Yellow Sigatoka resistance – Goldfinger mutagenesis trial

Posted on by Ingrid Jenkins

Yellow Sigatoka screening - Goldfinger mutagenesis trial

Katie Robertson, Jeff Daniells, David East and Carole Wright, Queensland DAF (Dec 2023)

The screening trial assessing Goldfinger mutant selections’ resistance or susceptibility to the fungal leaf disease, yellow Sigatoka is now complete. Most of the selections had a similar reaction to yellow Sigatoka as the Goldfinger control, however GMS 766 was significantly more resistant, while four others were significantly more susceptible. This latter result demonstrates that mutagenesis can significantly change a plant’s level of yellow Sigatoka resistance.

An estimated $25-30 million per year is spent on controlling yellow Sigatoka leaf spot (Pseudocercospora musae) Identifying varieties with better resistance to the disease, provided they were otherwise commercially viable, would be of great benefit to the industry by reducing input costs and reliance on fungicides. 

Over the 2023 wet season, a yellow Sigatoka screening trial was conducted at the South Johnstone Research Facility on the top-twenty Goldfinger selections which had been developed through DAF’s mutagenesis project.

The block had been nurse suckered to standardise plant development stage. Leaf disease control ceased a few months prior to nurse suckering to build up inoculum levels and ensure adequate disease pressure. Plants were assessed for severity of leaf spot disease on 3 occasions, at monthly intervals leading up to bunching. The assessment consisted of determining the Youngest Leaf Spotted (10 or more mature necrotic lesions (YLS), and the Youngest Leaf with greater than 33% of the leaf lamina destroyed by disease (YL33). The Total number of Functional Leaves (TFL) was recorded if the disease was not present or had not progressed to the >33% necrosis severity level. For the purposes of this article, only the YLS and TFL data will be discussed.

While Goldfinger has resistance to black Sigatoka, this is not the case for yellow Sigatoka, to which it is better described as having an intermediate reaction.  Of particular interest was how the new selections compared to Goldfinger and the possible effect mutagenesis may have had on the plants ’ reaction to this disease.  Figure 1 below shows the average YLS of Goldfinger was 6.4, which was similar   to fifteen of the twenty mutant selections. ‘GMS 766’ was the only selection that demonstrated better resistance to leaf spot than Goldfinger, having an average YLS of 7.9.  

On the other end of the spectrum, ‘GMS 602’, ‘GMS 211’, ‘GMS 145’, and ‘GMS 255’ had average YLS values of between 5.4 and 4.4, demonstrating  they were significantly more susceptible to the disease. Although not the original objective of the project, these results demonstrate mutagenesis can significantly change a plant’s level of yellow Sigatoka resistance. The dwarf Lady Finger selection ‘Dwarf Rossi’ was planted as a single guard plant at the beginning of each row of Goldfinger. 

The level of disease was comparable to the ‘Dwarf Rossi’ plants in the variety evaluation, allowing comparisons to be made between the two trials. It was not significantly different to Goldfinger in its reaction to yellow Sigatoka. 

Figure 1: Youngest leaf with 10 or more necrotic lesions (YLS) for the 20 Goldfinger Mutant Selections, standard Goldfinger, and the Lady Finger variety ‘Dwarf Rossi’ (average of 3 rating occasions; error bars represent +/-1 standard error ). The different coloured bars indicate varieties that were statistically similar (yellow) or significantly different (red/green) to Goldfinger (patterned fill).
This research has been funded as part of the project Improved Plant Protection for the Banana Industry (BA16001), which is funded by Hort Innovation, using the banana research and development levy, co-investment from the Department of Agriculture and Fisheries and contributions from the Australian Government. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture. 
Hort innovation logo

Yellow Sigatoka resistance – agronomic evaluation trial (October 2020)

Posted on by Ingrid Jenkins

Yellow Sigatoka screening - agronomic evaluation trial (October 2020)

Katie Robertson, Jeff Daniells, David East, and Carole Wright, Queensland DAF (Dec 2023)

The screening trial assessing new varieties’ resistance or susceptibility to the fungal leaf disease, yellow Sigatoka is now complete. Varying degrees of leaf spot resistance were observed among the Cavendish (highly susceptible), Lady Finger (intermediate resistance) and CIRAD hybrids (resistant).

