May 2009 – What a difference a mite makes!

Posted On 04 Aug 2009
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However, the Kenyan growers who benefit from natural all year round growing conditions also have no winter respite but spider mites are becoming less of a pest. Integrated Pest Management and the use of inundative Phytoseiulus persimilis programmes have relegated this pest to minor status on many of the top farms in Kenya, from Mt Kenya to Naivasha and Athi River and in the Western Highlands. Many rose breeders are now using Phytoseiulus in their show-houses as the quality of their roses is exceptional when biological rather than chemical control is sued. Leading propagators such as Stokman and Solo Plants have also shifted into biological warfare – providing customers with better quality plants.

Locally produced Kenyan Phytoseiulus is adapted to high temperatures and for the last five years has proved it is able to control mites very effectively even in the hot ‘summer’ season of December through to the following March – when temperatures in the greenhouse can peak at above 40 deg C sometimes. However, to achieve this, the grower needs to know how to use a compatible spray programme and have well trained managers and scouts. Failures may sometimes be blamed on high temperatures when in Kenya, Phytoseiulus controls mites even more quickly under these conditions. If growers humidify their roses, as is common practice – high temperatures should not be a reason to expect Phytoseiulus to fail to give control, unless the strain used is not produced under local conditions and not adapted to these temperatures.

More importantly, the cost of the biological programme is not only less expensive than pesticide programmes but once pesticide applications are reduced the grower benefits from higher yields, longer stems and bigger buds. The acaricides market in Kenya has collapsed as result of this ‘mighty mite’ and carefully developed IPM programmes which have stood the test of time!

Get to know your enemy – if you want to kill it.

Globally, the most common mite pest of flowers is Tetranychus urticae, known as ‘spider mite’, red spider mite’, and ‘two spotted mite’. Variation in the colour of mite pests leads to some confusion amongst growers about their identification, as does the plethora of its common names. T. urticae is sometimes referred to as the ‘red form’ or the ‘green form’. There other pest mites, such as T. cinnabarinus and T. evansi – very similar to the common T. urticae. There are also mites not in the Tetranychus genus such as the Bryobia mites, and the tarsonemid mites as well as the false spider mites such as Brevipalpus. All these mites attack ornamental crops.

It may seem academic to be able to identify the pest mite species, but the relevance to the flower grower of the identification of the pest mite is that some species of predatory mites have preferences for different pest mites and it is important to choose the correct predatory mite for the pest present. Some taxonomists describe the ‘red form’ of Tetranychus urticae as another distinct species, Tetranychus cinnabarinus (also known as carmine spider mite). Yet another species of mite, Tetranychus evansi, has been made a quarantine pest in the EU but it is virtually indistinguishable from .T urticae, unless examined by a taxonomist. T. evansi is present in many rose producing countries but is considered a problem in Europe because it does not hibernate in the winter, unlike T. urticae which stops feeding when temperatures drop in the winter.
Tetranychus urticae, spider mite has so many common names because when it feeds on the chlorophyll in the plant sap, it fills its intestines with a dark green pigment that looks like two black spots (two-spotted mite). If excessive feeding occurs, these two spots can merge and appear as one large dark spot. It can also turn a brick red colour when the night lengthens in autumn and temperatures drop (red spider mite).

The exact night/day length when hibernation is triggered can vary with longitude but the practical significance of this is that the hibernating form generally comes off the plant and sits out the winter either in the soil, under dead leaves or inside drip irrigation lines. The red pigment, which gives the mites its brick red colouration, is unpalatable to predators, drawing an end to biological warfare for a growing season. However, predatory mites, such as Phytoseiulus, do not have the ability to go into diapause and will resort to cannibalism, eating other Phytoseiulus, and eventually starve to death as the north and southern hemisphere winter draws in. This makes it necessary to re-apply them in the spring when the mites come out of hibernation, as night lengths shorten and temperatures rise. The female spider mites would have mated before they went into hibernation, as males tend to be scarce after the winter, because they are smaller and have less food reserves than female spider mites.
However, if all year round growing conditions are provided, either by the natural heat from ‘growing under the sun’ or by fossil fuel heating and lighting over the winter – the spider mites may not enter diapause and continue to breed. For this reason, the spider mite populations on crops grown on the Equator can be several times higher than those in the north and southern hemisphere crops. Badly managed roses on the Equator, where pesticide resistance is a problem have been recorded with more than 150 million pest mites per hectare – and not many saleable roses.

