- Author: Shimat Villanassery Joseph
The soil-dwelling springtail (Protaphorura fimata) is a serious pest of lettuce in the Salinas Valley of California. This springtail is ~ 2.5 mm long and white and has no jumping organ (furcula) and eyes. They feed primarily on the radicle of the germinating lettuce and broccoli seeds causing stunted seedling growth, and their high densities at planting is associated with poor lettuce stands. Most springtails are saprophytic feeders considered as beneficial organisms because they aid in the decomposition of decaying plant material, thereby contributing to the cycling of carbon and nitrogen, which in turn improves soil health and structure. These springtails typically known to feed on fungi in the soil.
There is no grower-friendly monitoring method to determine incidence and abundance of springtails in the lettuce and broccoli fields of California's central coast. Garden symphylan another soil-borne arthropod pest that occurs in vegetable fields, is commonly monitored using a potato slice bait. Potato slices are typically deployed on the soil surface, and they attract garden symphylan. The utility of potato slices for captures of springtail has not been investigated. Because potato slices are off-white in color, it might be difficult to quickly quantify lightly-colored arthropods such as springtail and garden symphylan on the potato surface. Beets are dark red in color and may provide background contrast and to help quantify lightly-colored organisms on it.
Recent research suggest that beet and potato attract springtail when placed in lettuce fields and these baits could be used to monitor in commercial field settings. It could only provide the presence or absence information, which might be still important to avoid unnecessary insecticide application with no or low captures on baits. Capture of springtails on the baits suggests that their populations are likely developing in the upper soil profile especially near the root system. Also, the incidence and abundance of springtail is subjected to soil moisture and their captures likely decline sharply as the upper soil layer loses moisture. It is likely that springtail populations multiply as the pre-plant irrigation is initiated. Monitoring the field using baits before or during the initial irrigation may provide an indication of active springtail population in the upper soil profile. Study also show that greater numbers of springtails were collected on beet one day after deployment than extended days of exposure. Perhaps deployment of baits for extended periods of time in soil may cause desiccation and make them less attractive to springtails.
Therefore, beet and potato can attract springtail in the soil and could be used for monitoring springtail.
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- Author: Shimat Villanassery Joseph
The garden symphylan (Scutigerella immaculata) (Figure 1), a white, highly mobile, centipede-like, 1/4 inch long soil arthropod, is a serious soil pest of several high-value crops in the Central Coast of California such as lettuce, strawberry, broccoli, cauliflower, artichoke and celery. The garden symphylan feeds on roots of both direct-seeded and transplanted crops alike causing severe stunting and plant mortality. Besides feeding on the roots, they also survive feeding on organic matter, and other soil dwelling fungi. The garden symphylans use the channels created by other soil organisms such as earthworms for vertical and lateral movement through the soil profile. Their seasonal movement in the soil is also influenced by soil moisture, and temperature. Incidence of garden symphylan infestation is mostly reported in heavier or clay soils with higher organic matter content than lighter or sandy soils. The garden symphylans spend their entire life in the soil and are well adapted to the subterranean habits. They lack eyes but have long antennae and thousands of sensory hairs on the body, which possibly help taste and feel the surroundings.
The garden symphylans are primarily managed using preventative insecticide application, although other tactics such as crop rotation, planting less susceptible crops, flooding the field, reduced tillage, and conservation of beneficial organisms have been suggested. Success and effectiveness of these non-chemical tactics were constrained by several factors such as lack of fit to the current production practices, susceptible crops being grown, varied topography, and enormous population size.
It is important to remember that garden symphylans are very difficult to manage because of their behaviors such as high mobility, and their adaptations to the soil conditions. They can move to deeper soil layers when conditions are not favorable in the upper soil layers (such as high temperature or low water content). Often, garden symphylans aggregate in high densities in certain spots in the field and the damage is concentrated in those spots (Figure 2). Thus, it is very difficult to predict their incidence and plant damage in the field.
One strategy to manage garden symphylans is to determine which insecticides would repel garden symphylans in the soil. This will at least provide some control until the seedling establish in the soil. Studies were conducted to establish relative efficacy of insecticides against garden symphylan based on repellency behavior and how many died. Based on the studies, Belay, Vydate, Mustang, Lorsban, Mocap, Aza-direct Leverage and Torac showed signs of repellency to garden symphylans. Aza-direct is the only organically approved insecticide that elicited repellency. The insecticides, Capture, Vydate, Belay, and Mustang caused 44 to 95% garden symphylan dead. 100% of the garden symphylans were killed when Torac was used. Torac is not registered on any crops at this moment. These studies provide guidelines on efficacy of insecticides against garden symphylans. For further reading please click the link below to access the published article.
- Author: Shimat Villanassery Joseph
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.
- Author: Shimat Villanassery Joseph
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. 49.5oF) to complete a generation, but has been estimated to be as high as 254 DD with a minimum temperature of 43.7oF. 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.