An estimated $25-30 million per year is spent on controlling yellow Sigatoka leaf spot (Pseudocercospora musae) in Australian banana plantations. Identifying varieties with better resistance to the disease, provided they were otherwise commercially viable, would be of great benefit to the industry, by reducing input costs and reliance on fungicides.

Over the 2023 wet season, a yellow Sigatoka screening trial was conducted at the South Johnstone Research Facility. This was after the agronomic assessments on 15 varieties had been completed in the 2020 Variety Evaluation and the block was nurse-suckered. 

Leaf disease control ceased a few months prior to nurse suckering to build up inoculum levels and ensure adequate disease pressure. Plants were assessed for severity of leaf spot disease on 3 occasions, at monthly intervals leading up to bunching. 

The assessment consisted of determining the Youngest Leaf Spotted (10 or more mature necrotic lesions) (YLS), and the Youngest Leaf with greater than 33% of the leaf lamina destroyed by disease (YL33). 

The Total number of Functional Leaves (TFL) was recorded if the disease was not present or had not progressed to the >33% necrosis severity level. For the purposes of this article, only the YLS and TFL data will be discussed.

Figure 1 shows four of the varieties had comparable disease severity to the very susceptible reference variety, ‘Williams’, which had an average YLS of 4.5. These included the TR4 resistant Cavendish ‘Asia Pacific #1’, along with the two tetraploid ‘High Noon’ selections (one with a ‘clean’ rachis [HNC] and one with a ‘dirty’ rachis [HND]) , and the Pendulous Lady Finger selection. 

The other four Lady Finger varieties demonstrated slightly better resistance and were statistically similar to ‘Pacific Plantain’ (YLS = 6.2), which has a documented intermediate level of resistance to leaf spot. Overseas, the hybrid ‘CIRAD 925’ has demonstrated both yellow and black Sigatoka (Pseudocercospora fijiensis) resistance (Risède et al. 2019). 

It was anticipated that the other three CIRAD hybrids may also possess this characteristic, which proved to be the case. Although no leaf spot was present, ‘CIRAD 925’ and ‘CIRAD 918’ only maintained around seven functional leaves throughout the assessments, with older leaves snapping due to petiole weakness, while ‘CIRAD L9’ and ‘CIRAD X17’ sustained over 10 functional leaves. 

The common leaf fungi, Cordana leaf spot (Neocordana musae) and banana leaf speckle (Mycosphaerella musae), were likely present on all varieties, but more obvious along the leaf margins of the CIRAD hybrids due to the absence of yellow Sigatoka. ‘Dwarf Ducasse’ was included as the resistant reference variety and supported an average of 13 functional leaves with no yellow Sigatoka present.

While the CRIAD hybrids remained unaffected by yellow Sigatoka (CIRAD X17 pitcured here), there were other common pathogenic fungi, primarily Cordana and banana leaf speckle present on some older leaves.
The TR4 resistant Cavendish 'Asia Paific #1', was as susceptible as Williams to yellow Sigatoka.
Figure 1 Youngest leaf with 10 or more necrotic lesions (YLS) for 15 varieties at South Johnstone (average of 3 rating occasions; error bars represent +/-1 standard error). If the YLS stage was not reached for a variety (i.e. the CIRAD hybrids and Dwarf Ducasse) the leaf number represents the total number of functional leaves present. Williams (very susceptible), Pacific Plantain (intermediate), and Dwarf Ducasse (highly resistant) were used as references to assess the disease reaction of the new varieties. The different coloured bars illustrate varieties that were statisically similar to the reference varieties (patterned fill).
References
Risède, J.-M., Achard, R., Brat, P., Chabrier, C., Damour, G., Guillermet, C., de Lapeyre., Lœillet, D., Lakhia, S., Meynard, P., Tixier, P., Tran Quoc, H., Salmon, F., Côte, F.-X. and Dorel, M. (2019). The agroecological transition of Cavendish banana cropping systems in the French West Indies. In The agroecological transition of agricultural systems in the Global South. Côte, F.-X., Poirier-Magona, E, Perret, S., Roudier, P., Rapidel, B., Thirion, M.-C, eds. (Agricultures et défis du monde collection, AFD, CIRAD, Éditions Quæ, Versailles). Pp. 107 – 126. https://agritrop.cirad.fr/592993/1/ID592993.pdf 

More information

This research has been funded as part of the project Improved Plant Protection for the Banana Industry (BA16001), which is funded by Hort Innovation, using the banana research and development levy, co-investment from the Department of Agriculture and Fisheries and contributions from the Australian Government. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.
Hort innovation logo