A keen grower will be making regular observations of the pest mite populations on the crop, recording the colour and pattern of spots which appear on the pest mites. If there is a change in the efficacy of the crop protection programme, whether it is a pesticide programme or a biological programme, these observations may help to explain the causes of a loss of control.
In Californian roses in the 1990s biological control with Phytoseiulus failed because the mite population shifted from T. urticae to predominantly T. evansi. Although Phytoseiulus will eat T. evansi, its reproduction rate is reduced, making it necessary to introduce much higher rates of Phytoseiulus to obtain control of this pest. Only high introduction rates of Phytoseiulus, in excess of one million per hectare in one application followed by top-ups in subsequent hotspots of the same level or more in the next 5 weeks, will be sufficient to prevent serious damage occurring as a result of poor control of T. evansi.

This is only possible if the Phytoseiulus is not too expensive – economics in the Northern and Southern hemisphere, and not biology, may limit the effectiveness of biological control for T evansi and other mite pests. Whereas, predators produced by commercial companies on the Equator are a very cost effective solution for local growers. There can be up to a five-fold difference in the retail price of the same predators, internationally. The ‘dribble method’ of some European predatory mite programmes, which introduce only a few thousand predatory mites per hectare per week, due to cost, are unlikely to prevent severe damage occurring if T evansi is a more important pest than T urticae on the crop. Fortunately predators are freely marketed throughout the world, as they are the same species used internationally and Kenyan predatory mites are already sold in many countries outside of Africa.

The battle of the predatory mites – which one is best?
There are a number of predatory mites which are raised commercially as biological control agents for mites in different parts of the world. It is important that a grower understands their individual strengths and weaknesses when designing an IPM programme. All predatory mites are not the same. Which one should be used and when? Can they be used together? These are seriously important questions.

The most successful predatory mite is Phytoseiulus persimilis, which is globally the most efficient predator for T. urticae spider mite control because there are four female Phytoseiulus per male in the population, whilst other predatory mites such as Amblyseius californicus have only two females for every male. Since females must eat more pest mites to produce eggs, it is a huge benefit for growers to choose a biological control agent which naturally has more females in the population. Even though Amblyseius californicus will live longer than Phytoseiulus – the overall egg laying capacity is much lower and therefore it is a less effective predator in high pest mite situations.

In addition to having 100% more females, Phytoseiulus also eats more spider mites per day than any other predatory mite. They will eat from 5 to 20 pest mite eggs per day and up to five motile stages of pest mites. According to Koppert, a female Phytoseiulus will eat 503 spider mites whereas a female A. californicus will only eat 156 spider mites. It is very important therefore that a grower does everything possible to ensure that nothing interferes with the rapid establishment of Phytoseiulus in the crop, especially if hot weather is approaching and the risk of pest mites increasing is imminent.
The two most important factors which can interfere with this are the presence of toxic pesticide residues and the presence of too many competing predatory mites from another species. Most Biocontrol suppliers can provide comprehensive information about the harmfulness of different pesticides and the length of time these harmful effects last – so it is not difficult to devise a programme which takes this into account. Forward planning by the grower will ensure successful introductions of Phytoseiulus, when the weather warms up.

The main practical advantage of Amblyseius californicus is that it has much more tolerance to pesticides than Phytoseiulus. Therefore if a grower uses a lot of non-compatible pesticides this may sometimes give it a seasonal advantage over Phytoseiulus – but over reliance on this attribute will lead to problems.

In California recently, excessive introductions of Amblyseius californicus into strawberry crops are reported to have made it very difficult to re-establish Phytoseiulus to control pest mites when the weather got hotter – crops suffered damage as a result. Growers are now re-thinking their strategies. The reason for this is that Amblyseius is a ‘generalist predator’ – meaning it will feed on more types of prey than Phytoseiulus which is a ‘specialist predator’. Consequently – Amblyseius californicus will even eat Phytoseiulus eggs and juveniles, if spider mite is in short supply (such as at the beginning of the growing season). It is very difficult to prevent pest mite populations from increasing in this situation, as Amblyseius is not good at controlling high pest mite populations. Phytoseiulus must establish fast and begin to breed in the crop. Researchers in Austria , Schausberger and Walzer (2001) have reported that Phytoseiulus may be able to detect the presence of Amblyseius in pest mite patches and avoid colonizing these areas. Growers in the Mt Kenya growing region have also observed that where there are many Amblyseius californicus it is not easy to establish Phytoseiulus. The Amblyseius needs to have died back to low levels, due to lack of food, before the grower tries to re-establish the Phytoseiulus. This requires careful timing, early introductions of low levels of Amblyseius in the low season for mites and a sufficiently long gap between these introductions and the anticipated introduction date for the Phytoseiulus when the mite season starts again.

However, if a grower has cleaned his crop up very well with Phytoseiulus, an alternative strategy is to monitor crops closely and apply only Phytoseiulus to any new hotspots of spider mite in the low season. They will be cleaned up very quickly by Phytoseiulus. However, this is only possible if the grower has a strong team of well trained scouts and a keen manager – on the look-out for new spider mite hotspots. It is not difficult.

