With head lettuce harvest approaching quickly, it would be wise to keep a look out for corn earworm (CEW). These sneaky worms have caused problems for PCAs and growers on the early fall head lettuce crops in the past few years. Last fall and spring, CEW abundance was lighter than it had been in the previous 2 years, but don’t let that fool you. Currently, CEW pheromone trap catches this fall have been much lighter than last fall or spring (see Trap Network) , one never knows what to expect in the field. Nonetheless, CEW can be very damaging in early fall head lettuce crops where once head formation begins larvae will usually bore into the head almost immediately upon hatching. Corn earworm is much more likely to bore into lettuce heads than other Lepidoptera larvae, rendering the heads unmarketable. Larvae may enter the head from any point on the plants, but can often be found burrowing in from the top half of the head. If fields are not watched closely, infestations may not be noticed until the head is harvested. Once inside the head, it is virtually impossible to control the larvae with insecticides. Thus, pay careful attention for newly oviposited eggs (laid singly) on lettuce plants. If you are beginning to find eggs and suspect that CEW are active in the field when plants are beginning to head or cup over, you should treat as soon as possible. Moreover, during November it is probably a good idea to prophylactically apply a pyrethroid, or methomyl when heads begin to form. The UA nominal threshold for CEW in head lettuce from early heading to harvest is 1-2 larvae / 100 plants. Repeated insecticide treatments may be required to maintain low population levels if heavy pressure is sustained near harvest. Most contact insecticides recommended for Lep larvae are active against CEW. Lab bioassays have shown that CEW larval mortality is most rapid when exposed to Lannate, 0.5 lb (>90% mortality in 1 hr after exposure) and pyrethroids, high rates (>90% mortality in 3 hrs), followed by Radiant, 5 oz and Coragen, 5 oz (>90% mortality in 6 hrs). By 24 hrs, mortality was 100% for all the treatments. For more information on CEW management and control recommendations see Corn Earworm Management on Desert Produce and the 2017 Lep Control Chart.

In response to the recent outbreaks of Diamondback moth (DBM) , Plutella xylostella in Yuma, we have established a pheromone trap network designed to monitor the activity and movement of adult populations of DBM. PCAs have had difficulty controlling DBM in cabbage, broccoli and cauliflower since October. Traps have been placed in Roll, Wellton, Dome Valley, Gila Valley and Yuma Valley in locations where cole crops are presently being grown or in areas where infestations were known to occur this fall.

Click here to see results of DBM pheromone trap network.

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.

When rain is a possibility, people begin to ask how much time they need before or after foliar applied herbicides are applied. In this area, herbicides are more likely to be washed off with irrigation water then they are by rain, but the same principles apply. As always, there is no universal answer and it will depend upon many factors. Almost all are rainfast in from 1 to 4 hours. Only 15 minutes is needed for some and others require eight hours. 2 to 4 hours is a good rule of thumb. Some of the most important factors affecting how fast the herbicide gets into the foliage are mode of action, leaf surface characteristics, surfactant use, environmental conditions and age of the plant. Plants that are small and actively growing absorb herbicides faster than those that are larger and more mature. Contact herbicides do not have to be translocated through the plant, work quicker and are not as sensitive to growing conditions. Water-based formulations are more easily washed off than are oil-based products. Herbicides need more time to get into plants that have a thick waxy cuticle or leave hairs. The addition of a surfactant that helps the herbicide spread and stick to the leaves reduces the time needed before rain or irrigation. All of these factors interact with each other and can affect how much time is needed.

When rain is a possibility, people begin to ask how much time they need before or after foliar applied herbicides are applied. In this area, herbicides are more likely to be washed off with irrigation water then they are by rain, but the same principles apply. As always, there is no universal answer and it will depend upon many factors. Almost all are rainfast in from 1 to 4 hours. Only 15 minutes is needed for some and others require eight hours. 2 to 4 hours is a good rule of thumb. Some of the most important factors affecting how fast the herbicide gets into the foliage are mode of action, leaf surface characteristics, surfactant use, environmental conditions and age of the plant. Plants that are small and actively growing absorb herbicides faster than those that are larger and more mature.

Results of pheromone and sticky trap catches can be viewed here.
Area wide Insect Trapping Network   VegIPM Update, Vol. 15, No. 15, July 24, 2024

Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:             
CEW moth counts increased in Roll and Yuma Valley; about average to previous years.
 
Beet armyworm:          
BAW moth counts increase in Tacna and Wellton; comparable to previous summers.
 
Cabbage looper::          
Cabbage looper numbers are low consistent with previous years and higher temperatures.
 
Diamondback moth:      
DBM moths were not caught in any trap; average for this time of the year.
 
Whitefly:                      
Adult movement continues to increase consistent with disking of late melon fields and volunteers.
 
Thrips:                          
Thrips adult activity continues to decline, comparable to mid-summer.
 
Aphids:                        
Aphid movement is absent and consistent with what we typically see during the summer.
 
Leafminers:                  
Adult activity about average for early July.