Sharan Muthukumar

Posted on by Sarah Williams

Sharan Muthukumar

The potential of banana diversity is what fascinates Sharan

Sharan is a Technical Officer working at the Department of Primary Industries (DPI), South Johnstone. Born in a town called Mettupalayam in southern India, Sharan moved to Australia to pursue his Master’s in Agricultural Science (Horticulture) at UQ Gatton campus. He has been working with DPI since September last year (2023). His primary research areas include the agronomic evaluation of new TR4 resistant Cavendish varieties and Race 1 resistant Lady Finger-like varieties. He’s also responsible for the maintenance of the Australian field collection of banana varieties at South Johnstone, which is a significant undertaking to ensure that the collection remains available for future research.

Sharan is passionate about working with different banana varieties and learning how to identify them. He finds the diversity of bananas fascinating and believes that exploring their genetic intricacies could reveal potential solutions to emerging challenges. One of the most exciting aspects of Sharan’s work is taste-testing different varieties, which allows him to experience firsthand the diverse flavours and textures they offer. In his free time, Sharan enjoys listening to music and going bushwalking. He’s excited to be a part of DPI and being able to make a positive contribution to the Australian Banana Industry.

Sharan meet a researcher
Sharan Muthukumar
Technical Officer
Department of Primary Industries
Centre for Wet Tropics Agriculture, South Johnstone, Qld

First ratoon observations and results (2020 agronomic trial)

Posted on by Ingrid Jenkins

First ratoon results - agronomic evaluation trial (October 2020)

By Katie Robertson & Jeff Daniells, Queensland Department of Primary Industries

Trends observed in the plant crop (see here) continued in the first ratoon. Williams outproduced the TR4 resistant Cavendish varieties. All the Lady Finger selections performed well compared to the standard Lady Finger, with either improved plant or bunch characteristics and no yield reduction.

Productivity declined in the short statured TR4 resistant Cavendish Asia Pacific #1, while GCTCV 106 continued to perform comparably to Williams.

The yield reduction measured in the plant crop of the TR4 resistant Cavendish variety Asia Pacific #1 (AP#1) was further amplified by the end of the first ratoon. AP#1 yielded 33% less than Williams over the 2 crops (29.3 versus 44.1 kg/year). This was due to their longer cycle time and a lower average bunch weight (26.7 versus 37.9 kg). AP#1 pseudostem height only increased by 8% between the plant crop and first ratoon (2.2 to 2.4 m), remaining significantly shorter than the other Cavendish varieties. Most other TR4 resistant Cavendish selections from Taiwan are taller than Williams. Finger length also remained shorter, with only 17% of AP#1 fruit falling into the premium size grade (22 – 26cm), compared to 56% for Williams. The GCTCV 106 selection – which originated from a vigorous plant identified in the previous 2018 South Johnstone variety evaluation (BA16001) – again performed comparably to Williams with regards to cycle time and yield (see table). Unfortunately, this variety has not demonstrated TR4 resistance in the Coastal Plains (Northern Territory) screening trials (see here for more).

The TR4 resistant Cavendish selection, Asia Pacific #1, remained shorter than Williams with reduced bunch weights, a longer crop cycle and shorter fruit lengths.

There were no yield differences among the six Lady Finger varieties, with some selections displaying reduced pseudostem heights and better bunch conformation.

All Lady Finger selections, except Pendulous Lady Finger (PLF), remained significantly shorter in stature than standard Lady Finger in the first ratoon (11 – 30% shorter). In the plant crop, PLF had yielded 19% more than Lady Finger per 12-months, but by the end of the first ratoon the yields compared over the 2 crops was not different. The standard Lady Finger experienced a 33% increase in bunch weight between the plant crop and first ratoon, going from 12.2 to 18.5 kg, while PLF only had 15% heavier bunches in the first ratoon (17.7 to 20.9 kg). The fruit in PLF bunches took longer to fill (22.3 weeks) than standard Lady Finger (17.1 weeks), and had issues with premature ripening, likely influenced by the timing of bunch emergence and the environmental conditions during fruit filling. Nonetheless, the PLF plants lived up to their name and continued to produce bunches with a more desirable conformation. The four other Lady Finger varieties did not yield significantly differently to the industry standard.

The yield of 'Dwarf Lady Finger' was comparable to standard Lady Finger but was 30% (or 1.4 m) shorter in stature.

