoid physical contact with infested plants as mealybugs easily adhere to clothing and implements.
• Practice good farm hygiene and clean all equipment that has been in affected fields.
• Consider the insecticides that are used in control of other insect pests to conserve natural enemies of mealybugs.

Trials in Pakistan and India show some promising results in terms of suitable insecticides. Insecticide trial work to control mealybug will commence in Queensland in the near future.

The main priority at this point is to try and limit further spread of the mealybug from and within the Emerald and Burdekin areas. With harvesting of crops to commence soon in the Emerald area, the Queensland Government is working closely with the cotton industry to develop wash-down and decontamination protocols for harvesting machinery. More information about these protocols will be provided in a blog next week.

Biological control
A survey in Pakistan recorded as many as nine species of predators feeding on Solenopsis mealybugs. These predators were present throughout the cotton season but their effectiveness was not sufficient to keep mealybugs under control.

In Australia, ladybird beetles and their larvae, and green lacewing larvae were abundant in mealybug-infested fields. One of the most conspicuous predators is the mealybug ladybird (Cryptolaemus montrouzieri), whose larvae look like an oversize mealybug. To date no Solenopsis mealybug parasitoids have been recorded in Australia

Meanwhile in India, a small wasp (Aenasius bambawalei) is reported to parasitise about 60 percent of the mealybug population under field conditions.

Article by Kate Charleston and David Murray.  Images by Zara Ludgate and Greg Kauter.