After climbing one great hill, one only realizes that there are many more mountains to climb’. This old adage attributed to one of African greats rings true in pest management. The past few decades has seen the rise and spread of thrips in greenhouse farming, hitherto a minor pest, to become the most destructive insect pests of the greenhouses. Thrips hits growers where it hurts most! Writes Simon Kihungu
Thrips are minute, slender insects, usually only a few millimetres long with piercingsucking- mouthparts and the ability to cause direct damage to flowers making it unmarketable for export.
Thrips have piercing-sucking mouthparts. They use a needle-like structure to puncture plant tissue and a second tube-like structure which is placed into the hole through which plant sap is extracted, subsequently causing direct damage to the crop.
In Flowers there are mainly two types of Thrip species that attack roses, carnations and other flowers. The western flower thrips (Franklieniela occidentalis) and Thrips tabaci also known as Onion thrip. These thrips species mainly feed on both leaves and flower petals with the majority of their damage to roses occurring throughout the growing period of the flowers.
Western flower thrips (Frankliniella occidentalis)
Western flower thrips are one of the most nortorious insect pest of greenhouse rose production worldwide. In order to develop controls, it is essential to understand the feeding behavior of these pests.WFT have piercing-sucking mouthparts, but they do not feed exclusively in the phloem sieve tubes. Instead, they feed within the mesophyll and epidermal cells of leaf tissues. More specifically, they feed on plants by inserting their tubular stylets into cells and withdrawing the cellular contents.
Monitoring
Monitoring is important for early detection of a thrips infestation; determining the numbers of thrips present in the greenhouse; detecting seasonal trends in thrips population over the year; and in assessing the effectiveness of management strategies implemented. Yellow or blue sticky cards are the easiest way to monitor for thrips. Gently tapping the terminal buds and flowers onto a white piece of paper and using a 10x magnifying lens to examine the insects that fall out is an easy way to check for thrips. Be sure to record population levels.
Effect of thrips
• Affects the quality of the flower
• The wounds inflicted serves as avenues for Botrytis cineria
• Reduced number of graderable and exportable stems
• Heavy investment on the farms chemical budget
• Less income for the farms
Reasons why managing Western flower thrips is difficult
- Small and easy to overlook
- Concealed mode of life (part of life in soil)
- Prefer feeding on flower parts
- Hide in flowers, buds, leaf buds/hard to spot and reach with pesticides
- Live on a wide variety of host plants
- Reproduce rapidly in warm greenhouses
- Hide in plant materials and transported worldwide
- The pest is physiologically and behaviorally resistant to a number of pesticides.
Management
Start clean: Making sure the young plants are clean is an important part of keeping a crop thrips-free. Transplants should be inspected for thrips before being placed in a greenhouse. Granted, this may be too labour intensive and time consuming especially when large quantities of plant material are involved but sticky card traps placed among the new plants for a day or two will quickly indicate the presence of thrips.
Exclusion: Staying clean would involve excluding thrips from the greenhouse as well as eliminating sources of thrips infestation. Where feasible, covering the openings to greenhouses is a very effective delivering a reduction of up to 70% in pest problems. Screening to exclude thrips must be very fine. Such screening, known as microscreen, has a maximum hole size of 192μm (0.037mm2). This, however significantly reduces air flow when placed over vents, and growers must modify ventilation systems to compensate for this.
Weeds and plant debris: Weed control is essential for a successful thrips control program. Certain weeds, particularly those in the composite and solanaceae families, and those with yellow flowers, attract thrips adults serving as refuge both for thrips and tospoviruses. As such weeds must be removed from both inside and around the greenhouse. It is also essential to immediately remove plant debris and bloomers from the greenhouse or placed into containers with tight-sealing lids, and not in the open as is common practice in most farms. Thrips adults will abandon desiccating plant material for the main crop.
If any of these actions are omitted, it may leave an opportunity for the Thrips to persist.
Blanket Sprays: When treating a greenhouse for western flower thrips, all flowering plants in the greenhouse must be treated. Certain pesticides repel these insects. When a grower sprays only part of his greenhouse crops (commonly known as spot spray) thrips can fly to nearby unsprayed plants. Some of the thrips will return to the sprayed plants shortly after the residue dries and the population will hardly seem to have been affected.
Clothings: Do not wear light blue, white, or yellow clothing into the greenhouse. Thrips may jump onto clothing and be carried to other benches or other greenhouses.
Traps: Use blue sticky traps or cards placed at top plant height to monitor populations.
Closed Greenhouses: Screen greenhouse vents and air intakes to exclude thrips from entering from outside.
Chemical Application: Pesticide treatments should be scheduled twice a week or no less frequently than five days apart. Use normal insecticide rates and treat more often, rather than heavy rates less often. The success of the insecticide application will depend on the application strategy (e.g. Target sprays) and the chemical residual activity. Use several pesticides on a rotation rather than using the same pesticide repeatedly.
Developing Spray Program In developing a spray programs, the grower MUST take into consideration the following:
Biological Mode of Action: Biological Mode Action of the insecticides e.g. contact, systemic and translaminar, etc
Target Stage: Target stage in the insect lifecycle e.g. larval stages, adults etc
Biochemical Mode of action: Biochemical Mode of action e.g. Acetylcholineesterase inhibitors, Nicotinic Acetylcholinereceptor agonists etc
Current resistance status: It is important to know the population status before developing your spray program
Use of Biological agents: Biological control of thrips relies on the use of natural enemies including predatory mites and pirate bugs, entomopathogenic (or insect-killing) fungi, and entomopathogenic nematodes.
In conclusion… Thrips has been, and still is, a difficult insect pest to control or regulate in greenhouse production systems leading many to believe that we have reached an impasse regarding its management. Dealing with thrips, therefore, requires a holistic approach integrating the methods detailed above. Key in this approach is knowledge about the biology of the pest, and indeed all other relevant pests, the crop (s), the cropping systems, pest management options etc.
Simon Kihungu is a Technical Sales Representative with Bayer East Africa