The novel hybrids from CIRAD continued to perform poorly in the first ratoon, in terms of bunch size and pseudostem height.

In the plant crop, 40% of the CIRAD hybrid 918 plants had snapped before harvest despite being similar height to Williams. The incidence of pseudostem snapping in this variety rose to 87% in the first ratoon, meaning there were not enough data plants to include in the analysis. The remaining three CIRAD hybrids had bunch weights 25 – 64% lower than Williams per 12-months for the 2 crops, and CIRAD 925 and X17 were taller by 10 and 17%, respectively. X17 has demonstrated a high level of resistance to TR4 in the NT (see here for more).

Where to next?

The trial block was nurse suckered at the beginning of the year to manage the cropping cycle, and the varieties were rated for yellow Sigatoka over the wet season. These results will be reported on later in the year.  

The TR4 resistant CIRAD hybrid X17 yielded 34% less than Williams per 12-months over the two crop cycles.

More information

This research has been funded as part of a project funded by Hort Innovation, using the banana research and development levy, co-investment from the Queensland Department of Primary Industries and contributions from the Australian Government. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.

 

Nectaries

Posted on by Better Bananas

Unravelling the mystery of banana tips

What is the black spot at the tip of my bananas? This was a question recently asked by a grower. The DAF extension team set out to investigate the issue and find out the cause.

It was suggested that the discoloration could be Mokillo disease, a bacterial infection that causes dry rot at the end of the finger, starting in the same area on the fruit that the grower was questioning. Mokillo typically only affects a few fingers in a hand, these fingers are often smaller and narrower or pinched at the tip. Mokillo infection will continue to move through the fruit over time and have a rusty red gummy appearance (internally). It was evident that the issue in question was not Mokillo given that the blackened areas were present in all examined fruit, and the symptoms did not match.

The black tip did not progress when fruit was ripened.
The black tip did not progress when fruit was ripened.
The black area at the tip of bananas in question.
The black area at the tip of bananas in question.

From a market perspective, the team firstly wanted to demonstrate that these black areas did not progress into the fruit pulp nor display Mokillo-like symptoms as it ripened.  The extension team took a sample of mature fruit and cross sectioned them at the different ripening stages. The black tips were present in ALL of the fruit and there was no progression into the fruit throughout the ripening process. Consequently, the DAF extension team was confident that this wasn’t Mokillo.

After this initial investigation, the query persisted as to the nature of this blackened internal tip. With a theory that the blackening was associated with the physical make-up of the fruit, a review of banana literature, including the basics of banana ‘anatomy’, found that this blackened area is called a nectary (Septal nectary). Nectaries are structures in plants, often found at the base of stamens (male flower parts) that provide food rewards for insect or bird pollinators and therefore play a role in the fertility of plants. Studies found that the nectary cells disintegrate/oxidise in Grand Naine when the flower ends are ‘unfurling’ allowing the reproductive structures (style and stamens) to be accessed by insects to pollinate. An interesting fact, is that this disintegrating/oxidising of the nectary acts as a ‘roadblock’ for the growth of the pollen tube towards the ovary. It has been suggested that this may be a contributing factor in why Cavendish is sterile and doesn’t produce seeds.

Nectaries female parts of banana flower
Floral structures of a banana finger showing oxidised/disintegrated nectary.
The project team cross sectioned fruit at different floral developmental stages and confirmed what previous research found, in that the blackening of the nectary in Williams Cavendish occurs when the flower end reproductive structures are exposed to pollinating insects.

Overall, these nectaries which have disintegrated/oxidised don’t appear to impact fruit quality. It is possible that these areas allow for secondary infections, like Mokillo, which explains some of the similar symptoms. However, further investigations would be needed to better understand the risks and conditions which may favour these infections. The DAF extension team will continue to keep tabs on the blackening of nectaries at ad hoc times throughout the year in the course of their work and make observations in other varieties.

