This tool was developed with support from Altria Client Services, Phillip Morris International, RJ Reynolds, and the Tobacco Education and Research Commission.
1. The estimate of historical TSWV incidence that you provide for your location is used as a reference for the tool's forecasting. The tool queries weather data to compute estimates for five years in the past, the average of these estimates is adjusted based on the value you provide, and forecasts are made relative to this adjusted average. The rationale for this adjustment lies in the fact that there is variation in cultivation practice that is sure to lead to variation in TSWV incidence, and providing risk estimates relative to your history accounts for this, assuming you continue to do what you did during the time that you observed TSWV incidence in the past.
2. The effect of using Imidacloprid as a tray drench in the greenhouse has been shown to reduce late-season TSWV incidence. If you used Imidacloprid in the past, then your estimate of historical TSWV incidence reflects this. If you use Imidacloprid again during the current year, then the tool's estimate of risk will be as accurate as possible relative to the effects of Imidacloprid use. Similarly if you did not use Imidacloprid during the years that constitute your observation of historical TSWV, and you do not use it during the current year, the tool will again be as accurate as possible. In other words, the tool estimates change in weather-driven TSWV risk, assuming cultivation practice does not change.
3. A seasonal "biofix" date is used as a point in time when degree day accumulations begin. Degree day accumulations are used to describe development of organisms such as plants and thrips. For thrips, the biofix date used depends on your location. For more southeastern (generally warmer) locations, biofix is later based on the expectation that thrips enter their state of overwintering later. For more northwestern (generally cooler) locations, biofix is earlier.
4. This tool was developed using data describing TSWV incidence in tobacco. TSWV infects many other crops, and the biology underlying the epidemiology of TSWV in other crops is similar to that of TSWV in tobacco, but not the same. This tool can be reasonably used to estimate relative TSWV risk in southeastern crops other than tobacco, but it should be noted that the tool is most numerically accurate for tobacco.
Acibenzolar-S-methyl: A plant activator, which in tobacco is used against blue mold and TSWV. It is sold under the trade name Actigard®. Actigard® can be applied either in greenhouse float water or as a foliar application in the field. When properly applied to a tobacco crop that was treated prior to or at transplanting with imidacloprid, Actigard® can reduce TSWV levels by an additional 10% to 30% over the reduction provided by imidacloprid.
Actigard: See acibenzolar-S-methyl.
Admire Pro: See imidacloprid.
Degree days: The models used to run this tool are based on degree days and precipitation. The total amount of heat required, between the lower and upper thresholds, for an organism to develop from one point to another in its life cycle is calculated in units called degree-days (°D). Sometimes called heat units, degree-days are the accumulated product of time and temperature between the developmental thresholds for each day. Figure 1 illustrates the relationship between time and temperature, and the accumulation of degree-days. One degree-day is one day (24 hours) with the temperature above the lower developmental threshold by one degree. For instance, if the lower developmental threshold for an organism is 51°F and the temperature remains 52°F (or 1° above the lower developmental threshold) for 24 hours, one degree-day is accumulated. A Celsius degree-day is not the same as a Fahrenheit degree-day because a Fahrenheit degree is smaller than a Celsius degree. It takes nine Fahrenheit degree-days to make five Celsius degree-days. DDc = 5/9 (DDf) and DDf = 9/5 (Ddc). Figure 2 shows that the areas under the temperature curve represented in Fahrenheit and Celsius units are equal, but the units differ. (Adapted from http://www.ipm.ucdavis.edu/WEATHER/ddconcepts.html)
Imidacloprid: A neonicotiniod insecticide, available in both soil applied and foliar formulations. The most commonly used formulation is Admire Pro (Bayer). Properly applied, imidacloprid can reduce TSWV incidence by 30% to 50%. Imidacloprid also provides control of aphids and flea beetles in tobacco.
Thrips: Thrips is both singular and plural. In general, the name thrips refers to members of the order Thysanoptera. Eight species of thrips are known to vector TSWV, and the tobacco thrips (Frankliniella fusca) is the most important vector of the virus in the southeast. The western flower thrips (Frankliniella occidentalis) is occasionally an important TSWV vector in the southeast. Thrips must feed as newly hatched larvae to acquire TSWV and afterwards can transmit the virus for the duration of their lifespan.
Tobacco thrips: Tobacco thrips (Frankliniella fusca) are the most important vector of TSWV in the southeast. Unlike their name suggests, tobacco thrips feed on many plants, including weeds and crops. The model used by this website calculates tobacco thrips densities and predicted flight times.
Tobacco thrips flight: Each year tobacco thrips spend the winter on weed hosts, some of which are infected with TSWV. In late winter and early spring, tobacco thrips populations increase on these weeds. As the populations grow, the thrips disperse to other plants by flying. As the thrips disperse to new plants they spread TSWV. Each spring the tobacco thrips flight typically spans 4 generations of thrips. The time of each generation’s flight is determined by the cumulative effects of winter and spring temperatures. The warmer the winter and spring temperatures, the earlier in the spring each generation occurs. The 1st and 2nd generation tobacco thrips flights typically involve low numbers of thrips flying to other weed hosts and are responsible for very little of the TSWV spread to tobacco. The 3rd generation occurs after tobacco is transplanted but while the crop is still young and highly susceptible to infection by TSWV. The 3rd generation flight is most often responsible for the greatest amount of TSWV spread to tobacco and for the greatest loss in yield. The 4th generation tobacco thrips flight typically occurs in late May or early June after the tobacco plants have developed a high level of natural resistance to TSWV. For this reason, the 4th generation flight is of concern in tobacco only if it occurs within 5 weeks following transplanting. Area specific degree day models that use location specific temperatures and 15-day temperature forecasts to predict the timing of each thrips generation flight.
Tobacco thrips population size: The size of the tobacco thrips population that develops each spring is largely determined by winter and spring rainfall and temperatures. As temperatures increase thrips grow and reproduce faster resulting in larger populations earlier in the spring. However, rainfall can suppress thrips population growth or even dramatically reduce thrips populations by killing young thrips. The effects of rainfall on tobacco thrips populations are complicated and depend on the amount of rainfall that occurs, when it occurs, and how long rainfall events last. The models that are used ot predict topacco thrips population size account for the complex effects of rainfall and temperature on thrips population growth.
Tomato spotted wilt virus (TSWV): A virus spread by several thrips (Thysanoptera) species and known affect several hundred species of plants, including tobacco. TSWV infections are most severe in tobacco when they occur in young plants (within 4 to 5 weeks after transplant). Severe TSWV infections result in plant death or near complete economic loss.