Watch out for thrips on your farm

Ornamentals Advisory Blog

There are over 8,000 species of thrips world-wide, belonging to the order of Thysanoptera. They are minute insects, only a few millimetres in length and have long, slender bodies, which enables them to enter even tightly closed flower buds. However, only a few species are serious pests of

crops. Most pest thrips belong to the family Thripidae.

The most significant pest species are:

  • Thrips tabaci, Onion thrips
  • Frankliniella occidentalis, Western Flower Thrips (WFT)

Concerns have been raised about the spread of the palm or melon thrips (Thrips palmi) world-wide due to international traffic in plant material. Recently thirty-five outbreaks have been reported from Holland, where orchid flowers from SE Asia are the suspected carriers. It has been recorded on a range of plants including, cucumbers, peppers, aubergines and chrysanthemums. In order to monitor the spread of this ‘new’ pest, it has been designated a notifiable pest.

Although resistance to many insecticides has already been reported for Thrips palmi, (as with WFT), it is likely that the current biological controls used for thrips control will be as useful against this species.

Floriculture Kenya -  Thrips

Identification

Most adult thrips are elongate, slender, minute (less than 1/20 inch long), and have long fringes on the margins of both pairs of their long, narrow wings. Immatures (called larvae or nymphs) are oblong or slender and elongate and lack wings. Most thrips range in color from translucent white or yellowish to dark brown or black. A few species are brightly colored, such as the distinctive reddish-orange larvae of the predatory thrips, Franklinothrips orizabensis and F. vespiformis. Feeding results in various tissue responses, including scar formation and distorted growth. Behavior, body appearance, and host plants help to distinguish among thrips species.  It is more important to distinguish among thrips species in situations where integrated pest management methods are used. For example, each species of natural enemy preys on and helps to control only certain species of thrips or other pests. Certain thrips occur on many different plants but damage only a few of the plant species on which they are found. Identifying the species of thrips may reveal that it is harmless in certain situations and no control action is needed.

Life Cycle

Thrips hatch from an egg and develop through two actively feeding larval stages and two nonfeeding stages, the prepupa and pupa, before becoming an adult. Late-instar larvae change greatly in appearance and behavior and are called prepupae and pupae, even though thrips do not have a true pupal stage.

Females of most plant-feeding species lay their elongate, cylindrical to kidney-shaped eggs on or into leaves, buds, or other locations where larvae feed. The pale prepupae and pupae of most species drop to the soil or leaf litter or lodge within plant crevices or galls. Greenhouse thrips pupate openly on lower leaf surfaces; while pupae (and eggs) of some gall-making species, such as Cuban laurel thrips and myoporum thrips, occur on leaf surfaces but are enclosed within distorted plant tissue. Thrips have several generations (up to about eight) a year. When the weather is warm, the life cycle from egg to adult may be completed in as short a time as 2 weeks.

Damage

Western flower thrips is primarily a pest of herbaceous plants; but high populations can damage flowers on woody plants, such as roses. Rose petals may develop dark streaks and spots from feeding injury that occurred before the buds opened, or the flower buds may deform and fail to open. Western flower thrips also vectors Impatiens necrotic spot virus and Tomato spotted wilt virus, which can severely damage or kill certain vegetable crops and herbaceous ornamentals.

Thrips feeding can stunt plant growth and cause damaged leaves to become papery and distorted, develop tiny pale spots (stippling), and drop prematurely. Infested terminals may discolor and become rolled. Petals may exhibit “color break,” which is pale or dark discoloring of petal tissue that was killed by thrips feeding before buds opened. On some plants thrips can cause severe stunting to the early season flush of terminal growth.

Resistance

The need for regular sprays and the risk of virus has led to very intensive spray programme. Inevitably thrips have developed resistance to many of the insecticides used to control them in the past. Even if the thrips population is not resistant to chemicals, most of the chemicals used to control thrips are not compatible with the biological controls for other pests. Spraying for thrips control may therefore disrupt the biological control of other pests.

Virus transmission

Thrips carry viruses, which they transmit during feeding. Tomato spotted wilt virus (TSWV) is the most serious problem, which can infect a very wide range of plants, not just tomatoes. It infects over 550 species from about 70 families including many ornamental and vegetable crops. Transmission can occur in only a few minutes and thrip can be infectious for life.

