No cure for tospoviruses is currently available. The current recommendations for controlling tospovirus epidemics are to control thrips and to rogue infective plants. Monitoring thrips transmission of tospoviruses with petunia indicator plants gives growers important information for making pest management decisions and directing methods of suppression where they will have the greatest impact.

To understand how monitoring with indicator plants works, one needs to understand the complex relationship between tospoviruses and the thrips vectors. Most importantly, an individual thrips can only infect a plant if it has acquired the virus as an immature. Infective adult thrips can transmit the virus to healthy plants by feeding for as little as 15 minutes, and they retain the ability to transmit the virus throughout their adult life. A thrips that did not feed on an infected plant while immature cannot acquire or transmit the virus as an adult, even if it feeds on infected plants as an adult.

Petunia indicator plants show distinctive local lesions when infective thrips feed on them. Lesions appear as small brown to black spots of the leaves, as shown in the upper leaf in Figure 1. These lesions look very different from the whitish feeding scars, seen on the lower leaf in Figure 1, left by noninfective thrips. Local lesions result from a hypersensitive response which is the strategy used by the petunia as protection from the virus. In a hypersensitive response, the tissue around the virus entry site dies rapidly preventing the virus from spreading and causing a system wide infection in the plant. Local lesions are apparent on petunias about 3-7 days after feeding by an infective thrips. Characteristic lesions are shown in Figures 2 and 3. Notice how the dead tissue in the center of the lesion has turned tan and is surrounded by a dark border.

Figure 1. Petunia lesions caused by the feeding of infectious thrips can be seen on the top leaf, while the white feeding scars caused by noninfectious thrips can be seen on the bottom leaf. Photo by Jack Kelly Clark. University of California, UCIPM.

Figure 2. A close look at a lesion as it first appears on the petunia leaf. Photo Leslie Campbell, UC Davis.

Figure 3. An older lesion on a petunia leaf. As the lesion ages, its center changes from black to tan. Photo by Leslie Campbell. UCD.

Not at all petunias are created equally when it comes to serving as indicator plants. Research at UCD has shown that ‘Carpet Blue’, ‘Summer Madness’, and ‘Burgundy Madness’ rapidly produce easily recognized lesions. Other plants, such as fava beans, have been evaluated as indicator plants, but the most reliable cultivar of fava beans, ‘Little Toto’, is no longer available. Petunias are an excellent choice as an indicator, because the plants do not support thrips development and seldom become systemically infected. As a result, the plants do not serve as a source of the virus or additional thrips.

Grow the petunias in an area that is isolated from thrips and tospovirus sensitive plants to avoid lesions during production. Otherwise, it will be unclear whether the lesions observed on the petunia indicator originated in the petunia propagation area or the production area being monitored. Plants can be used while there are still relatively small, with approximately 10-15 leaves in a 3 1/2 inch pot. lowers must be removed from the plant before placing them at the monitoring station. This is important because the thrips are more attracted to the flowers than the foliage, and the petals do not express local lesions.

Position the petunias at or slightly above the crop canopy. Use self-watering containers so the petunias don’t dry out in the field or greenhouse. Place the plants on or near a blue surface. We have had success using nonsticky blue cards, placing the plants in aluminum plates painted blue, placing the plants on blue sheets of plastic, or painting the self-watering containers blue.

We have demonstrated the efficacy of monitoring for infective thrips using petunia indicator plants in conjunction with directional sticky traps. In our trials, monitoring stations were placed at the edge of the crop, and among the crop. Each station contained directional sticky traps (north, south, east and west facing traps) and a plant stand for the petunias (Figure 4). The traps and plants were changed weekly.

Figure 4. A typical trapping station with directional sticky traps and a petunia plant on a blue background. Photo by Karen Robb, UCCE San Diego Co.

Petunia indicator plants used in conjunction with the sticky traps have provided valuable insight about the direction from which thrips enter the field or greenhouse. For instance, in one trial, the greatest numbers of thrips were consistently caught on the north facing sticky traps and the first lesions were detected on petunia at trapping stations at the north end of the field. These observations directed our attention to the area north of the fields where a large block of TSWV infected, thrips infested malva was discovered. The grower quickly focussed on removing the malva from his field and from the surrounding areas to the north. Directing this control effort to a specific area made it feasible and the result was a dramatic decrease in spread of TSWV to the grower’s flower production area.

The question has been posed, ‘Why monitor with indicator plants at all? Why not just spray the crop regularly or when thrips are found on sticky traps?’ Although many growers have tried routine spraying, they often find that they still have problems with INSV or TSWV. Sticky trap counts alone do not necessarily reflect the number of infective thrips present, nor do they reveal their source. In our trials, we have found that there is no relationship between the average number of western flower thrips collected on sticky traps and the average number of lesions found on petunias (Figure 5). This is because only the infective thrips in the population can cause lesions and these are the only thrips important to virus spread. Since one thrips can infect several plants, it is not surprising that low levels of infective thrips can reflect a high level of virus. In our trials, peak lesion numbers occurred in areas where the western flower thrips populations were relatively low (see Block 6 on Figure 5). Conversely, peak numbers of western flower thrips have been observed where there were relatively few lesions observed (See blocks 3 and 5 on Figure 5).

Use of the petunia indicator plant/directional trap system alerts the grower to the presence of infective thrips and helps locate their source. In our experience, removal of these sources greatly reduced virus incidence. For example, in our trials with field grown flowers, the number of infected plants dropped from more than 70% to less than 1% in one year.

Figure 5. Comparison of average numbers of western flower thrips (the red bars) to the average number of lesions detected on petunia indicator plants (the black line) in each of seven production blocks monitored. Block 7 is the most northern block, block 1 is most southern.

The petunia indicator plants can be used for more than one week if they do not show any lesions. Care should be taken to remove any flowers or buds that have developed during the week. As mentioned previously, flower petals are more attractive to the thrips than the foliage, but do not express local lesions.

It is not necessary to use directional traps with this system. You will still gain important information, i.e. are infective thrips transmitting virus to the crop, even if you don’t use the directional traps. However, the directional traps do provide information regarding the direction from which the thrips are entering the field or greenhouse and can direct you to the source of the thrips.