What you should know about western flower thrips

Western flower thrips, Frankliniella occidentalis, is one of the most destructive insect pests found in commercial greenhouses. Its damage potential is not only due to its ability to directly cause damage to greenhouse-grown crops by feeding on leaves and flowers, but also because it may vector the tospovirus; impatiens necrotic spot virus. Once a plant is infected with impatiens necrotic spot virus there is no cure, and it should be immediately removed from the greenhouse.
 

Major pest; reasons why are many

There are a number of factors associated with the biology and life cycle of western flower thrips that make it a major insect pest for greenhouse-grown horticultural crops. These include high reproductive capacity, broad host range, rapid life cycle, resistance to insecticides, feeding habit, small size, and virus vector. Because western flower thrips can vector impatiens necrotic spot virus, the tolerance for this insect pest is ‘near zero.’ Therefore, producers routinely apply insecticides to suppress populations of western flower thrips in order to prevent crops from getting infected with impatiens necrotic spot virus. However, the “selection pressure” affiliated with regular use of insecticides can result in resistance development in western flower thrips populations.

Impatiens necrotic spot virus has a broad plant host range that includes many horticultural crops and common weeds. Weeds that are known to harbor impatiens necrotic spot virus include oxalis (Oxalis sp.), chickweed (Stellaria media), bittercress (Barbarea vulgaris), prostrate spurge (Euphorbia supina), and jewelweed (Impatiens capensis). The symptoms commonly observed on infected plants are necrotic spots and/or concentric line and circular patterns. Symptoms of impatiens necrotic spot virus may be expressed one to two weeks later depending on temperature and plant type.

Some plants can harbor symptomless infections or symptoms that are atypical for this virus. Detection using commercially available serological methods is highly recommended. Impatiens necrotic spot virus is classified in the genus tospovirus and its plant host range overlaps with that of tomato spotted wilt virus, another tospovirus. In fact, plants may harbor infections of both viruses simultaneously.
 

Vector potential and virus Infection

Western flower thrips is the primary vector of impatiens necrotic spot virus but other thrips species have been reported to transmit the virus including F. fusca, F. intonsa, and F. schultzei.

The virus is transmitted in a circulative manner, meaning that the virus must circulate through the insect body before transmission occurs. Impatiens necrotic spot virus is acquired by the vector (western flower thrips) through their mouthparts when they feed on infected plants.

The virus first infects the insect gut where it replicates. Then the virus accumulates in the gut tissue and spreads internally within the insect vector. In order for transmission to occur, the virus must get to the insect feeding structures, namely the salivary glands.

The virus is secreted, along with saliva, into a new host plant and transmission occurs.

During the acquisition phase, the virus is ingested by the nymphs (1st and 2nd instar) and can then be transmitted by late 2nd instar nymphs and/or adults during feeding. There is a latent period between acquisition and transmission of the virus in which the virus must infect gut cells, cross tissue barriers, and then reach the salivary glands in order for transmission to occur. Adults are the primary transmitters of tospoviruses because they are more mobile.

Furthermore, adults may feed on virus infected plants and sustain midgut infections; however, adults that acquire the virus cannot transmit it because their salivary glands do not become infected with the virus.

The western flower thrips nymphal stages acquire the virus by feeding on an infected host plant with viral acquisition taking approximately 15 to 30 minutes. Efficiency of viral acquisition increases with longer feeding duration on an infected plant.

Due to the complexity of the insect vector and virus infection cycle, nymphal stages that acquire the virus typically develop from eggs that were deposited into that plant.

Therefore, plants that are hosts for the vector and virus are the most important plants to monitor for impatiens necrotic spot virus infection.

The ability to acquire and transmit the virus decreases when the virus is obtained at a later nymphal stage.

The virus is transmitted when adults feed, which generally requires five minutes to infect plants, but longer feeding times can increase the efficiency of transmission. Once infected with the virus, western flower thrips remain infective throughout their life-span (30 to 45 days) with the virus replicating (making copies of itself) within the body of the western flower thrips.

Despite the fact that the virus may replicate or multiply inside the body of adult western flower thrips, transovarial transmission does not occur, which means that the virus is not passed on to the offspring or young.

In addition, not all western flower thrips that have the virus are able to transmit it, as a certain threshold of virus must be present in order for transmission to occur.
 

Vigilance is critical

In conclusion, it is important to routinely monitor crops and manage western flower thrips populations as effectively as possible.

Doing so will help avoid substantial problems with impatiens necrotic spot virus on your greenhouse-grown horticultural crops.

Raymond A. Cloyd, is a professor and extension specialist in Horticultural Entomology/Integrated Pest Management at Kansas State University - Department of Entomology. Anna E. Whitfield is associate professor of Plant Virology at Kansas State University -Department of Plant Pathology.