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.
Widely accepted definition of a living organism “A living organism has a cellular structure and is manifest by growth through metabolism, reproduction, and the power of adaptation to the environment through changes that originate internally”. Viruses are not cellular and do not metabolise, but they reproduce and adapt.
A virus is a set of one or more nucleic acid template molecules, normally incased in a protective coats of protein or lipoprotein and is able to organize its own replication but only within a suitable host cells. Record of plant viruses do not go as far as human viruses, but plant viruses have caused considerable loss in agriculture system.
One of the most common virus we see in agriculture system in todays world is Cucumber mosaic virus(CMV). CMV belongs to family Bromoviridae. The genome size of cucumber mosaic virus (see pic) is about 8000 to 9000 nucletotide bases (1 base=1 letter of AGTC). The genome size of Covid19 Coronivirus is about 30,000 bases and the genome size of human DNA is 6.4 billion bases.
CMV has a very wide host range and is transmitted by aphids in nonpersistent manner (stylet borne). This means that the aphids acquire the virus particle in their stylet within seconds of feeding in infected plants, hop on to next plant and start feeding on next plant. The virus is transmitted to the next plant immediately.
Next is incubation period. Viruses cause systemic infection. It can take anywhere from few days to few weeks from initial entry of the virus to symptom exhibition in your plants. The severity of symptoms varies depending on many factors. The age of plant (infection stage), the general plant vigor (health), varietal susceptibility, conducive environment (viruses express better in colder weather than hot weather), a plant that has already been infected with other viruses (preesisting condition) are to name a few.
Attachment – the virus attaches itself to the outside of a new plant cell
Penetration – the protein pushes the nucleic acid strand into the plant cell
Replication – the viruses’ nucleic acid uses the plant cell DNA to make many new nucleic acid strands and protein sheathes
Assembly – the nucleic acid and protein assembly into millions of new virus copies
Release – the viruses leave the cell – at this stage the cell is normally dead and bursts releasing the viruses
Transmission – the viruses move using a vector to new cells to infect.
When you see the symptoms in your plants, the first thing you have to understand is virus infection is systemic. The best you can do to manage the virus is to limit the transmission (flatten the curve). Some viruses need a vector for transmission like insects and nematodes. Some viruses are mechanically transmitted from one infected plant to another. Washing field tools between plants/field whenever possible limits the transmission of virus. Soap, bleach, and disinfectants reduce transmission by protein denaturalization of the virus.
Controlling Disease and Weeds with Band-Steam – Yuma Trials Show Good Promise
In previous articles (Vol. 11 (13), Vol. 11 (20), Vol. 11(24)), I’ve discussed using band-steam to control plant diseases and weeds. Band-steaming is where steam is used to heat narrow strips of soil to temperature levels sufficient to kill soilborne pathogens and weed seed (>140 °F for > 20 minutes). The concept is showing good promise. This past season, three trials were conducted examining the efficacy of using steam for disease and weed control in Yuma, AZ. In the studies, steam was applied in a 4-inch-wide by 2-inch-deep band of soil centered on the seedline using a prototype band-steam applicator (Fig.1). The band-steam applicator is principally comprised of a 35 BHP steam generator mounted on top of an elongated bed shaper. The apparatus applies steam via shank injection and from cone shaped ports on top of the bed shaper.
Trial results were very encouraging as the prototype applicator was able to raise soil temperatures to target levels (140°F for >20 minutes) at viable travels speeds of 0.75 mph. Steam provided better than 80% weed control and significantly lowered hand weeding time by more than 2 hours per acre (Table 1). Results also showed that Fusarium colony forming units (CFU) were reduced from 2,600 in the control to 155 in the 0.75 mph and 53 in the 0.5 mph treatments, respectively (a more than 15-fold reduction). A significant difference in Fusarium wilt of lettuce disease incidence was not found, however disease infection at the field site was low (< 2%) and differences were not expected. At 0.5 mph, fuel costs were calculated to be $238/acre which was considered reasonable and consistent with the values reported by Fennimore et al. (2014).
An unexpected finding was that plants in steam treated plots appeared to be healthier and more vigorous than untreated plots (Fig. 2). This trial is still in progress and it will be interesting to see if this improved early growth translates into increases in crop yield.
In summary, early trial results are showing good promise for use of band-steam as a non-herbicidal method of pest control. We plan on conducting further trials in this multi-year study. If you are interested in evaluating the device on your farm and being part of the study please contact me. We are particularly interested in fields with a known history of Fusarium wilt of lettuce and/or Sclerotinia lettuce drop that will be planted to iceberg or romaine lettuce.
As always, if you are interested in seeing the machine operate or would like more information, please feel free to contact me.
Acknowledgements
This work is supported by Crop Protection and Pest Management grant no. 2017-70006-27273/project accession no. 1014065 from the USDA National Institute of Food and Agriculture, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council. We greatly appreciate their support. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
A special thank you is extended to Mellon Farms for allowing us to conduct this research on their farm.
References
Fennimore, S.A., Martin, F.N., Miller, T.C., Broome, J.C., Dorn, N. and Greene, I. 2014. Evaluation of a mobile steam applicator for soil disinfestation in California strawberry. HortScience 49(12):1542-1549.
Click link below or picture to see the band-steam and co-product applicator in action!
Carryover of Vegetable Herbicides to Wheat Grown in Rotation
Almost all the herbicides used on lettuce, cole crops and melons have restrictions on how soon wheat can be planted in rotation after they have been used. Experience has demonstrated, however, that safe intervals can vary considerably based upon many factors and are almost always much longer than they need to be. The most important factors are rate applied, irrigation practices and tillage. For example, when Kerb used to be banded at 2 to 4 lbs. per acre after planting and incorporated with furrow irrigation, it was common to see treated strips across wheat fields which followed. This is uncommon now that lower rates are Chemigated. We still see some Balan injury at ends of fields or in overlaps especially when sudan is planted. Wheat it not very sensitive to Prefar and carryover injury is uncommon.