Diamondback Moth Control and Resistance Management 2016
What started off as a moderate insect season this fall has turned into one of the heaviest worm seasons on produce crops we’ve experienced in many years. Since mid-September, cabbage looper (CL), beet armyworm (BAW) and diamondback moth (DBM) populations have been consistently abundant in various areas throughout Yuma county. For the most part, CL and BAW have been relatively easy to manage with our standard insecticide products. In some cases, however, spray intervals have been shorter than normal due to overlapping egg lays and warm nighttime temperatures driving development of multiple overlapping generations in fields. Given the warm weather this fall and the abundance of alternate hosts for them to develop on all summer, the presence of these heavy infestations is not unusual. However, what is unusual is the emergence of DBM as a fall pest of cole crops. It’s been my experience, that DBM has become a fall pest within the past 7-8 years. Prior to then, I considered DBM a spring pest, occurring in fields once the weather warmed up. Because DBM attacks only cole crops (Cruciferae), they do not survive in any great abundance during the summer due to the lack of host crops. However, it is not unusual for fall transplanted cole crops to arrive in fields with DBM, regardless of the origin of the transplants. This certainly would explain how DBM populations can become so quickly established in September and October. Under ideal temperatures (~85 °F), DBM can complete a generation in a little over two weeks, as opposed to about 3 weeks necessary for CL and BAW. Thus once established, DBM populations can rapidly build up multiple generations in the field. Management of DBM in cole crops often requires intensive insecticidal management, more so than what is generally required to control BAW and CL. In some growing locations (e.g., Florida or Hawaii), protection of cole crops often requires multiple spray applications, sometimes as often as twice per week to break the DBM cycle. Unfortunately, intensive management can lead to insecticide resistance which DBM has a long history of. I’ve become concerned this fall because I’ve had numerous reports from PCAs who have had difficulty in controlling DBM in transplanted cole crops; transplanted cabbage and cauliflower in particular. In some cases, there have been reports of our standard insecticide products not providing adequate efficacy against DBM. It impossible to explain for sure why this poor field performance has occurred as many operational and biological factors (i.e., application timing and frequency, rates, choice of product, weather and resistance) ultimately determine the level of efficacy. Although resistance is unlikely, we will be collecting and bioassaying a few of these populations just to make sure this is not one of these factors. For the short term though, PCAs should remain diligent in their worm control using the standard products (Radiant, Proclaim, Coragen, Exirel, Intepid) at labeled rates and by ground whenever possible. Also it is important that PCAs practice sound insecticide resistance management (IRM) by rotating modes of action following each application. This is particularly important with the Diamide group of insecticides (IRAC group 28) because these products can be applied as both foliar sprays and soil injections and multiple generations of Lep larvae can potentially be exposed to variable doses of this chemistry. The most effective way to delay the onset of resistance by worms in leafy vegetables is to consider the recommendations provided in the guidelines recently updated entitled Insecticide Resistance Management Guidelines for Lepidopterous Larvae in Lettuce.
This study was conducted at the JV farms at Gila Valley. Lettuce variety ‘Guapo’ was seeded, then sprinkler-irrigated to germinate seed on September 19, 2023, on double rows 12 in. apart on beds with 42 in. between bed centers. Rest of the irrigation was supplied by furrow irrigation or rainfall. Treatments were replicated five times in a randomized complete block design. Each replicate plot consisted of 25 ft of bed, which contained two 25 ft rows of lettuce. Plants were thinned on October 9, 2023 at the 3-4 leaf stage to a 12-inch spacing. Treatment beds were separated by single nontreated beds. Treatments were applied by incorporating in soil before seeding or with a tractor-mounted boom sprayer that delivered 50 gal/acre at 100 psi to flat-fan nozzles spaced 12 in apart.
Month
Max
Min
Avg
Rain
September
100
71
86
0.71 in
October
93
61
77
0.00 in
November
80
51
65
0.08 in
December
71
44
57
0.82 in
Fusarium wilt (caused by Fusarium oxysporum f. sp. lactucae ) rating was done in the field by observing the typical symptom of lettuce wilt. Confirmation was done by cutting the cross section of roots. Disease scoring/rating was done on December 6, 2023.
The data in the table illustrate the degree of disease control obtained by application of the various treatments in this trial. The disease pressure was extremely high in 2023, and most treatments showed little or no control against the disease. The treatments that showed some activity were Bexfond, Cevya, Rhyme, and Serifel. Plant vigor was normal and phytotoxicity symptoms were not observed in any treatments in this trial.