On the other hand, Phytoseiulus does not eat Amblyseius californicus eggs as it is a specialist predator and more likely to be cannibalistic if there are not enough pest mites about to eat. This is a distinct advantage on ornamental plants which are to be exported to the EU market as the crops, if properly managed can be completely free of both Phytoseiulus and mites. There is a greater risk however, that Amblyseius californicus could remain on exported crops, if they have been applied in large numbers, since they can survive for longer periods without food and are less likely to resort to cannibalism. Amblyseius californicus is regarded as an exotic species in the UK and only permitted to be used under glass under license from DEFRA and not on outdoor crops.

Amblyseius californicus has some advantages, in that it can survive for 18 days without food, when Phytoseiulus can survive for 6 days without food. Their reproduction rates are very low under these conditions. It is unreasonable to assume that when spider mites are not present in large numbers that Amblyseius will survive in sufficient numbers to provide good control in spider mite hotspots which may develop in hot weather. Phytoseiulus will always be needed to prevent mites becoming a problem again in hot weather.

Amblyseius californicus is a generalist predator and will feed on a wider range of pest mites and even pollen – whereas Phytoseiulus is a specialist predator of Tetranychus species. Although Amblyseius californicus has been reported as being able to feed on thrips – it is not an effective predator for this pest and will lay very few eggs if this is the main source of food. Compatible pesticides will still be needed to control thrips if other more suitable predators such as Amblyseius cucumeris are not deployed for thrips control (see next month’s article on IPM of thrips).

Equally, although Amblyseius cucumeris may occasionally eat spider mites it is far less effective than Phytoseiulus and is not considered an economically or biologically effective predator for spider mites. Amblyseius californicus is usually a more expensive predator to purchase than Phytoseiulus and is therefore not economically feasible to use where formerly Phytoseiulus has provided good control, such as roses and carnations. In Kenyan roses, Amblyseius californicus occurs naturally in low populations, in some areas the natural population is equivalent to application rates in excess of 100,000 per ha per week – and this happens for free – just by natural migration into the crop – once the use of non-compatible sprays is reduced.
Syngenta Bioline has recently introduced an innovative slow release sachet containing a breeding colony of a new predatory mite, Amblyseius andersoni, which feeds on many types of arthropod prey and pollen. Predatory mites continue to emerge over several weeks from this waterproof sachet as it is hung in the crop and in much higher numbers than if it was applied as a ‘loose product’. These sachets are called Gemini sachets and are a patented product of Syngenta Bioline.
Another new Amblyseius on the scene is Amblyseius swirskii. If present in the crop, it will eat only the younger spider mites, according to Koppert BV. They also report that A. swirskii is hindered by spider mite webbing and consequently avoid spider mite hotspots.


Other natural enemies of spider mites

There are a number of other predators of spider mites that may occur naturally or can be purchased from commercial producers such as Feltiella acarisuga. This is a predatory midge which needs high numbers of mites in order to keep it reproducing in the crop. Care needs to be taken in the integration of compatible pesticides, including fungicides such as milbemectin which will kill more than 50% of Feltiella present in the crop. Very little published information is available from any bio-control producer on the pesticide sensitivity of Feltiella. Koppert’s side effects list records that >75% would be killed by a spray of spinosad, and that similar toxic effects will last for 2-3 weeks after a spray or drench of imidacloprid. This would severely limit the pesticide options for thrip control, which is usually a problem at the same time as spider mites.
Another predator is Stethorus punctillum – a small black ladybird type beetle, which feed only on spider mites. This can occur naturally in many crops around the world.

