As is typical this time of the year, the weather has finally broken and it feels like “winter” has arrived in the desert. With cooler temperatures, you can expect a steady decline in insect abundance until temperatures begin to warm up again. Last fall, I felt that the insect pressure on produce and melons crops in the Yuma was as heavy as I’d seen in many years. This year it didn’t seem quite as heavy, but I’ve heard many PCAs complain of heavy insect pressure, particularly during November which was unusually warm. A quick look at both recent and historic data on pest abundance recorded from our research plots here in the Yuma area suggests that insect pressure this fall was a little lighter than last year, but still heavier as compared to previous years. First, whitefly adult numbers on fall melons and produce were extremely high in August, but monsoon rainfall in early September appeared to suppress their numbers for a couple of weeks thereafter. Much like last fall however, untreated melons plots on the Ag Center wilted and died rapidly as a direct result of heavy whitefly infestations. Sticky trap captures of whiteflies near cantaloupe fields from Wellton to Texas Hill, were lower than last fall, but were still higher than the previous four years. However, numbers of adults caught on traps in the Roll/Tacna area were higher in late-August this fall than we’ve ever recorded. Similarly, CYSDV incidence at harvest in cantaloupe fields in these areas was very high this fall. In all cases, high traps catches were found in melon fields in near proximity to cotton fields. Based on counts from untreated lettuce at the Yuma Ag Center, beet armyworm and cabbage looper population abundance this fall was slightly lower than last fall. However, worm pressure the past two years was higher than we’ve seen the previous 5 years. Populations began infesting plots in early September, and remained steady throughout October and into early November. Egg deposition and larval development has only recently declined significantly. Corn earworm numbers were lower than what we observed last year, but could still easily be found in untreated plots. Finally, Bagrada bug infestations were abundant in untreated broccoli plots at YAC for the 4th consecutive year. As expected, low-moderate population appeared in early September, but increased steadily through September and peaking in early October. Overall, the peak numbers observed were not as high as either 2010 or 2012, but much higher than in 2011. The infestation levels in our untreated broccoli plots this year remained at damaging levels throughout October and into mid-November. This is the first year we have seen Bagrada bugs this abundant in November. In contrast, similar to last fall, western flower thrips population numbers have been relatively low, but unlike last fall, we’ve been picking up winged green peach aphids and colonies on sticky traps and in untreated lettuce plots since early November. How these aphid and thrips numbers will translate into potential population pressure in January and February is unknown, but you should anticipate their abundance as usual. I’m often asked why we see such differences in insect pest numbers each year. It’s obviously a very complex question, and I don’t have a good answer. The bottom line; insect abundance and outbreaks are dictated by many abiotic, biotic and crop management factors within our cropping system. Moreover, it’s nearly impossible to consider all the factors necessary to draw a reliable conclusion. Nonetheless, graphics showing these recent trends in Whitefly, CYSDV, Lep Larvae and Bagrada abundance can be found at Pest Abundance on Desert Produce and Melon Cops in 2013.
We are on the final section of virus transmission. Virus transmission by insects is one of the most efficient and economically important transmission in agriculture. When you have insects in your crops, not only you are losing your crops because of feeding/chewing by insects, a lot of insects also act as a vector of plant viruses.
Seven out of 29 orders of insect feeding on living green land plants are vectors of plant viruses.
Insect transmit viruses in 4 distinct modes:
Non persistent transmission: The insects can acquire the virus in a matter if seconds/minutes and they are immediately viruliferous. The virus in retained in the stylet of the insect and are transmitted to the next plant the insect feeds on. The virus is retained in the vector only for few minutes and is lost after insect molting. Most viruses transmitted by aphids are non persistent. So when you see few aphids in your melon field and see cucumber mosaic virus symptoms 1-2 weeks later in your field, don’t be surprised. Aphids are efficient vectors, and since viruses are systemic it takes anywhere from few days to 2-3 weeks for the plants to show symptoms. Thus it is very important to manage insects in the field even if you don’t think the ‘pressure’ is not as high.
Semi-persistent transmission: The insects can acquire the virus in minutes/hours and there is no latent (incubation) period in the insect. The virus can stay in the insects foregut for hours and is lost after insect molting. Some species of aphids and whiteflies fall in this category. Example: Cucurbit yellow stunting disorder virus in melons transmitted by whiteflies.
Persistent circulative: Insects have to feed on virus infected plants for hours/days to acquire the virus and the virus has to incubate for hours/days in the insect. After insect can transmit the virus for weeks. Virus can be present in the vectors hemolymph but there is no multiplication of virus in the insect body. Vectors in this transmission includes: Aphids, leafhopper, whiteflies, treehopper.
Example: Beet curly top virus transmission by beet leafhopper
Persistent propagative: Insects have to feed on virus infected plants for hours/days to acquire the virus and the virus has to incubate for hours/days in the insect. After insect can transmit the virus throughout its lifespan. The virus can multiply in the vector system and often times the virus particles are also passed on to the insect offspring. Tomato spotted wilt virus is transmitted on persistent propagative manner by 9 different species on thrips.
Save the Date : 2024 Plant Pathology Workshop
When: August 29th 8AM-12 PM ( breakfast and Lunch provided by Gowan Company and BASF)
Where: Yuma Ag Center, 6425 W 8th Street
What will covered: Plant Pathology program Updates, past season field trial results (we
have some exciting results to share), Q&A to help better Plant pathology program,
Industry panel discussion for all your industry related questions! See you in few weeks!
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.
Corn earworm:
CEW moth counts increased in the past two weeks particularly in Wellton and Roll.
Beet armyworm:
Trap counts beginning to increase in most locations especially in dome Valley and the south Yuma Valley.
Cabbage looper:
Cabbage looper increased in all locations particularly in Wellton, Dome and N. Yuma Valleys consistent with reports of eggs in the in the field.
Whitefly:
Adult movement increasing in all locations and particularly in Dome/Wellton; about average for this time of year.
Thrips:
Thrips adult movement remains about average for early September.
Aphids:
Aphid movement has been absent since early June typical with high summer temperatures, and the monsoon flow.
Leafminers:
Adult activity remains similarly low, above average for this time of season.
Diamondback moth:
First record of DBM adults since early June. Are appearing sooner than last season, likely from high altitude winds associated with Tropical Storm Nora experienced last week.
