Thrips

General

Thrips are a growing problem in the cultivation of Anthuriums and Phalaenopsis. Whereas thrips were rare in Phalaenopsis a few years ago, they have become increasingly common in recent years. There is an increasing number of exotic thrips species, which require a specialist approach. The variety of chemical agents, on the other hand, is increasingly limited. In addition, some thrips populations are resistant to chemical agents. The emphasis in recent years has therefore been on a preventive biological approach, plant resistance, and the targeted use of chemicals. The application technology is also attracting more and more attention. The growing problem of thrips in outdoor crops along with the limited range of pesticides is putting extra pressure on greenhouse crops. In March 2019 a lot of attention was given to this issue, but thrips remains a problem: growers are still too often surprised by the increase in thrips and they also experience difficulties with their control. Thrips pressure often varies by season. Outdoors and in unheated greenhouses, they will hibernate below 12°C. Thrips do not fly in at this time. This is why a good control in the autumn is important, as this will prevent thrips pupae being deposited in the substrate. Even in warm crops, depending on the climate, this will get rid of thrips for a few months. In this article we discuss the different thrips that can be found in Phalaenopsis and Anthurium. In collaboration with Koppert Biological Systems we will also explain the possibilities of biological control.

Recognition Identifying thrips is specialist work that is done on the basis of their microscopic characteristics, such as the number of antennae, the colour of the wings, and their hair growth. It is important to know which thrips (or types of thrips) are found in greenhouses. Each type of thrips requires a different approach. This depends on the following factors: • Time that thrips are active • Sensitivity of thrips to certain pesticides • Behaviour and spreading • Placement of egg deposits or pupae • Fly in at certain times of the year • Life cycle This information is particularly important when choosing the right agent if chemical or biological intervention is required. Most thrips pupate in the substrate. If thrips pupate on the leaf (Echinothrips), this may mean that the usage of nematodes or introducing soil predatory mites has less effect.

Life cycle The life cycle of thrips consists typically of the following stages: • Egg • Larva (2 stages) • Pupa (2-3 stages) • Adult The duration of the life cycle can be very variable. Firstly, temperature has a major influence on the duration of the life cycle of thrips. In general, thrips develop from egg to adult in three weeks at 20°C.

At 30°C, however, it only takes 10 days. In addition, the crop on which they live has a major influence on the duration of their life cycle. One crop can be much more nutritious than another.

The life cycle of Echinothrips. Source: Ada Leman, research entomologist, Wageningen UR Greenhouse Horticulture.

The life cycle of western flower thrips. Source: Ada Leman, research entomologist, Wageningen UR Greenhouse Horticulture.

The location of thrips at certain times of the life cycle may vary. The different stages can be found in the soil, on the leaves, or on the young buds and flowers. The table shows where the different thrips pupate.

Prof. Dr. Ir. G.J. Messelink of the Glasshouse Horticulture Business Unit at WUR has produced a thrips recognition chart, which can be used to identify the most common species. It is available at this link.

Scouting

For optimum control and to prevent increasing damage, it is important that the thrips are detected quickly. Discovery begins with searching, or scouting. This takes time and is therefore costly. Unfortunately, scouting is an indispensable part of the approach. To prevent or limit damage to the crop, scouting is necessary at least once a week. Scouting also builds up knowledge. The first time may be difficult, but with experience thrips damage can be recognized faster and often it is easy to see which thrips are involved. Scouting for thrips in orchids can be tricky. Not all thrips can be observed on sticky traps. This depends mainly on the level of activity. Species such as orchid thrips or Echinothrips are not very mobile and with these species good crop monitoring is important. The thrips are often found in the young parts of the plant and are light-sensitive. In Phalaenopsis, they are often found on the underside of the youngest leaf, on the stalk. For species that are mobile, sticky traps can be used. Blue sticky traps are specific for thrips, which makes counting them easier. When five traps per 1,000 m2 are hung, a good picture can be formed of the number of thrips and their 'hotspots'. Chemistry or biology can therefore be used in a targeted way.

Control Thrips control in general is still often underestimated. The egg and pupal stage cannot be controlled chemically. Thrips in this phase will hatch anyway, and the population can continue to develop. It is therefore important that this kind of cycle is taken into account, and that one or two applications of insecticides will not be sufficient. Spraying only on Saturdays also has a minimal effect, as there are often fewer than seven days between thrips stages. The length of the stages depends on the time of year, the temperature, and the type of thrips. Furthermore, spraying one product after the other only has a moderate effect and creates a lot of resistance. With the use of blue sticky traps, scouting, and biological and chemical support, good thrips control can be achieved.

Spray pressure and maintenance A common mistake is an excessively high spray pressure. Some growers spray with a pressure of 15 bar or more. This creates a very fine mist, which is pushed violently towards the crop. The drops are so fine and light, however, that they don't get very far. At best, the spray liquid only reaches the outer parts of the crop and drips off easily. This is especially important when dealing with thrips that are hidden in the crop. At a lower pressure (around 6 bar), crop penetration is better. Also,even slight contamination of the nozzle can lead to poor dispersal. With water sensitive paper, the effect of a spraying session can easily be monitored. Crop penetration and droplet size can thus be easily made visible. Many crop protection agents today are contact agents, especially in the case of biological agents, and the correct spraying technique is therefore of even greater importance.

Insect netting Keeping insects out by means of good entrance control of incoming planting material is of great importance. Keeping insects out by using insect netting also helps. To keep thrips out, netting will never be 100% effective, but there have recently been developments in new netting techniques that to some extent address the disadvantages of current insect netting. Insect netting can be used not only in ventilation windows but also in the pad of the pad fan installation.

Crop resistance to thrips Crops that are resistant or less susceptible to insects can complement a sustainable approach with biological control well. Varieties that are less attractive to thrips or which contain substances that prevent thrips growth will slow down the population build-up. This makes it easier to achieve a biological equilibrium between the pest and a pesticide. This allows the biological approach to be minimized. Anthura is investigating the possibilities of finding insect resistance in our genetics and possibly cross-breeding it. Hopefully this will result in thrips resistant varieties in the future.

There are several types of thrips that can be found in both Anthuriums and orchids. The most important are listed below.

Dr. Manfred Ulitzka – Thrips-iD

Chaetanaphothrips orchidii – Orchid / Anthurium thrips

Dr. Manfred Ulitzka – Thrips-iD

Frankliniella occidentalis – Western Flower thrips – Californische thrips

Dr. Manfred Ulitzka – Thrips-iD

Echinothrips americanus

Dr. Manfred Ulitzka – Thrips-iD

Thrips tabaci - Onion thrips

Dr. Manfred Ulitzka – Thrips-iD

Dichromothrips corbetti