References

  • Soares, T.L.; Souza, E.H.; Costa, M.A.P.C.; Silva, S.O.; Santos-Serejo, J.A. In vivo fertilization of banana. Ciênc. Rural 2014, 44, 37–42.
  •  dos Santos Silva, M.; Santana, A.N.; dos Santos-Serejo, J.A.; Ferreira, C.F.; Amorim, E.P. Morphoanatomy and Histochemistry of Septal Nectaries Related to Female Fertility in Banana Plants of the ‘Cavendish’ Subgroup. Plants 2022, 11, 1177.
This article has been compiled as part of the National Banana Development and Extension Program (BA19004) which is funded by Hort Innovation, using the banana research and development levy, co-investment from the Department of Agriculture and Fisheries and contributions from the Australian Government. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.
Hort innovation logo

Agronomic evaluation of Lady Finger-like varieties – plant crop results

Posted on by Sarah Williams

Plant crop results - agronomic evaluation of Lady Finger-like varieties

(Trial establised December 2022)

Lady Finger-like varieties from Brazil under trial at South Johnstone

By Jeff Daniells, Katie Robertson, Sharan Muthukumar and Carole Wright (August 2024)

Latest update...

Several Fusarium wilt Race 1 resistant Lady Finger-like varieties are being assessed for agronomic performance at the South Johnstone Research Facility. The plant crop data and a summary of the results is available below.

The first ratoon harvest is already underway with harvest commencing in May 2024. The trial will be completed after the first ratoon harvest. Local taste panel results for varieties will be available then. Results from Panama disease Race 1 screening available next year will help guide any future on-farm studies. 

Those who attended the September 2023 field walk had an opportunity to see plant crop bunches of these Lady Finger-like varieties. A video field walk through the block, prepared the 2023 Industry Roadshows in NSW at about that time, is available.

Background

According to the ‘Australian Horticulture Statistics Handbook 2022/23’ the annual production of Lady Finger banana in that year was 11,228 t compared to 363,023 t of Cavendish, so Lady Fingers represent only about 3% of overall banana production. However, the farmgate value of Lady Finger production can be estimated as worth about $30 million, which because of the higher price paid for Lady Finger per kg of fruit, represents about 5% of the overall banana industry. But whatever way you want to look at it, the Lady Finger industry is an important industry in its own right.  In New South Wales the Lady Finger industry is proportionally much more important than it is overall for Australia.

In the April 2024 edition of Australian Bananas (see here pp 26-27) we drew attention to the constraint that Fusarium wilt Race 1 has placed on Lady Finger production in Australia. Varieties with resistance have been sought and evaluated over time, but while many have been identified with resistance, none have so far measured up as replacements for the Race 1 susceptible Lady Finger in the marketplace. So where possible, affected growers have sought new ground not infested with the pathogen.

In 2019/20 some new Lady Finger selections, Lady Finger hybrids and Sugar hybrids were imported from banana breeding programs in Brazil to evaluate in Australia. As described in the April article in Australian Bananas, some of these are being screened against Race 1 on the Atherton Tablelands. Here in this article, we present results from the plant crop of their agronomic evaluation at South Johnstone. This evaluation is part of the project BA21002 – ‘New varieties for Australian banana growers’.

Trial results in plant crop

Tissue cultured plants of 3 Lady Finger selections, 4 Lady Finger hybrids and 2 Sugar (Silk) hybrids were field planted in December 2022 at South Johnstone DAF. Pacovan, SCS451 and the hybrids all came from Brazil. The 6 hybrids from the program of EMBRAPA are all reported to be resistant to both Panama disease Race 1 and yellow Sigatoka. Japira and Pacoua are also reported to be resistant to black Sigatoka. SCS451 is from the program of EPAGRI and was purported to have tolerance to Panama disease Race 1. Bunch emergence commenced in May 2023, and harvest was completed in December 2023.

Pacovan Ken
Pacovan Ken were more pendulous than Lady Finger
Plantina lady finger hyrbid
Platina is shorter in stature than the other Lady Finger hybrids
Pacovan
Bunches of Pacovan were more pendulous than Lady Finger

Unfortunately, all except one of the Lady Finger (Improved) control plants were tissue culture offtypes. Thus, there are serious limitations when wishing to compare the new selections with the industry standard, Lady Finger. Nevertheless, reliable comparisons can be made amongst the new varieties. The agronomic and yield results from the plant crop are presented in the table below. Relative comparisons from previous studies suggest that for the Lady Finger data included in the table, the pseudostem height is indicative, but the duration from planting to harvest would usually be a little slower resulting in bunch weight/12 months being more in line with that obtained for SCS451.

Table: Selected yield and plant characteristics of varieties in the plant crop

Summary of results

A summary of the results and observations made are as follows:

  • The highest yielding varieties (per unit of time) were Pacovan, Pacovan Ken and SCS451, which were all significantly higher yielding than Tropical, Princesa and Japira. 

