Pest Status and Economic Losses to Insects in Desert Lettuce: A 12-year Summary 2016
For the past 12 years, the UA Vegetable IPM Team has annually surveyed PCAs and growers to document insect pest activity, economic costs of insect management and insecticide usage in Arizona lettuce through interactive workshops held each April. The information collected during these workshops can be very useful to the lettuce industry by providing “real world" information on insect pest status and insecticides usage that can be used for addressing state and federal regulatory issues, prioritize research and educational activities, and translate the PCA’s activities into economic terms for their growers. In a recent VegIPM update (Jun 8), we discussed interesting trends in insecticide usage on desert lettuce based on the results of these workshops that can be found at Insecticide Usage on Desert Lettuce. Just recently we summarized the remaining data collected in the workshop surveys. Summarized over the past twelve-years (2004-2016), the survey data provides “real world” information on the pest status of key insect species, economics of insect management, and estimates of yield losses in lettuce. A quick analysis of the summary clearly indicates that insect management in desert lettuce is very intensive, can be expensive and is generally effective. Specifically, surveys completed by PCAs represented a large proportion of the lettuce acres grown in the Yuma, and neighboring Bard/Winterhaven growing areas. Not surprising, the surveys showed that annually 100% of the acres are scouted for desert lettuce pests and lettuce fields are visited >4 times per week. In terms of pest status, the Lepidoptera larvae complex (beet armyworm/ cabbage looper/corn earworm), western flower thrips, aphids and seedling, soil pests represent the most economically important insects attacking desert lettuce crops based on the % of treated acres, numbers of spray applications, and yield losses associated with each pest. In terms of yield losses, average estimated loss from any one pest never exceeded 1% in either fall or spring lettuce. Given the heavy insect pressure we experience each season, PCAs and growers should be commended on their capability of maintaining insect losses to such low levels over a 12-year period. However, it is important to note that this was accomplished in part, by the considerable expense and time invested by PCAs in scouting for these pests, and certainly by the significant investment in insecticide spray and application costs used each season to prevent these key pests from causing economic damage to lettuce crops. Bottom line: IPM in desert lettuce crops is effective, but at a cost. For a copy of the complete, summarized report go to: Insect Losses and Management on Desert Lettuce: 2004-2016.
Plant viruses cannot penetrate the intact plant cuticle and cellulose cell wall that acts as barrier to infection. The virus overcomes the problem by either avoiding the need to penetrate (example seed transmission) or by using the wound in plants as infection site, or transmission by insects, nematodes or fungi as a vector.
Mechanical transmission involves the introduction of infective virus or viral RNA into the wounds of plants. Viruses such as Tobacco mosaic virus (TMV), Potato virus X are highly stable, and reach high concentration in plants. As you all know TMV can readily contaminate hands, clothings, and implements and can be spread by worker. TMV can even spread mechanically by tobacco smokers as the virus is present in cured tobacco leaves.
Mechanical transmission is of great importance. In field and greenhouse, great amount of caution has to be implemented to not transmit the infection. Field sanitation, tool sanitation is very important to avoid the spread of virus.
However, in experimental world mechanical transmission is a very useful tool to study viruses. Mechanical inoculation of virus to a heathy host plant is done for assays, to produce local lesions, in the propagation to of viruses for purification, in host range study, diagnosis, and to understand the interaction between virus and susceptible cells.
Seed transmission: About 1/7 th of the known plant viruses are transmitted through seeds. Different viruses have different host ranges (the plants that they can infect). Tobacco mosaic virus, Cucumber mosaic virus are some viruses with a very wide host range and they may not be seed transmissible in all plants they infect. Seed transmission plays a huge role in virus epidemiology. Not only they can be a primary source of infection, leading to an epidemic in the field upon conducible environment, seed transmission is an effective way for long distance travel of the virus, thus introducing the virus to new places. You have heard of USDA regulations/restrictions on different crops, from certain foreign countries to avoid introduction of infected seeds/plant materials.
Seed transmission can occur simply by contamination of seeds, as in tomato seeds by Tobacco mosaic virus. This can be readily inactivated by seed treatments.
