Managing Lygus Bugs in Strawberries

Transcription

1 Managing Lygus Bugs in Strawberries Production Guideline by Frank Zalom, Jianlong Bi & Pat Thompson Issue 3.1 June 2011 The California Strawberry Commission Production Guidelines are produced in cooperation with scientists who conduct research related to strawberry production. These guidelines are a tool for growers, providing critical scientific background information on diseases and pests common to strawberry production in California. For copies of this guideline or others in the series, visit Production Guideline by: Frank Zalom Department of Entomology University of California, Davis fgzalom@ucdavis.edu p Jianlong Bi Cooperative Extension Monterey County jbi@ucdavis.edu p California Strawberry Commission Lygus bugs are a serious pest in coastal production areas from Camarillo to Watsonville. In central coast fields where strawberry fruit are harvested past May, lygus populations continue to build through the summer months. More recently, lygus have become well established in summerplanted off-cycle strawberry systems found in Ventura County, reaching damaging levels on both summer and fall plantings. Figure 1. An adult lygus bug. (These photographs are by Jack Kelly Clark, and are copyrighted by the Regents of the University of California. They are provided by the UC Statewide IPM Program for use in this publication.) Identification Adults (Figure 1) have wings, and are about 0.25 inch long, oval, and rather flattened. They are greenish or brownish and have reddish brown markings on their wings. Near the center of their back is a distinct, but small, yellow or pale green triangle. Eggs are not easily seen because they are small and are inserted into plant tissue including Figure 2. Lygus bug nymphs showing spots on back. (These photographs fruit and leaves. First and are by Jack Kelly Clark, and are copyrighted by the Regents of the University of second instar nymphs California. They are provided by the UC Statewide IPM Program for use in this publication.) are pale green and look similar to aphids, but move faster, have red tips on their antennae and lack the paired cornicles or tubes located near the tip of the abdomen of aphids. The larger third and later instars (Figure 2) are green and have five black dots on their

2 back - two on the segment immediately behind the head, two on the next segment, and one in the middle of the abdomen. A common nonpest species that may be confused with lygus is Calocoris spp. (Figure 3). They have two prominent black dots on the back, just behind the head, and dark wing tips. Lygus adults have no black dots on the back. Nymphs and adults of Calocoris are longer and narrower than lygus. Other related species that may be confused with lygus are big-eyed bugs (Geocoris spp.) and false chinch bugs (Nysius spp.). Life history and damage In central coast areas, adult lygus bugs typically overwinter from September to January on flowering plants and legumes outside of strawberry fields (Zalom et al. 1990). Cover crops including clover, fava beans and vetch are good lygus hosts. Some lygus overwinter on summer-planted strawberries that become infested during the fall migration and on second-year strawberries which had become infested during the first year. Only the adults can fly from one host to another since the nymphs lack wings. Three seasonal populations of nymphs occur on central coast strawberries, the first in May or early June, the second in late June or early July, and the third in late July or August, depending on location and temperature conditions. Adults from the third generation overwinter, and will lay eggs in the spring contributing to the next season s problem. In the Oxnard area, lygus populations cycle throughout the year. Adults readily move between strawberry plantings and from alternate hosts such as flowering ornamental plants, beans, weeds, and possibly caneberries. Figure 3. Calocoris looks similar to a lygus bug, but does not damage strawberries. (These photographs are by Jack Kelly Clark, and are copyrighted by the Regents of the University of California. They are provided by the UC Statewide IPM Program for use in this publication.) Figure 4. Cat-facing caused by lygus bug feeding. (These photographs are by Jack Kelly Clark, and are copyrighted by the Regents of the University of California. They are provided by the UC Statewide IPM Program for use in this publication.) Lygus has sucking-type mouthparts, which it uses to feed on the developing strawberry seeds. When a seed is damaged, the flesh behind it does not develop properly. This creates the characteristic discoloration and malformation called cat-facing (Figure 4). Poor pollination also causes cat-facing, which is one reason that cat-facing symptoms alone should not be used as the basis for applying insecticides. Damaged strawberry fruit will not recover. It is necessary to suppress lygus before the damage occurs. Both nymphs and adults will damage fruit. 2 California Strawberry Commission Production Guideline

