Frequently asked questions: Managing silverleaf whitefly and mealybug in cotton

The TIMS committee has approved a 30-day window for the application of pyriproxifen (Admiral) to control whitefly (SLW) in all cotton growing districts/areas in Australia, beginning in the 2017-18 season:

Q: Why is a 30-day window necessary?

• Pyriproxyfen (Admiral) is a cornerstone product for SLW control; it is currently under significant resistance selection pressure. 
• Testing by DAF researchers in 2017 found resistance to pyriproxyfen in multiple samples from multiple regions; this has highlighted the need to limit pyriproxyfen use to no more than 1 application per season in cotton (of any age).
• In 2017, in some high yielding crops with dense canopies, aerial application of pyriproxyfen was found to be deposited on top 1/3 of the canopy thereby potentially contributing to variable performance of the product.
• Experiences from the 2016 and 2017 seasons show that pyriproxyfen was being applied too late for effective SLW control - well after canopy closure and often well into the open boll stage.

Q: What does a 30-day window mean for growers/consultants?

• A single application of pyriproxyfen in a season applied within a narrow window of 30 days within a defined area/region/valley (area-wide management (AWM) approach) aims to prevent multiple generations being exposed to the product.
• To get the most out of pyriproxyfen, it must be used before its efficacy is limited by spray penetration of the canopy; this is particularly important for high yielding crops with dense canopies – use pyriproxyfen effectively or risk losing it to resistance.
• The 30-day window should ideally be positioned such that pyriproxyfen has sufficient time (at least 3 weeks, more like 4) to do its job (trigger a population crash) before the onset of open bolls.
• Adoption of a 30-day spray window does not mean that everyone has to use it – only use it if you have an in-crop SLW population that is growing at a significant rate (see below).
• If you miss the opportunity to spray (fall outside the window) because the whitefly presence in your crop is low and your crop is significantly younger than others around you, refer to the Cotton Pest Management Guide 2017 for other insecticidal options; note that these will give good results so long as their efficacy is not limited by canopy density.
• If you are in an AWM group that is working well, timely treatment of significantly increasing SLW populations by others in your AWM group within the 30-day window will minimise the risk of their SLW migrating to your crop(s) later in the season.

Q: How do I determine if I have to spray for SLW in the next 10-14 days?

• SLW problems in cotton crops are generally home grown; if one excludes mass immigration of adults from external sources (weeds, other crops), only populations that have been building up slowly but surely for weeks within a crop (lower in the canopy) are likely to explode if exponential population growth occurs, facilitated by suitable environmental conditions (sustained high temperatures, no rainfall, low or nil mortality from parasitoids/predators).
• Sample for adults at the 5th node as per current guidelines (even if you don’t have confidence in this recommendation) and, equally importantly, sample for red-eye nymphs (RENs) at the 8th node leaf from the terminal down (9th and 10th node leaves can also be used):
  • The 8th node leaf is easy to locate using the 1-5-8 rule 
  • Locate the petiole of the 1st fully unfurled terminal leaf (size of a 50 ₵ piece or larger); the petiole of the 5th node leaf is directly across and down the main stem, the petiole of the 8th node leaf is directly below (and in line with) that of the 5th node leaf.
  • With a little practice, the 8th node leaf can be located by eye without touching the plant (9th/10th nodes are deeper within the canopy and more difficult to locate/sample).
  • Count number of RENs on 30 whole leaves.
  • Calculate the % increase in RENs over the total number of checks.
  • Repeat at least twice a week for each management unit.
• If the rate of increase of RENs is ≥ 2-3% per check (every 3-4 days) then you have a growing population that may require treatment; use adult (5th node) and SLW REN (8th node) density information to make a spray decision.
• The amount of honeydew on lower canopy leaves and bolls is another useful indicator of whitefly activity. A pictorial guide to what low, moderate and high levels of honey contamination look like can be found in CottonInfo's Managing silverleaf whitefly in Australian cotton booklet. The pictorial guide should be used in conjunction with the results of adult and REN monitoring, as outline above, to arrive at a spray decision.

An actual whitefly scenario from the 2017-18 cotton season:

Crop was planted thick (20 seeds/m) and developed a dense canopy in boll filling stage. At around 8-9 weeks from defoliation, the crop was sprayed with a soft option product for mirid control at 1-2 mirids/m. The following week mirid density increased to 5-6/m and the crop was sprayed again, this time with bifenthrin (Talstar). Around two weeks later the crop was sprayed with Admiral for whitefly which were building up low down in the canopy. At 10 days after Admiral application (2-3 cracked bolls and around 3 weeks from defoliation), no whiteflies found in the top half of the canopy but honeydew secretion appeared to be increasing slightly in the lower canopy with large numbers of whitefly eggs laid on lowermost leaves.