  • The tallest variety was Japira and the shortest in stature was SCS451. Several aspects of crop management are more difficult with the taller varieties.

  •  Japira, Pacovan Ken and Pacovan had significantly longer fruit on the 3rd hand, while fruit of Tropical and Princesa was significantly shorter.

More information...

This research has been funded as part of the project New varieties for Australian banana growers (BA21002), which is funded by Hort Innovation, using the banana industry research and development levies and contributions from the Australian Government. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture. The Queensland Government has also co-funded the project through the Department of Agriculture and Fisheries.

Chemical treatment of mites

Posted on by Better Bananas

Chemical treatment of spider mites

Restricting the use of chemicals that cause mite population flares

Some chemicals are associated with mite flares. This can be due to several reasons but primarily it is because these chemicals either encourage the mites to lay more eggs (the neonicotinoids, e.g. imidacloprid) or eliminate natural predators (the synthetic pyrethroids, e.g. bifenthrin). Where possible, avoid using these chemicals or if they must be used, time their use to the low-risk periods for mite flares, such as winter.

Correct application of miticides

Firstly, it’s important to check to ensure that live mites are still present and it’s not residual damage that’s still visible. With only a limited number of miticides available to the banana industry, it is important for treatment efficacy and long-term availability of these products that they are applied correctly.

  • Avoid using neonicotinoids for control of Banana weevil borer and Banana rust thrips (e.g. imidacloprid), particularly if hot dry conditions are expected. These chemicals can cause mite flares as they encourage mites to lay more eggs.
  • Avoid using broad spectrum pyrethroids (e.g. bifenthrin) as these products will remove the predator (beneficial) population. There is also the risk of spider mite populations developing resistance to these chemicals.
  • Miticides will not provide instant results and monitoring after spray applications is required as it may take 2-3 days before the mites begin to die.
  • Apply miticides in the cooler parts of the day as the leaves will close up during the middle of the day making full leaf coverage difficult to achieve. Mites are generally found on the undersides of the leaves therefore it is important that the leaves are open at the time of application.
  • Apply miticides with at least 400 L/ha and up to 600 L/ha of water to ensure good coverage. Poor coverage will result in limited mite mortality and may create chemical resistance problems.
  • Rotate between the available modes of action, chemical groups and abide by the restricted number of annual uses for each product to minimise the chance of chemical resistance issues.
  • Knockdown miticides (e.g. fenbutatin oxide) will only control nymphs and adults and therefore may require a follow up application 10–14 days later to control mites that have hatched from the eggs.
  • Some miticides are referred to as ovicides (e.g. clofentezine) meaning they only control the eggs, preventing them from hatching. These must be applied with a knockdown miticide to control the adult population. Follow label instructions for resistance management as ovicidal miticides have a history of resistance development if over used.

Actives registered for control of spider mites in bananas

Always check the current registration status of chemicals before use by visiting the Australian Pesticides and Veterinary Medicines Authority website (Click here) and always follow label directions.

Download this information as a factsheet

For more information contact

The Better Bananas team
Department of Agriculture and Fisheries
South Johnstone
Email betterbananas@daf.qld.gov.au 

 
This information is adapted from: Pinese, B., Piper. R 1994, Bananas insect and mite management, Department of Primary Industries, Queensland 
This information has been prepared as part of the National Banana Development and Extension Program (BA19004) which is funded by Hort Innovation, using the banana industry research and development levies and contributions from the Australian Government. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture. The Queensland Government has also co-funded the project through the Department of Agriculture and Fisheries.

Natural predators of spider mites

Posted on by Better Bananas

Natural predators of spider mites

Natural predators are beneficial insects, that actively hunt and consume specific pest species. Spider mites have many natural predators including lady beetles, predatory mites, rove beetles, and predatory thrips. These natural predators need to be protected through conscientious spray programs (avoiding disruptive sprays) and some can be purchased from suppliers for augmentative releases to address spider mite population flares. Here we investigate two of the key natural predators and how they work to control spider mites.

Stethorus

The small, shiny, black mite-eating ladybird beetle or Stethorus is one of the most important predators of spider mites in bananas. Three species of Stethorus occur in bananas, but the main species is Stethorus fenestralis. All three species appear identical to the naked eye and all species are specialist spider mite predators.

