Lygus bug (Lygus herperus) (Figure 1), usually a sporadic pest on vegetable crops, is now a major pest of several vegetables this year especially on celery, lettuce, and radicchio in the Salinas Valley. 

On celery, the feeding injury appears as lesions toward the base of the mature stock and young foliage in the center (Figure 2-3). On lettuce and radicchio, lygus bug feeding injury appears toward the bottom mid-rib area of the leaf (Figures 4-8). 

Lygus bugs are highly mobile meaning they can move from field to field until they find a resourceful food source. Perhaps, lettuce and celery are not the nutrient rich diet to lygus bugs but these crops could provide much needed moisture and refuge. High populations of lygus bug could develop on weed hosts in the unmanaged areas such as ditches, side of the roadways etc. Also, alfalfa or beans could serve as hosts. When weed plants dry out or mowed, lygus bug adults tend to leave those hosts seeking food and water elsewhere and even seek temporary refuge in the lettuce or celery fields. Those female lygus bugs settled in the vegetables not only cause feeding injury but also lay eggs. A lygus bug female can lay on average 150 eggs for its life time in an ideal laboratory conditions. This suggests that a few lygus bug females settled in lettuce or celery can develop into colonies. Any disturbance to the favorable hosts such as mowing the weeds in the unmanaged areas or mass cutting of alfalfa could trigger lygus bug adult movement. It is important that alfalfa growers pay attention when they cut the alfalfa crop. Sequential or staggered alfalfa cutting is advisable because any area wide cutting will disturb them and will cause lygus bug adults flee the alfalfa field. Maintaining the alfalfa crop succulent with adequate water and fertilizer is advisable.     

Management of lygus bug involves repeated use of insecticides particularly pyrethroid insecticides and Lannate (methomyl). Among those registered insecticides on celery such as Vydate (oxamyl) and Malathion have comparatively longer pre-harvest intervals (PHIs) than pyrethroid insecticides; thus, they are used during the early phase of the plant development. Among the pyrethroid insecticides registered on vegetables, Mustang (zeta-cypermethrin), permethrins and lambda-cyhalothrin (Warrior II) are widely used. Growers restrain from using Mustang because of maximum residue level (MRLs) restrictions imposed by certain export markets. There are several generics of pyrethroid insecticides available in the market. Repeated use or exposing pyrethroid insecticides to same generation of lygus bugs may lead to insecticide resistant lygus bug populations. Lack of insecticide coverage could also result in ineffective spray results. As indicated earlier, lygus bug adults are highly mobile and they could move as the spray equipment approaches. If nymphs exist in the field, they could easily hide in the crown areas of the plant. This suggests that there could be several reasons behind inadequate control of lygus bug.  Monitoring the field is critical to reduce the establishment of lygus bug colony in the field for timely management. 

Figure 1. Lygus bug adult on Lettuce

Figure 2. Lygus bug feeding on older celery stock

Figure 3. Lygus bug feeding on the young celery growth

Figure 4. Lygus bug feeding along the mid-rib areas of lettuce leaves

Figure 5. Lygus bugs settle in base of the lettuce plant

Figure 6. Lygus bug feeding injury on lettuce

Figure 7. Lygus bug injury on radicchio1

Figure 8. Lygus bug injury on radicchio2

As summer continues to heat up, keep in mind that regulations remain in effect to reduce the volatile organic compounds (VOCs) that can be emitted into the atmosphere by pesticides and other harmful chemicals and contribute to the amount of ozone or smog in the environment.  

Calculators from the Department of Pesticide Regulation (DPR) that determine the VOC emissions from fumigant and non-fumigant pesticides before application are available to help growers, pest control advisers, and pesticide applicators comply with the regulations.  The UC Statewide Integrated Pest Management (IPM) Program provides a link to these calculators from each of the treatment tables in the UC Pest Management Guidelines.  Click on the Air Quality – Calculate emissions button.