3 Monitoring and Management Successful lygus management requires an understanding of its biology and life cycle, and is best achieved through a combination of control approaches and careful monitoring both within strawberry fields and on its alternate hosts. Monitoring is particularly important to determine if and when to take a control action. The most effective treatment timing targets nymphs soon after egg hatch after overwintering adults have invaded the strawberry field when the greatest percentage of the lygus present are nymphs. Monitoring should begin in February to detect the first appearance of lygus nymphs on possible host plants around the field to establish the first biofix for the lygus degreeday (DD) model. These alternate hosts include second year strawberry fields and summer plantings in Ventura County. Monitoring in fall planted strawberry fields should begin in February in south coast growing areas to early April in Santa Maria and the central coast to detect when adults first appear. Summer plantings should be monitored for adults at the initiation of flowering. Establishing when adults first enter a field also serves as the second biofix for the lygus DD model, and is an indicator that nearby infested weed hosts must be destroyed immediately. Monitoring the field regularly after this time will establish whether or not lygus densities are economically important and require treatment. The lygus DD model is designed to predict when lygus nymphs will be present in strawberries. Controlling lygus nymphs is preferred because they can t migrate from the weed or alternate crop hosts being destroyed and they are also more susceptible to control using insecticides. Degree-days are units representing heat accumulation and they can be used to predict the time required for some event to occur in the life cycle of lygus and other insects after the occurrence of some biofix (a specific event which establishes that point at which DD calculation should begin). In any 24 hour period, the amount of heat above a minimum developmental threshold can be expressed as DD for that day, and these DD are summed over time. Lygus requires a minimum temperature of 54O F to develop, and this is the temperature above which DD are calculated. Below 54O F, lygus eggs and nymphs do not develop, and adults do not tend to feed or reproduce. There are two primary periods when lygus migrate from weed and host crops into strawberries, and DD can be used to predict when peak egg hatch occurs following each migration. On the central coast the migration of the overwintered adults often begins in April. Not all fields will have damaging levels of lygus at this time. If treatment levels are exceeded, apply the first spray 252 DD from the date you found the first adult (biofix #2) in the strawberry field. This will generally be from May to early June. The second treatment period is at 799 DD (late June/early July) from the date the first nymphs (biofix #1) were found in weeds. A third treatment period corresponds to the emergence of nymphs that come from adults established in a field and those that have migrated to strawberries during the summer and it is about 799 DD (early August) after the first treatment period. After this time in a typical season, lygus populations will include large numbers of all life stages, so use of degree-days has little value. The lygus model is on the UCIPM Program website: Degree-days can also be easily calculated by hand or with commercially available data loggers using local weather information. In Ventura County, migrations of adults occur to both the summer plantings and fall plantings at some time after initiation of flowering. Apply the first spray at 252 DD from the date you find the first adult in the strawberry field. The second treatment period is at 799 DD from the date the first nymph was found in the strawberry field. On summer plantings that have been chopped, a treatment at 252 DD after chopping is warranted if lygus were present the previous fall. Monitoring for the presence of lygus in weed and many alternate hosts can be conducted using a sweep net by vigorously pushing the net through the upper third of the plant canopy and examining its contents for the presence of lygus nymphs and adults, or shaking the upper portion of the plants over a beating tray. To quantify the abundance of lygus in the strawberry field, it is necessary to systematically use either a beating-tray or a Managing Lugs Bugs in Strawberries 3

4 hand held vacuum device described in University of California Publication 3351, Integrated Pest Management for Strawberries, 2nd Edition (Bolda et al. 2008). A recommended beating tray consists of a 12-inch embroidery hoop with a sturdy piece of white cotton cloth in the hoop, or some other tray of this approximate size. Divide the field into four equal areas to be monitored separately. Walk along a 200- foot section of each area sampling one strawberry plant every 10 feet by placing the embroidery hoop at the base of the plant and striking the plant three or four times so that any insects are knocked onto the tray (Zalom et al. 1993). Be careful to distinguish lygus from aphids, Calocoris, and beneficial insects such as bigeyed bugs (Geocoris spp.). The treatment level is one lygus nymph or adult in 20 plants using this sampling method (Zalom et al. 1990). Consider treating only a portion of the field if lygus populations are localized. Systematic monitoring for lygus can also detect infestations of aphids and worms which are dislodged onto the tray. Weed Control and Trap Crops Overwintering adult lygus bugs lay eggs in weeds and non-host crops from late fall through March that will hatch in February or March, depending on location and weather. Many flowering plants attract lygus, but they also serve as a source of adults that may migrate to strawberry fields. Monitoring lygus in weeds along roadways and field borders at this time is important to determine when lygus nymphs are present. Control infested weeds while the lygus are still nymphs, before they emerge as adults. The adults may remain on weeds that are still attractive, but when the weeds dry or are disturbed they will migrate to other hosts including strawberries. Insecticides applied to weeds to prevent movement are not very effective. Legumes, including alfalfa and fava beans planted adjacent to strawberries should be mown or disked while lygus are still nymphs. Alfalfa, in particular, needs to be managed throughout the season to prevent adult movement. Riparian areas, floodplains and fallow fields nearby strawberries with large stands of such common species as mustards, pepperweed, lupines, filaree, lambsquarters and common groundsel are especially problematic as a source of lygus adults when they are present throughout the season (Walsh et al. 1995). A related approach is to actually grow plants to attract lygus adults (Figure 5) in or adjacent to strawberry fields, but this requires careful monitoring and management to prevent an even greater problem from occurring. This approach targets local populations of lygus, and may not prevent longer range lygus migration from drying foothill weeds. In concept, adult lygus will preferentially lay eggs on these plants. Nymphs emerging in spring must be controlled before they become adults and move into the strawberry fields. If the plants are allowed to flower later into the season, careful monitoring of nearby strawberry fields is essential so that appropriate actions can be taken against lygus. Figure 5. Flowering borders can attract lygus and their natural enemies, but they can also be a source of lygus adults if not managed properly. 4 California Strawberry Commission Production Guideline