The second type of seed transmission occurs when the virus is present in the embryo tissue that can happen prior to fertilization or takes place at pollination. Pea seed-borne mosaic virus is a well studied plant virus in this category.
Pollen Transmisison: Some viruses are transmitted from plant to plant via pollen. As in seed transmission, pollen transmission has two mechanisms, gametic infection of embryo and direct infection of mother plant.
Vegetative propagation: An important horticultural practice, and unfortunately a very effective method for perpetuating and spreading viruses. In clonally propagated plants, an infected mother plant which could be asymptomatic could be used to make hundreds and thousands of daughter plants, which will all have the virus. Any vegetative parts such as bulbs, corms, runners, and cutting will be infected.
Grafting: Essentially a form of vegetative propagation, once the organic union has been established and plants (Scion and Stock) function as a single plant. In experimental front, grafting is used as a virus transmission methods, when all other methods fail.
Controlling Fusarium Wilt of Lettuce Using Steam Heat – Trial Initiated
Earlier this week, we initiated a trial examining the use of band steam for controlling Fusarium wilt of lettuce. The premise behind this research is to use steam heat to raise soil temperatures to levels sufficient to kill soilborne pathogens. For Fusarium oxysporum f. sp. lactucae, the pathogen which causes Fusarium wilt of lettuce, the required temperature for control is generally taken to be > 140°F for 20 minutes. Soil solarization, where clear plastic is placed over the crop bed during the summer, exploits this concept. The technique raises soil surface temperatures to 150-155˚F, effectively killing the pathogen and reducing disease incidence by 45-98% (Matheron and Porchas, 2010).
In our trials, we are using steam heat to raise soil temperatures. Steam is delivered by a 35 BHP steam generator mounted on a custom designed elongated bed shaper (Fig. 1). Preliminary results were encouraging. The device was able to increase the temperature of the top 3” of soil to over 180°F at a travel speed of 0.5 mph as shown in this video of the machine in action (shown below). These temperatures exceed that of those known to control pathogens responsible for causing Fusarium wilt of lettuce (> 140°F for 20 minutes).
Stay tuned for final trial results and reports on the efficacy of using steam heat to control Fusarium wilt of lettuce.
If you are interested in evaluating the technique on your farm, please contact me. We are seeking additional sites with a known history of Fusarium wilt of lettuce disease incidence to test the efficacy and performance of the device.
References
Matheron, M. E., & Porchas, M. 2010. Evaluation of soil solarization and flooding as management tools for Fusarium wilt of lettuce. Plant Dis. 94:1323-1328.
Acknowledgements
This project is sponsored by USDA-NIFA, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council. We greatly appreciate their support.
A special thank you is extended to Cory Mellon and Mellon Farms for allowing us to conduct this research on their farm.
Weeds are one of the most visible of all agricultural pests. They can’t move or hide and once established often stick up over the crop. Just one weed in a 10 acre field is annoying to look at. With insects and diseases, the damage is often more visible than the pest. That is not the case with weeds. A moderate weed infestation is approximately 10 weeds per square foot. If a herbicide produces 90% control, that leaves 1 weed per square foot or 43 weeds per acre. Without an untreated check, this can look like the herbicide failed! It is easy to leave an untreated spot in a field and it is well worth doing. Many applicators do so unintentionally because of skips, powerlines and other causes. They help determine crop injury and weed control. Here are some examples of what various levels of control looked like from one of our cole crop trials:
Corn earworm:
CEW moth counts remain low across all locations; average for this time of the season.
Beet armyworm:
Trap counts decreased in all locations, and a little below average for late-January.
Cabbage looper:
Cabbage looper trap counts remained low in all locations; below average for January.
Diamondback moth:
Adult activity decreased across all locations, except the North Gila Valley where trap is adjacent to with nearby brassica seed crops. Overall, activity is a little below for this time of year.
Whitefly:
Adult movement remained low in all locations consistent with previous seasons.
Thrips:
Thrips adult movement beginning to increase slightly in most locations last week but increased sharply in Roll. Activity about average for January.
Aphids:
Aphid movement low in most locations, increased slightly in N. Yuma and E. Gila Valleys. Trap captures slightly below average for this time of season.
Leafminers:
Adult activity increased in many areas, particularly in the Yuma Valley; above average for this time of season.