Californian flower thrips: a recipe for control
The Californian flower thrips frankliniella occidentalis is the number one problem pest in the production of cut roses in Germany, often complicating the provision of healthy, high quality flowers. The control of this pest has become increasingly difficult in recent years, so that the plant protection service in Hamburg has been researching alternative methods to chemical plant protection for a long time now.
An infestation of Californian flower thrips is roses. Photo: Juliane Braun
A plant protection strategy for measures to combat Californian flower thrips
An infestation of Californian flower thrips is roses. Photo: Juliane Braun
To bake a cake you need a bowl, mixer and ingredients but also a recipe, i.e. baking instructions. In the plant protection sector this means: we need both the various plant protection measures and a strategy that leads to optimum application of the measures. The strategy aims to help to combine the plan protection measures optimally and to carry them out at the correct time, in the correct quantity and at the correct intervals.
For example when cultivating cut roses in northern Germany the plants’ dormant season in the winter and the staggered timing for heating greenhouses must be taken into account. But before presenting the ‘recipe’ for combating thrips we should look more closely at the equipment and ingredients.
It is not usually possible to bake a cake without a bowl, mixer and cake tin. When combating Californian flower thrips these are the fundamental measures that must be observed during cultivation. They include healthy young plants, an optimum cultivation process, hygiene and regular monitoring.
Healthy young plants and operational hygiene
It is important to check for infestation with pests (primarily spider mites, whitefly, thrips, aphids and powdery mildew). All the buds on the young plants should be removed. If pest infestation is found then the pallets with the young plants can be treated quickly prior to planting. Both beneficial insects and biological or chemical preparations can be used at this point. After planting the roses should be checked regularly (at least once a week) for infestation with pests. The installation of glue panels can help.
Even if most gardeners find it difficult: infected rose blooms should always be removed as they represent a source of infection for plants that are still healthy. The flowers should preferably be collected in a sealed container (bucket, apron or similar) and left on paths or close to the roses. The containers could always be kept closed when picking the flowers as the adult thrips can easily escape from open containers. The closed containers may not be disposed of in compost but rather with household waste. Weeds must be removed, whatever the cultivation system as they are often used by pests as intermediate hosts or a place to spend the winter.
Monitoring: constant control of rose stocks
Monitoring means the regular observation of rose stocks – even if this is a time-consuming activity for businesses. Constant checking of rose stocks for infestation with thrips (and other pests) is essential, regardless of whether biological, integrated or chemical plant protection is used. The objective is to be able to react to an infestation in good time with a plant protection measure.
Coloured glue panels, for example, can be helpful for this observation (yellow: thrips, winged aphids, whiteflies, cicadas; blue: thrips). These checks should not always focus on the same parts of the rose stock but should rather look at different varieties and positions (beginning, middle, end of the row). Areas of infestation focus may also be marked with rods or similar so that partial treatment can be carried out or so that the areas of focus can be treated more intensely when beneficial insects are distributed.
The controls also serve to assess the success of plant protection measures so that correction can be undertaken as necessary. It is also sensible to entrust one or several persons with the monitoring work full-time.
Beneficial insects, chemical plant protection and natural preparations
Certain ingredients are necessary for every kind of cake. For the combat of thrips these ingredients include the use of beneficial insects, preparations of natural origin and/or chemical plant protection agents. A combination of the above procedures is possible. It is standard practice in many businesses but this is linked to certain regulations and/or dependences.
If it is discovered that a stock of roses is infested with thrips then it is important to know what kind of thrips they are. The thrips on the glue panels or, better, the thrips from a removed flower should be identified by the relevant plant protection service laboratory. The relevant measures can then be taken, depending on which thrips is diagnosed.
Using beneficial insects to combat frankliniella occidentalis
In the production of cut roses the use of robbery mites has proved to be the best method to combat Californian flower thrips. Other adversaries such as predatory bugs, franklinothrips vespiformus or lacewing larvae have not prevailed in this crop. The nematode steinernema feltiae (products: nemasys F or nemaflor has not proven effective in tests at the plant protection service in Hamburg.
When using the robbery mite Amblyseius cucumeris, which primarily eats the young larvae stages, it is important to use them early when the infestation level is low. The repeated use of high quantities (more than 500 animals per square metre every two to four weeks) is necessary for the effecting combat of trips infestation in cut roses.
There are some agents now with effective ingredients of natural origin. They can supplement a strategy against Californian flower thrips, above all through a change in active ingredients. They include, for example, Neem Azal-T/S and PREV-AM.
Treating cut roses with mineral or rapeseed oil
In recent years winter treatment with mineral or rapeseed oil has become popular again in Hamburg businesses. After the plants are cut back in January or February they are treated once before heating, sometimes also a second time. As the Californian flower thrips not only overwinter in the soil as pupae but also in their adult form the thrips can be suppressed in spring alongside the development of spider mites, whiteflies and aphids.
It is important in the use of all preparations to combat frankliniella occidentalis that the block spraying (three to five spraying periods) is carried out in rapid succession (every three to five days) with alternating groups of effective substances and into the growing rose stock. When the buds have developed colour this treatment is no longer useful as the existing thrips can easily hid in the buds. Even systemic preparations only penetrate the buds in very small quantities, so that there is hardly any combating effect. It must also be ensured that the agents are applied under the leaf as the thrips stages are also located on the underside of the leaves.
If there is already an infestation with Californian flower thrips in the first flush of flowers then a block spraying with plant protection agents should be planned because the use of beneficial insects often fails to have the desired effect at this point. If work is planned with beneficial insects from the second flush onwards then pesticides with long-term effects (such as liquid Mesurol) should be avoided in the first spraying sessions.
Individual combat recipes
Besides the pure ingredients that you need to bake a cake it is often important to read the recipe first to know the order in which order and quantities the ingredients need to go into the bowl. For a cake the recipe is enough in order to achieve success. Unfortunately there is no standard recipe for combating thrips that leads to healthy roses.
Different measures must be combined and adapted and modified depending on the infestation situation and climatic conditions. A horticulture business usually needs to write its own recipe and try it out, which is normally also not suitable for use every year.
No complete solutions for flower thrips
All the available measures must be used when combating Californian flower thrips. Measures should be taken at several points (climate conditions, combating pupae in the soil/substrate and the larvae and adult animals on the plants).
No complete solution for Californian flower thrips will be found in the near future. Every business with its own rose varieties and cultivation systems must combine its own ingredients and write its individual recipe. The important thing is to carry out the selected measures very consistently and carefully.
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