Stethorus numbers increase following mite flares, as mites provide ample food supplies that allow Stethorus’ populations to flourish and eventually bring the mite levels back under control. Stethorus are high-density predators, meaning they are attracted to mite hotspots.  Interestingly, both adult and larval Stethorus beetles primarily feed on mites, making them very effective predators against these pests.

Stethorus adult 2
Adult Stethorus beetle

The life cycle of Stethorus

There are four distinct stages in the life cycle of Stethorus and it is important to recognise each of these stages. The elongated, translucent to pale brown eggs are laid singly under the leaves, either on or close to the mite colonies. The eggs are about 0.2 mm long and can easily be distinguished from the smaller spherical (and usually more numerous) mite eggs. Mite eggs are not visible to the naked eye, and a hand lens would be necessary to view them in the field.

Larvae are hairy and vary in colour depending on their age. Larvae go through four stages of maturation, each separated by a moult. Young larvae are pale cream becoming dark grey at maturity. Fourth-stage larvae eventually stop feeding when they are about 2 mm long and attach themselves to the leaf where they pupate.

The pupae are black, hairy and about 1 mm long. Pupae may be found anywhere on the underside of the leaf; however, they tend mostly to be found close to or on the midrib. The pupal stage is easily seen on a leaf, as a skin remains after the adult emerges. To determine whether a pupa is alive or is simply an empty pupal skin, smear it gently with a finger. A wet streak will indicate it was alive and if no wet streak is produced, then it was an empty skin.

Adults are shiny black, almost circular beetles about 1 mm long. They also occur on the underside of the leaf. Where there is a high incidence of mite infestation, there may be more than fifty adults under one leaf, although there are usually less than ten when mite populations are in check.

Looking after your Stethorus population

Broad spectrum insecticides are a major cause of mite flares because they destroy beneficial predators like Stethorus. Avoid using these chemicals (e.g. products containing bifenthrin) to control mites. Check your leaves to see if you have Stethorus present and get a gauge on the population levels. Although research specifically in bananas hasn’t yet been undertaken to determine how many Stethorus need to be present to control spider mites, they can keep spider mite populations in check when spider mite pressure is low.

Stethorus egg
Stethorus egg
Stethorus larva
Stethorus larva
Stethorus pupa
Stethorus adult
Stethorus adult

Californicus

Some predatory mites are commercially available for purchase to apply in the field. The more common predatory mite species Neoseiulus californicus (‘Californicus’) is described as an ‘aggressive and robust mite’.

Californicus mites are less than 1mm long and are pear-shaped. Their colouring is dependent on diet but can be clear to pink or orange. Eggs are clear to white, oval-shaped and similar to that of the eggs of the two-spotted spider mite, but distinctively larger. Females can lay up to 4 eggs per day, eggs tend to be laid on the underside of the leaves along veins or on leaf hairs.

Adult Californicus can consume up to 5 adult spider mites daily and can live for up to 20 days. These beneficial insects can even flourish even when prey is scarce, as they are also able to consume alternate food sources such as pollen or other small insects. However, research has shown that reproduction and developmental rates are increased when Californicus exclusively feed on spider mites.

Californicus is known for its resilience to differing environmental conditions. They remain active in both warm and cool temperatures and can survive well in both high and low humidity better than most other predatory mites. However, their optimal conditions are between 16-32˚ C with a relative humidity range of 40-80%. In optimal conditions (30˚ C), their lifecycles can be as fast as 4 days, almost twice as fast as that of their prey. Californicus are also less sensitive to pesticide residues which enables faster re-establishment after chemical applications.

The use of predatory mites as a biological control for spider mites has been trialled on commercial banana farms in Far North Queensland. Some growers release the predatory mites monthly as a preventative treatment for mite flares. For more details on how to release and release rates, contact a commercial biological service provider. The Association of Beneficial Arthropod Producers Inc. has a list of Australian suppliers. 

Download this information as a factsheet

For more information contact:

The Better Bananas team
Department of Agriculture and Fisheries
South Johnstone
Email betterbananas@daf.qld.gov.au 

 
This information is adapted from: Pinese, B., Piper. R 1994, Bananas insect and mite management, Department of Primary Industries, Queensland 
This information has been prepared as part of the National Banana Development and Extension Program (BA19004) which is funded by Hort Innovation, using the banana industry research and development levies and contributions from the Australian Government. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture. The Queensland Government has also co-funded the project through the Department of Agriculture and Fisheries.