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Tunyalee Martin, tlamartin@ucanr.edu

Jodi Azulai, jlazulai@ucanr.edu

Romy Basler, rbasler@ucanr.edu

www.ipm.ucdavis.edu

During this time of year, there are various weather issues that affect lettuce. Frost damage can often occur during the cold, clear nights in the winter and spring. Interestingly, frost damage on lettuce causes some classic symptoms that are easily recognizable and some that are a bit surprising. Different types of symptoms can be caused due to differences in the severity and duration of low temperatures, as well as the age and location of the leaf tissue. One of the more common symptoms of frost damage on lettuce is when the outer leaf cuticle separates from the underlying cell tissue; this can result in a bronzed look to the leaf, which is probably due to the death or damage of the surface epidermal cells (Photo 1). More severe frost conditions may cause necrotic interveinal lesions as well as necrotic spotting on the leaves (Photos 2 and 3); such symptoms could resemble those caused by heat damage or pesticide burn. Occasionally the tips of young leaves are killed, and as the leaf continues to grow, the leaf may curl around this dead area (Photo 4).  Sublethal frost damage tends to occur towards the edges of the leaves and causes thickening and roughening of the texture of the leaf tissue (Photos 5 and 6). Lettuce is mostly tolerant of temperatures near freezing for a short period of time, though plants will grow more slowly. Freeze damaged leaves that are not removed from heads may break down, decay, and cause post harvest issues.

Growers and others in the field should be aware of how very low temperatures can also damage other crops, such as cauliflower transplants (Photo 7), that are present during this time of year.

Photo 1. Bronze appearance of the surface of the lettuce leaf from light frost damage

Photos 2 and 3. Severe frost damage can cause necrotic interveinal areas as well as spotting on the leaf

Photo 4. Tip of the leaf killed by frost, but the tip of the leaf continued to grow around the affected area

Photos 5 and 6. Thickening, deformity and roughening of leaf texture

Photo 7. Cold temperature damage on leaf of a cauliflower transplant

My name is Shimat Joseph and I’m the new Entomology Advisor for Monterey County. Recent years, there have been reports from growers and pest control advisors regarding maggot problem on lettuce. I’m planning to conduct a survey to determine maggot species and their biology on lettuce. For this survey, I’m looking for suitable lettuce fields that meet the following criteria:

1. A history of maggot problem.
2. Have recently been planted (but seeds haven’t sprouted yet) or will be planting with lettuce in the near future, so that I could initiate sampling from the beginning.   

If so, please contact me at svjoseph@ucdavis.edu or (831) 759-7359. Thanks!

Bagrada bug, Bagrada hilaris (Fig. 1) is an invasive stink bug native to old world countries of southern Africa, Middle East and Asia. It was first detected in Los Angeles County, California in 2008, now well established in Ventura, Los Angeles, Orange, San Bernardino, Riverside, San Diego and Imperial Counties, but recently detected in Santa Barbara and San Luis Obispo Counties. Also, it is a serious pest and has been detected in 4 more states (Arizona, New Mexico, Nevada and Utah) in the U.S.

ATTN: Recently, bagrada bug adults were found on Chinese or napa cabbage in Santa Cruz County.

  Although this bug feeds on a wide range of hosts, we are more concerned because the bug prefers cruciferous hosts (Family: Brassicaceae) including broccoli and cauliflower, which are grown as rotation crops in the Salinas Valley. It is believed that other major crops especially lettuce and spinach are NOT a suitable host for bagrada bug. At the same time, bagrada bug can survive on cruciferous weeds such as mustard species (Brassica sp), wild radish, London rocket, short pod mustard and shepherd’s purse, as well as the insectary crop sweet asylum. Mustard weeds species are very common in the Salinas Valley along ditches, roadsides and even along the edges of agricultural fields. Other species of mustards such as white mustard (Sinapsis alba) and Indian mustard (Brassica juncea) are grown as cover crops. It is clear that given the abundance of mustard family weeds and crops, there is a readily available source of habitat for this insect in the Salinas Valley.

 Bagrada bug adult is often confused with harlequin bug. Adult of harlequin bug is orange with black and white marks, whereas bagrada bug adult is black with orange and white marks; and adult harlequin bug is about 3 times larger than bagrada bug (Fig. 2). Eggs of harlequin bug are white with horizontal, black strips, whereas bagrada bug has no strips but has a “dirty” white appearance.

   It is believed that bagrada bug overwinters as adult in the cracks and crevices in soil or on plants. Generally, female bug is larger in size than male. Eggs are laid on the underside of leaves, cracks and crevices in soil or on hairy stems. There are five nymphal stages for bagrada bug. Typically, bagrada bug is found in aggregation with various nymphal stages and adult rather than individuals (Fig. 3). Because Salinas Valley has relatively mild temperature through year, it is expected that the development of bagrada bug would be prolonged compared with its populations in the warmer regions where it has been established. This also indicates that, if the bug is established, the number of generations of bagrada bug would be fewer in the Valley than in the warmer locations such as southern California or in the desert regions. Normally, its population size is small during early spring to mid-summer but eventually increases in size during later summer or fall.

Damage to host plant is varied but could be severe. Severe economic loss has been reported when injury occurred during early developmental stages of the crop such as coldyledon or < 4 leaves stages. Injury on leaves appears initially as small puncher marks, which turn into white patches as leaves expand (Fig. 4a). In broccoli and cauliflower, the economic injury occurs when the bug feeding kills the apical meristematic tissue of young seedling, which later result in “multiple heads” (Fig. 4b) or sometimes “blind heads” (without a head). Severe feeding also depletes the nutrient reserves of the plant leading to desiccation or wilting.

  At this point, preventing the dispersal of bagrada bug to the Salinas Valley is the key strategy. Growers often move plant materials including transplants to the Valley for production from the regions where the bug has been established. Special care should be given to inspect the plant materials while moving them. Monitoring for bagrada bug during mid-day hours might increase the probability of finding them as the bugs typically hide and stay in the cracks and crevices or on the underside of leaves when the temperature is on the cooler side. Cruciferous weeds in the drains, river bottoms, edges of the field or near residential area increase the risk of establishment. Based on the insecticide efficacy studies conducted in University of Arizona, pyrethroids and neonicotinoids are effective in reducing bagrada bug infestation and injury. For organic growers, none of the products are efficacious but pyrethrin and azidirachtin are suggested.

  If you detect bagrada bug in Monterey, Santa Cruz and San Benito Counties, please do not hesitate to contact me at svjoseph@ucdavis.edu or (831) 759-7359.

For more reading, please visit the links:

http://cisr.ucr.edu/bagrada_bug.html

http://www.plantmanagementnetwork.org/pub/php/brief/2010/bagrada/

http://ucanr.org/blogs/blogcore/postdetail.cfm?postnum=8438

http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=4047