Should the grower have sprayed again for whitefly and if so with what?

Let’s analyse the situation step by step. The denseness of the canopy undoubtedly limited the penetration of the two mirid sprays and restricted product deposition to the upper section of the canopy. This created a chemical barrier that prevented adult whiteflies from entering the upper canopy and resulted in the whitefly population being forced into the lower canopy. Admiral applied by air will typically be deposited into the upper third or half of the canopy but is still highly effective because whitefly adults move up and down the plant during the day (eg., move down when it’s hot and up when it’s cooler), and in doing so pick up a dose of Admiral. If something like a chemical barrier from sprays targeting other pests changes the behaviour of adults and stops their movement cycles within the canopy (weather extremes can also have a similar effect), then Admiral may be less effective or even fail.

In the above scenario, the grower should wait at least another 10 days to determine whether or not Admiral has been effective (sprayed populations crash typically 3-4 weeks after application). It is not uncommon for treated populations to actually increase before crashing. While waiting for an outcome, there are indicators that will tell us what is likely to happen. One of Admiral’s effects is sterilisation of females. Eggs laid after contact with Admiral will not hatch. Therefore, in the above scenario, if the eggs laid on the lowermost leaves hatch and the small nymph population is clearly increasing, that is a clear indication that the efficacy of Admiral has been limited by the structure of the canopy AND sprays targeting other pests.

If the Admiral spray is deemed to have failed, refer to zone 3C table (page 29 Cotton Pest Management Guide). If another spray is warranted it would be important to consider if the canopy density would again limit coverage and whether control would be satisfactory. Products suitable for use in zone 3 are listed in the threshold matrix on page 30 Cotton Pest Management Guide. So, what else can be done in the scenario above? Keep a close eye on the level of honeydew deposition on leaves and open bolls (use pictorial guide; see online link above), and defoliate early if necessary.

With Solenopsis mealybug (MB) widespread throughout central Queensland and confirmed in WA, NT and VIC, it was only a matter of time before the pest turned up in southern Queensland and New South Wales cotton growing districts. It has now been confirmed in the Macintyre valley and the Gurley/Bellata Districts south of Moree. Growers and consultants are encouraged to remain vigilant in monitoring for this pest and please report any sightings to their local CottonInfo REO so that the movement of the pest can be monitored.

Q. What do growers/consultants need to look for?

Growers and consultants are encouraged to keep an eye out for MB infestations, most commonly manifested as patches of dead plants - hot spots. MB adults are 3-4 mm long and have two characteristic dark longitudinal bare spots across their thorax and abdomen (look like black spots). MB will build up in very large numbers on the tops of a small group of plants and form a “hot spot” of dead or dying plants. They are more likely to be prevalent in crops that are stressed, so particular attention should be paid to these areas (eg tail drains, lighter soil). As well as looking in crops, it is worthwhile scouting any volunteers, or hosts (eg pigweed or fleabane) that are near cropping areas. MB have a very wide host range, so it may be useful look in surrounding vegetation including gardens, to give an indication if they are in the area.

Q. How do I manage for SM?

• Beneficial insects are highly effective in keeping mealybug populations in check; chemical insecticides should be used for control as a last resort if beneficials are absent or at very low densities and population is growing.
• DO NOT USE BROAD SPECTRUM PRODUCTS FOR MANAGEMENT OF OTHER PESTS AS THIS IS LIKELY TO FLARE MEALYBUG.
• There is currently a permit for Transform (sulfoxaflor) PERMIT NUMBER – 85052 and for NSW ONLY, an emergency permit for APPLAUD (plus other registered products containing Buprofezin 440g/L as their only active constituent) PERMIT NUMBER – PER84127. Refer to and follow permit instructions.
• Good coverage and the use of appropriate adjuvants (as per label and research recommendations) is critical for effective control with chemical insecticides; depending on plant size, higher water volumes, up to 250-300L/ha will increase efficacy.
• Research has shown that sequential applications 14 days apart may be required to achieve a satisfactory level of control using Transform (sulfoxaflor).
• Thorough crop destruction/residue management as well as control of potential weed hosts will help to reduce risk for future seasons.