Take steps to reduce VOCs.  Avoid emulsifiable concentrate (EC) formulations as they release the highest VOC emissions.  Pesticide control advisers and growers can also reduce VOC emissions by employing IPM practices such as using resistant varieties, traps, exclusion, and biological control.  When using pesticides, spot-treat and seek low-emission materials.  Solid formulations, such as granules or powders, are best.

Check the fact sheet on the DPR web site for the most up-to-date-information on VOC restrictions and regulations.

Among several species of thrips that invade vegetable crops, western flower thrips [Frankliniella occidentalis (Fig. 1)] is the most destructive species of thrips in the Salinas Valley. They can cause severe feeding injury to all stages of plant development. Early feeding can cause severe stunting or reduce plant development whereas; late feeding can cause visible feeding patches - affecting marketable yield in both the instances. Severe feeding injury is usually associated with very high populations of thrips on the crop. It is likely that recent early surge in thrips populations in the Salinas Valley is related to warmer (high day temperatures) and dry winter. 

Thrips is a tiny insect (less than 3 millimeters) and prefers to stay and feed within tight protected areas of the plant such as, between veins or near mid-rib or within the layers or stacks of lettuce leaves or celery stems. Thus, thrips injury was detected in those tight areas of the plant. It is likely that colonizing at those tight areas provide protection from insecticide sprays.

Thrips has a “piercing-sucking” or “punch and suck” mouthpart. Mouthpart of thrips is referred as mouthcone (Fig. 2). Thrips typically feeds using two structures of its mouthcone: (1) a mandible and (2) stylets. As illustrated in the Figure 2, thrips uses the mandible to pierce or punch the plant cell wall and stylets (or needles), which often form a single tube, sucks the liquid food from the plant cell. This feeding apparatus allows thrips to feed on liquid food on a surface or within a plant cell.

As indicated, thrips can cause significant crop loss once its population increases to very high levels. Thrips injury on lettuce may appear as brown streaks, or scarring on the leaves (Fig. 3). If examined closely using magnifying glass, it appears like punctured plant cells and the content removed (Figs. 4 and 5). On celery, the thrips feeding injury is similar but the injured cells appear as raised ridges (Fig. 6). When attacked at the younger stages of the plant development, for e.g., on the growing tips of the cotyledons (Fig. 7), the feeding could deform the true leaves that develop later (Fig. 8).    

In addition to feeding injury, western flower thrips are able of transmiting plants viruses (tospoviruses) such as Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV). In the family: Thripidae, there are 1710 species of thrips but only 14 thrips species are currently reported to transmit tospoviruses.  Both larval and adult stages of thrips vectors actively feed on the host plants but only early larval instars can acquire tospoviruses and later instar larvae and adults can transmit tospoviruses after a latent period. Adult thrips can acquire tospoviruses, but they do not transmit them because virus could not multiply to sufficient numbers. Also, tospoviruses are not transmitted when the thrips lay eggs into the plant.  Thus, each new generation of thrips vectors must acquire the virus as larvae. The weed plants outside the field can be the reservoir for tospoviruses and when the larvae feed on them, they acquire the virus. In the field, larvae feeding on the tospoviruses-infected lettuce plants may also aid virus acquisition. The thrips carrying the virus just need to feed for 10-15 minutes to transmit the virus to uninfected plants.

Typically, bean-shaped eggs are inserted by female western flower thrips into the leaf. Within 5 days, eggs hatch to first instars. If the temperature stays at 86ºF, first instars molt into second instars. This can happen in couple of days in the Salinas Valley. Second instars develop into prepupae within 4-5 days.  Most of the prepupae drop to the soil and emerge into adults within 3 days at 86ºF. Pupal stage is the only non-destructive stage of the thrips. Clearly, thrips development is associated to temperature. Adult females lay about 50 eggs and can live up to 4-5 weeks at 86ºF. So, in Salinas Valley due to milder temperature range, western flower thrips may live longer than 5 weeks. Western flower thrips requires a minimum 194 degree days (DD) (min. temp. 49.5^oF) to complete a generation, but has been estimated to be as high as 254 DD with a minimum temperature of 43.7^oF. Western flower thrips can lay eggs with and without mating. The mated female thrips (fertilized) tend to produce more female offsprings than males whereas, unmated female thrips tend to produce more male offsprings than females.