5 Insecticides Insecticides are commonly used for controlling lygus. The registered insecticides are more effective on the younger nymphs than on older nymphs or adults, making lygus monitoring critical for their most effective use. Insecticides currently registered for lygus control reflect relatively few modes of action, so they must be used judiciously to avoid developing pesticide resistance. The organophosphates, malathion and naled (Dibrom, Amvac), and the pyrethroids, bifenthrin (Brigade, FMC and Bifenture, UPI) and fenpropathrin (Danitol, Valent), have been used for decades resulting in many local populations that are resistant to these chemical classes. Lygus in second year fields that were treated with pyrethroids the previous year are especially likely to be resistant. Novaluron (Rimon, FMC), an insect growth regulator that targets only the young nymphs, has proven to be effective when applied early season when nymphs are emerging from the eggs laid by adults that are already present in fields or from those that have entered first year fields from their alternate hosts. Nymph emergence can be determined by monitoring and by using degree-days. It is suggested that the use of pyrethroids be saved for later in the season, or applied in a tank mix with one of the organophosphates or a neonicotinoid such as acetamiprid (Assail, UPI) or thiamethoxam (Actara, Syngenta). Current research is focused on registering products with different modes of action. However, their use and treatment timing will likely differ from the currently registered products that directly kill nymphs. Product recommendations change over time, so refer to the University of California s Publication 3339, Pest Management Guidelines: Strawberries: for current information. al. 1994), most of which are custom made. More efficient machines maximize airspeed at the hood, but without damaging the plants. If bug-vac use is initiated after lygus densities become moderate to heavy, their use will not reduce damage to acceptable levels. Biological Control A parasitic wasp, Anaphes iole, which attacks lygus eggs, has been shown to parasitize 50% or more of lygus eggs in field cages. Economically acceptable results are unlikely from augmentative releases in conventional production because the parasite does not control lygus adults moving into fields or lygus nymphs already present in the field, and damage occurs at lygus densities below that which can be achieved by the parasite alone. This species and a recently released parasitoid, Perstenus relictis, may be useful in reducing lygus populations on untreated hosts in central coast strawberry growing areas. Naturally occurring predators that feed on the nymphal stages of lygus bug include big-eyed bugs (Geocoris spp.), damsel bugs (Nabis spp.), minute pirate bugs (Orius tristicolor), and several species of spiders. Bug-vacs Growers have been experimenting with suction devices (bug-vacs) to control lygus bugs for many years with variable results. Research has shown that an efficient bug-vac used regularly after lygus adults are first detected in strawberry fields can reduce adults by 55-80% and nymphs by 9-50% relative to untreated checks, but bug-vac efficiency can vary considerably depending on the machine (Pickel et Managing Lugs Bugs in Strawberries 5

6 References Bolda, M.A., O. Daugovish, S.A. Fennimore, S.T. Koike, K.D. Larson, D.B. Marcum, and F.G. Zalom (technical coordinators) Integrated pest management for strawberries, 2nd ed. University of California, Division of Agriculture and Natural Resources Publication Pickel, C., F.G. Zalom, D.B. Walsh, and N.C. Welch Efficacy of vacuum machines for Lygus hesperus Knight (Hemiptera: Miridae) control in coastal California strawberries. J. Econ. Entomol. 87(6): Walsh, D.B., F.G. Zalom, N.C. Toscano, and E. Borden Lygus bug hatch on winter weeds in Watsonville and Santa Maria. The Pink Sheet, California Strawberry Commission Zalom, F.G., C. Pickel, D.B. Walsh, and N.C. Welch Sampling for Lygus hesperus Knight (Hemiptera: Miridae) in strawberries. J. Econ. Entomol. 86(4): Zalom, F.G., C. Pickel, and N. Welch Recent trends in strawberry arthropod management for coastal areas of the western United States, pp In L.T. Wilson, N.J. Bostanian & T.J. Dennehy, [eds.], Monitoring and integrated management of arthropod pests of small fruit crops. Intercept Press, LTD. Winborne, England. California Strawberry Commission P.O. Box 269 Watsonville, CA p f California Strawberry Commission 6