Pest control should aim to preserve the natural enemies that include:

• Predators such as common ladybeetles, Cryptolaemus ladybeetle, lacewings, smudge bugs and earwigs
• Parasitic wasp – Aenaisus bamabwalei, a parasitoid of Solenopsis mealybug, is reasonably wide spread and very effective in suppressing populations where it occurs.

Q. What do I do if I find SM?

• Correct identification. While there is a high probability that you have found Solenopsis mealybug, there are other similar looking species that occasionally occur in cotton but rarely cause crop damage. The beat sheet blog link below includes good photos to help with ID. However, if you are unsure email a photo to either Simone Heimoana, CSIRO Narrabri or Richard Sequeira, QDAF, Emerald.
• Mark the plants they are on, as they can be difficult to locate when in low numbers. Mealybug colonies can disappear between checks due to predation. Re-checking marked plants will enable you to judge the efficacy of any natural enemies that are present and also provide an indication of the species present when it comes to making future spray decisions and selecting options that best conserve them. Tagged colonies also provide an indication of the potential disruption that a spray decision might have had as mealybug will quickly increase in numbers when natural enemies are removed.
• Look for the presence of natural enemies. It is highly likely that there will be other colonies elsewhere in the same field. Evidence of predation/parasitism and whether or not the solenopsis colony is growing or dwindling will provide a picture of what is occurring in your field on a broader scale.
• Come Clean Go Clean. While mealybug can be transferred by wind or water, people and machinery movement can rapidly expand the rate of spread. Wash down machinery going from heavily invested hot spots to fields without any mealybugs. Clothes may also carry bugs such as mealybugs. Aim to visit fields with mealybug last, and brush clothes down before hopping into vehicles. 

Q. Where do I go for more information?

• CottonInfo mealybug webinar
• The Cotton Pest Management Guide has the most up to date information regarding the relative impacts of the different insecticides currently registered in cotton. 
• The beat sheet blog is also a good source of information and photos of key beneficials.
• Richard Sequeira, QDAF, has been leading a program of extensive pesticide and ecology research on mealybug and can be contacted on 07 4991 0810 or via richard.sequeira@daf.qld.gov.au if you have concerns.

An actual multi-pest scenario from the 2017-18 cotton season:

Early sown rainfed crop ran out of moisture and stopped growing. The crop was carrying mealybugs for much of its duration. Mealybugs were scattered throughout the crop; no evidence of dead plants or hotspots – just individually infested plants. Beneficial insects were present throughout – ladybird larvae and lacewing eggs continually present and kept the MB population in check but did not “cleanup”. Following rain in February, the crop has begun to grow again and the grower is keen to produce a top crop which means another 8-10 weeks of growth. The fresh growth is being attacked by mirids, mites and MB inside squares and bolls. The beneficial population was hit by prolonged hot weather and is only just recovering. The fresh growth is starting to attract SLW from within the crop and from surrounding finishing crops and is highly likely to require a whitefly spray in the near future.

While the above is clearly a difficult situation brought on by the grower’s (justifiable) need for an economic return and hence the need to re-grow the crop, it is nevertheless a nightmare scenario for the consultant/crop manager and for which there are no easy options. Mirids and MB are immediate threats; beneficials may eventually clean up the MB population but there is a strong possibility that this may happen too late to prevent significant crop damage. Chemical control is going to be a very expensive option that has to include two sprays for MB (sequential, 12-14 days apart), one (or two) for mites and at least two for whitefly control given that the crop will attract immigrant SLW from other crops for the next few weeks.

Transform is currently the only registered option (available under permit) for MB control in Queensland and New South Wales; it has good efficacy on mirids and MB. Two sequential applications of Transform 12-14 days apart are required to get satisfactory control of MB. Applaud (buprofezin) is a population suppressant (not a curative option) available under emergency permit only in NSW. Research has highlighted another product with good efficacy on solenopsis MB in cotton but it does not have MB on the label and hence is not a legal option at this time. An application for registration of this product for use on MB is currently before the APVMA. Even reasonably “IPM friendly” insecticides will have moderate impacts on most beneficial insects and the cumulative impact of 5-6 sprays (including the miticide) of moderately harmful products will undoubtedly be devastating to the beneficial insect community in this crop.

The “damned if you do – damned if you don’t” scenario above is clearly one to be avoided and actively discouraged but is sometimes a reality and must be dealt with. Given the complexity of the scenario, specific advice on a spray program is beyond the scope of this article and is best obtained from relevant cotton industry resources and personnel.