Thrips are weak flyers but they have fringed wings which help them to get airborne and glide short and long distances. Thrips can stay airborne for about 24 hours in the cooler conditions and can remain without feeding and drinking. They get quickly desiccated if they stay longer in the air. The dispersal of thrips is largely depending on temperature, light, and wind.

To prevent direct feeding injury and viral transmission, it is important that we manage thrips on the crops using the tools such as targeted insecticide sprays. Recent insecticide efficacy studies against western flower thripssuggested that insecticides such as Radiant, Entrust, Lannate, Exirel and Beleaf have decent activity against western flower thrips. Other products, Gladiator and Torac are effective but are not registered for use. Please read the Monterey County crop note (May edition) for details on insecticide efficacy trials. It is important that the growers restrain from repeated use of insecticides within same IRAC class (http://www.irac-online.org/documents/moa-classification/?ext=pdf) in a given season instead rotate insecticides with distinctly different modes of action to reduce development of resistance.

Fig. 1. Adult western flower thrips (Photo: D. Riley).

Fig. 2: Mouthpart of thrips (mouthcone) showing structures, a mandible and maxillary stylets used while feeding on plants.

Fig. 3. Thrips feeding injury on lettuce.

Fig. 4: Thrips feeding injury on lettuce (close-up).

Fig. 5: Thrips feeding injury on lettuce (even closer).

Fig. 6: Thrips feeding injury on celery.

Fig. 7: Thrips feeding injury on young seeding of Swiss Chard.

Typically, spinach crops in the Salinas Valley and other coastal regions rarely are infected with viruses. However, that changed this spring 2015 due to a number of virus disease outbreaks on coastal spinach. Two viruses, Impatiens necrotic spot virus and Cucumber mosaic virus,have been confirmed in a number of spinach plantings. Symptoms caused by the two viruses are similar and cannot readily be differentiated in the field. All such reports have occurred on the larger, older plantings destined for processing (freezer spinach). To date no virus diseases have been reported yet on baby leaf or other fresh market spinach. This development on spinach is consistent with virus outbreaks on lettuce and other leafy vegetables grown on the coast (see Blog entry for April 9).

1. Impatiens necrotic spot virus (INSV): Diseased spinach plants are severely stunted and short. Leaves are at first yellow and later can become tan to whitish in color. Leaf texture is leathery and the foliage appears crinkled and deformed (Photos 1 and 2 below). Symptoms can be seen on both the older, outer leaves as well as on the newer leaves near the center of the plant growing point. The central growing point of the plant is severely affected. INSV is vectored by thrips.

Photo 1. INSV on spinach

Photo 2. INSV on spinach

2. Cucumber mosaic virus (CMV): On spinach, CMV also causes significant plant stunting, poor growth, and yellow to tan foliage. Leaves are leathery in texture and are crinkled and deformed (Photos 3 and 4 below). Symptoms occur on both the older, outer leaves as well as on the newer leaves near the center of the plant growing point. The central growing point of the plant likewise is severely damaged. CMV is vectored by aphids.

Photo 3. CMV on spinach

Photo 4. CMV on spinach (healthy leaf on right)

The dry spring is contributing to early drying up and senescing of weeds and hillside vegetation; this early decline of surrounding vegetation could be driving thrips and aphid populations into fields earlier than normal. In addition, growers and PCAs reported moderate to high thrips and aphid pressure in the early spring. This abundance of vectors certainly accounts for these early outbreaks of virus disease.

Plants suspected of being infected with viruses can be sent for analysis to the UC Cooperative Extension diagnostic laboratory in Salinas.