Band-Steam Applicator for Controlling Soilborne Pathogens and Weeds in Lettuce
Steam sterilization of soils is commonly used in plant nurseries and greenhouses for effective control of soilborne pathogens and weed seeds. The technique, however, is highly energy intensive as the entire soil profile is heated. This is too costly and slow to be practical for field scale vegetable production. To reduce energy consumption and cost, use of band-steaming, where steam is applied only in the area where it is needed – in the plant root zone, is proposed. In this method, narrow strips of soil centered on the seed line are treated with steam rather than the whole bed.
Over the course of the last year, we developed a prototype band-steam and co-product applicator that is designed to raise soil temperatures in a band 2” deep by 4” wide to levels sufficient to control soilborne pathogens (140 °F for > 20 minutes) and weed seed (150 °F for > 20 minutes). The device is principally comprised of a 35 BHP steam generator and a co-product applicator mounted on top of a bed shaper (Fig.1). The apparatus applies steam via shank injection and from cone shaped ports on top of the bed shaper. An exothermic compound can be co-applied via shank injection and/or a banding spray nozzle. The rationale behind co-applying an exothermic compound with steam is that exothermic compounds react and release heat when combined with water, thereby reducing energy requirements and increasing travel speed.
Preliminary testing of the device this spring in Yuma, AZ were very promising. Trial results showed that application of steam alone effectively raised soil temperature in the center of the seed line to levels required for effective pest control (140 °F for more than 20 minutes). Use of the exothermic compound increased soil temperature by about 10 °F. A video of the device in action can be found at the link provided below.
We are currently evaluating the device in field trials with lettuce in Salinas, CA. Target pests in these experiments conducted in collaboration with Steve Fennimore, UC Davis, are soil pathogens which cause Sclerotinia lettuce drop and in-row weeds. Future articles will report the findings of this research.
This fall, we will be replicating these tests in Yuma, AZ and also investigating the effectiveness of band-steam for controlling Fusarium oxysporum f. sp. lactucae which causes Fusarium wilt of lettuce. Heat has been shown to effectively kill Fusarium oxysporum spores and control Fusarium wilt disease. As an example, soil solarization, where clear plastic is placed over crop beds during the summer, raises soil temperatures to 150-155˚F at the soil surface, effectively killing the pathogen and reducing disease incidence by 45-98% (Matheron and Porchas, 2010).
These projects are sponsored by USDA-NIFA, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council. We greatly appreciate their support.
If you are interested in seeing the machine operate or would like more information, please feel free to contact me.
See the band-steam and co-product applicator in action!
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.
Sprangletop has become increasingly widespread in Arizona mostly because of its growth habits and tolerance to many commonly used herbicides. It is in the Leptochloa genus which is derived from the Greek words leptos (thin) and chloa (grass). There are more than 150 species of sprangletop worldwide but only three in Arizona and two in Yuma County. The two that are the most common in the low desert are Mexican Sprangletop, which is Leptochloa uninervia and Red Sprangletop, Leptochloa filiformis. A third species, Bearded Sprangletop, Leptochloa fascicularis, is more common at higher elevations of 1500 feet or higher. It is not uncommon to find both Red and Mexican Sprangletop in the same field and it is not hard to distinguish them when they are side by side. Red Sprangletop has a light green leaf blade which is similar in width to watergrass and barnyardgrass. It has very fine hairs and very small and fine branches and spiklets. It also has a long membranous ligule. The name Red refers to the leaf sheath, which is characteristically red, rather than the seed head. Mexican Sprangletop has a thinner leaf blade which is darker green or grayish in color and similar in appearance to common bermudagrass. The seed head is distinctly coarser than that of Red Sprangletop. Side by side, leaf color and size of the seed make it easy to distinguish these two. Both of these grasses are classified as summer annuals, but they grow more like perennials in the low desert. Sprangletop does very well in the hottest part of the summer and typically germinates from seed during the hottest period between July and September. Once established, however, it often survives through the cold winter months. It grows into clumps that often appear to be dead during the winter. New shoots commonly grow from these established crowns the next season. When this occurs, preemergent herbicides such as Trifluralin or Prowl are ineffective. Some Sprangletop plants stay green and grow through the winter. Many of the postemergence, grass specific herbicides that control many grasses are ineffective on Sprangletop. This also has contributed to the spread of these weeds. Sethoxydim (Poast) and Fluazifop (Fusilade) do not control either Red or Mexican sprangletop. Only Clethodim (Select Max, Select, Arrow and others) is the only one of these grass herbicides that is effective and only at the highest labeled rates. Two applications are often necessary to achieve season long control.
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