Biopesticides for mites
Certain entomopathogenic fungi such as certain strains of Beauveria bassiana (Naturalis) and have been mass produced for the control of a range of pests, including mites in ornamentals. Since these are fungal agents care needs to be taken when integrating their use with fungicides for plant diseases in the same crop.
IPM plan for mite control
For a successful IPM programme for mites the following plan will provide a useful guide.
1. Obtain specialist training for scouts and managers before embarking on a biological programme to ensure the distribution and number of mites in the crop can be reliably recorded before introducing Phytoseiulus. Scouting method should quantify the number of both mites and predators in a crop.
2. Check the pesticide history of the crop against published pesticide sensitivity information for the bio-control agent which will be applied – to ensure there are no harmful residues in the crop before applying Phytoseiulus.
3. Plan which compatible pesticides will be used for the anticipated pests and diseases which may occur after the Phytoseiulus has been applied – seek expert advice and have these in pesticides stock to cover at least the first 8 weeks after the Phytoseiulus has been applied.
4. If levels of pest mites are above 20 million per hectare do not start the bio-control programme and use compatible acaricides sprays to bring down the level of mite pests. Seek advice on choice of acaricides and do not use one with longer than 2 weeks persistence of any harmful effect.
5. Check that the theoretical ‘clear date’ from the previous pesticide history is truly safe, by applying Phytoseiulus in a mite hotspot before purchasing large numbers of Phytoseiulus to apply to the crop. Only start the Phytoseiulus introduction if there are no harmful pesticides and only very low numbers of competing predators such as Amblyseius californicus.
6. Purchase good quality Phytoseiulus in sufficient numbers to make an even introduction over the entire crop – minimum 1 million per hectare plus extra predators for hot spots. Aim to get as close to an introduction ratio of 1 Phytoseiulus for 20 mites as possible.
7. Plan pesticide applications (including fungicides) to make sure that nothing needs to be applied for 10 days after the first introduction of Phytoseiulus – to give it a chance to breed in the crop.
8. Scout the crop at least weekly to determine if additional Phytoseiulus is needed – take advice from experienced IPM advisors and top up as quickly as possible if the ratio does not reach 1:20 (one Phytoseiulus for 20 mites) within five weeks of application.
9. The mites should be completely eliminated within 6-8 weeks if no non-compatible sprays have been used and any hotspots have been found and treated immediately.
10. After the house has been cleaned up and the crop is free of mites, the scouts must be more vigilant. A ‘hotspot’ is now a plant with even ONE mite on it. These areas should be marked and treated with Phytoseiulus as quickly as possible. Do not spray these areas, as pesticides will not be as effective as Phytoseiulus.
11. Organise to have your crop also scouted every month by an independent experienced advisor – to check that your scouts are finding the mite hotspots effectively. In this way you should no longer have problems with mites.

Table 1: Attributes of different pest mites

  T. urticae T. cinnabarinus T. evansi
Other common names  red spider mite,
two spotted mite,
spider mite, glasshouse spider mite.
carmine spider mite,
red spider mite, carnation mite,
cotton spider mite
(often confused with the red form of T. urticae)
tobacco mite
eggs Eggs laid in clusters, spherical, translucent, pale in colour, becoming yellowish as mature, red eye spots can be seen inside eggs Eggs laid singly, amber in colour, often with a distinct pale brownish spot or traces of red. Eggs are rust red.
newly hatched larvae Six legs. Pale to yellowish, yellowish green after feeding. Six legs. Yellow to orange in colour, turns green on feeding.  
older larvae 1 Eight legs. Yellowish green with dark spots. Round bodies, short legs. Eight legs. Yellowish green with dark spots. Round bodies. Short legs.  
adult Females (summer form) yellowish to greenish in colour with two black spots. Females (summer form) are dark red with two black spots. Males are straw coloured. Adult varies in colour from light orange to dark red or brown but lacks two dark spots. Longer legs than other Tetranychus species.
hibernating form 2 Orange or red Pale red or purple No diapause
Feeding symptoms Whitish or yellowish stippling/spotting on upper surface of leaves from feeding damage. Toxic saliva causes severe chlorosis and death of leaves, particularly on tomatoes. Toxic saliva causes serious chlorotic spots
Severe webbing may kill plant
Min temp 12 °C   10.3 to 14 °C
Max temp 40 °C   38 °C
Optimum temp 30 to 32 °C   34 °C
development time egg to adult < 7days at 30 °C
10 days at 24 °C
25 days at 15 °C
  46 days at 15 °C
Eggs per female 3 10 eggs per day
100 eggs in 2 weeks at 25°C
129 eggs at 24 °C 120 to 250 eggs
notes     EPPO A2 quarantine pest
Most effective biological control agent Phytoseiulus persimilis Amblyseius andersoni Phytoseiulus longipes
or
Phytoseiulus persimilis in high application rates

Note 1: The colour of mites can differ depending on the host plant. The colour can also change if the mite is going into temporary summer diapause or full winter diapause (hibernation)
Note 2: The timing of diapause is variable and affected by latitude as well as temperature and day/night length.
Note 3: The number of eggs laid is significantly affected by the host plant and also the cultivar.

References:
“Mites of Greenhouses – identification, biology and control” Zhi-Qiang Zhang CABI Publishing www.cabi-publishing.org ISBN 0 85199 590 X
“ Combined versus single species release of predacious mites: predator-predator interactions and pest suppression” P. Schausberger and A. Walzer in Biological Control 20: pp 269-278 (2001)
“Knowing and recognizing – the biology of glasshouse pests and their natural enemies” M.H. Malais and W. J. Ravensberg Reed Business Information ISBN 905439126 – X

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