Weed reservoirs

Weeds such as chickweed and groundsel may be important reservoirs of TSWV infection. Tomato spotted wilt virus Symptoms caused by TSWV are varied and not always diagnostic. They can include stunting, leaf mosaics, mottling, vein clearing, chlorotic spotting, leaf discoloration, ringspots, leaf and stem necrosis, flower breaking, and general distortion of the plant.

Susceptible plants

Severe loses from TSWV can occur in cineraria, cyclamen, and chrysanthemum. Other very susceptible plants are:  alstromeria, begonia, dendrathema, dahlia, fuchsia, impatiens, pelargonium, primula, ranunculus, gloxinia, and verbena. Individual plants are sometimes used as indicator plants in other crops or the presence of thrip carrying TSWV. They must be regularly inspected, so as not to pose a threat to the crop.

Management

Thrips are difficult to control. If management is necessary, use an integrated program that combines the use of good cultural practices, natural enemies, and the most selective or least-toxic insecticides that are effective in that situation.

IPM of thrips

Thrips must be considered as part of an overall pest spectrum. If biological controls are required for other pests because of resistance reasons – it is necessary to adopt IPM for thrip as well. Most of the thrip pesticides are not compatible with beneficial insects – there is no option other than IPM.

Physical controls

Screens

Physical screens in glasshouses can prevent thrips migration. This may cause problems with diseases since it could increase the period of leaf wetness by reducing ventilation. Engineering solutions can overcome these problems. When combined with UV inhibitors, netting can also reduce the population growth and distribution of thrips as well as other sap-sucking pests.

Mass trapping

Flying adult thrip are attracted to blue sticky traps, particularly in the range 410-480 nm. Mass trapping of leaf miner is practised to some extent in Europe using large yellow traps. Mass trapping of thrips using Tanglefoot (glue) on blue sheets has been used on greenhouse vegetables in Japan. The largest catches are made when the traps are placed just above the canopy. Flower odours, such as geranoil, eugenoil and anisaldehyde have been used in trials to enhance the attractiveness of traps.

Drowning

Flooding the soil can drown thrips larvae in the soil.

Cultural controls

Nitrogen fertiliser adjustments. High levels of nitrogen application increase the growth rate of thrips populations. Adjustments to the fertiliser programme may slow this down and make biological control more feasible.

Reduce carryover from soil phase

The number of adult thrips emerging from pupae in the soil will be reduced if the control of this pest remains a priority after harvest. This could be achieved by managing biological control post-harvest or promoting biodiversity (by using organic mulches) in the soil to encourage predation of the soil phase or deep ploughing or destruction of the haulm.

Trap plants or bankers

Plants, which are more attractive to thrips, could be identified and used to pull thrips out of a crop or to attract them away from new plantations. Alyssum, dill, anise, lantana and verbena have been identified as suitable plants for this purpose. Thrip are more attracted to plants in flower and to colors. Thrip could also be attracted to artificial sources of pollen in dispensers or to honey water strips.

Repellents

Hot pepper wax and garlic barriers are reputed to make thrip come out of their hiding places and thereby, increase the efficacy of biological or chemical controls.

Biological controls

Due to the increasing resistance of thrips to insecticides, several natural enemies and microbials are now reared commercially for mass release into protected crops. These include:

  • Amblyseius cucumeris
  • Amblyseius degenerans
  • Hypoaspis miles
  • Orius laevigatus
  • Orius majusculus
  • Lacewing
  • Beauvaria bassiana
  • Verticillium lecanii
  • Entomopathogenic nematodes

Chemical controls for thrips

Improper timing of application, failure to treat the proper plant parts, and inadequate spray coverage when using contact materials are common faults that can prevent potentially effective insecticides from providing control.

Before using a synthetic insecticide, get to know about the biology of your thrips species and the characteristics of available products by reading the label recommendations. If chemical insecticides are used, combining their use with appropriate cultural practices and other methods will improve the pest control.

Thrips control products recommended by Syngenta:

Actara, Dynamec, Karate Zeon, Match and Pegasus.

Note: Always read the pesticide Label YOURSELF before using a pesticide and decide for YOURSELF what the legal restrictions relating to its use are. Always check for phytotoxicity if using a new pesticide or a familiar pesticide on a new variety. Adjuvants can affect phytotoxicity of a pesticide.