- Author: Shimat Villanassery Joseph
Growers in the Salinas Valley facing an irregular lettuce stand are usually uncertain about what caused the problem and often blame the factors such as poor seed quality, planting error, irregular irrigation timing or distribution, high salt levels in the soil or water, soilborne pathogens of seedlings, bulb mites, and garden symphylan feeding for the losses. Several springtails were collected from the soil associated with lettuce and it is not clear if they were feeding and contributing to the irregular lettuce stand. Often, this springtail is misidentified as garden symphylan.
Recently, I found a large number of a subterranean springtail (Protaphorura fimata) (Fig. 1) in the monitoring potato slice traps deployed in Salinas lettuce fields. I did a series of laboratory and field studies to determine its pest status in lettuce. This springtail (P. fimata) is less than 2.5 mm long, white in color and lacks eyes. Unlike other springtails, this springtail lack a furcula (jumping organ), and when disturbed it does not jump instead curls up. Other similar species of springtails primarily reproduce parthenogenically meaning they reproduce without mating; however, sexual reproduction is also seen on this one (P. fimata). This species (P. fimata) seems to be widely distributed in Europe, but has not been previously reported from the U.S.
Springtails occur in diverse habitats worldwide and are generally considered as beneficial arthropods because they aid in the decomposition of decaying plant material by feeding, thereby contributing to the cycling of carbon and nitrogen which in turn improves soil health and structure. This springtailis primarily known to feed on soil fungi but also feeds on live plant roots. Other springtails in the same family have been associated with feeding damage to germinating sugar beet seeds, sugarcane, poppy seeds and weed seeds (Plantago major). Foliage-feeding springtails (lucerne flea and garden springtail) attack several plants including Lucerne (Medicago sativa), clover (Trifolium sp.), sugar beet (Beta vulgaris) and bean.
The major objectives of the present study were to document the ability of springtail (P. fimata) to injure germinating seeds of lettuce in laboratory and field and characterize the feeding injury of springtail on germinating seeds and seedlings of lettuce.
In the laboratory, I conducted experiments in plastic petri dish with and without soil and sprigtails. Then I recorded the ungerminated lettuce seeds due to feeding injury, total number of feeding injury sites, and number of germinated seedlings with distinct feeding injury. I also documented the location (e.g. leaf, stem, plant crown or root) of the feeding injury on the plants. I conducted a field study too. The assumption of the field study was that the repeated use of maximum label rate of selected insecticides at early stages of plant development would suppress springtails and protect the seeds or seedlings from feeding. Two commonly used pyrethroid insecticides were applied (by the grower) three times: 2 days before planting, at planting, and 20 days after planting. Applications were made using a commercial tractor mounted sprayer. Two pyrethroid insecticides used were Mustang (4 fl oz per acre) and Warrior II (1.6 fl oz per acre). Both the Mustang and Warrior II were tank mixed and applied at 2 days before planting and 20 days after planting but only Warrior II was applied at planting to conform to the label. An adjuvant, Widespread Max (2 fl oz per acre) was added with all the applications. I used bait slices to monitor the springtails at weekly intervals. They were placed in the soil at 1.5 inches deep along the seed line and were covered with disposable white plastic bowls. At the end of each 2 days exposure period, beet root slices were removed, placed into plastic bags and transported to the laboratory. In addition, plant samples were collected to assess the plant growth. Please read the full Journal article for details.
Results demonstrate that this springtail (P. fimata) can feed on germinating lettuce seeds or young seedlings, resulting in reduction in lettuce growth (Figure 2).Springtails attacked seeds and young seedlings alike. In the laboratory, springtails directly fed through the seed coat (pericarp) of a few seeds. This is possibly due to the moistening of the pericarp, enabling springtails to feed through the softened coating (Fig. 3a and b). In some instances, springtails fed on the growing radicle of the germinating seeds (Fig. 3c). However, most of the feeding at the seed radicle or elsewhere did not entirely sever it (Fig. 3d), which allowed the seedling to survive but affected the normal development of the plant. Moreover, most of the feeding injury was evident at the crown area rather than on leaf, stem, or root (Fig. 4).
In the Salinas Valley, before the lettuce seeds are planted, fields are watered deeply and irrigations continue for at least three weeks after planting. I observed that the springtaildensity increased when the field was recently irrigated or after a rain event. This cultural practice which maintains high moisture levels for seed germination on the sub-surface profiles of the soil might be favoring faster buildup of springtailpopulations. In the field trial, the springtailcaptures were greater immediately after irrigation in the untreated beds than insecticide treated beds which was reflected in reduced number of springtails and in the untreated beds (Fig. 5).
In conclusion, this study clearly demonstrates that springtail (P. fimata) is an important pest of lettuce and is capable of reducing the crop stand. Incidence of high populations of springtail could be detrimental to germination of seeds in the field (Fig. 6 and 7). Springtails could be effectively suppressed to a large extent with early applications of insecticides directed to the seed line. Monitoring is the key to determine the presence and population size of springtail. Currently, I'm conducting for laboratory and field studies to determine the efficacy and application timing of insecticides.
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