Curative Control of the Peachtree Borer Using Entomopathogenic Nematodes

The peachtree borer, Synanthedon exitiosa (Say 1823), is a major pest of stone fruit trees in North America. Current management relies upon preventative control using broad-spectrum chemical insecticides, primarily chlorpyrifos, applied in the late summer or early fall. However, due to missed applications, poor application timing, or other factors, high levels of S. exitiosa infestation may still occur and persist through the following spring. Curative treatments applied in the spring to established infestations would limit damage to the tree and prevent the next generation of S. exitiosa from emerging within the orchard. However, such curative measures for control of S. exitiosa do not exist. Our objective was to measure the efficacy of the entomopathogenic nematode, Steinernema carpocapsae, as a curative control for existing infestations of S. exitiosa. In peach orchards, spring applications of S. carpocapsae (obtained from a commercial source) were made to infested trees and compared with chlorpyrifos and a water-only control in 2014 and 2015. Additionally, types of spray equipment were compared: nematodes were applied via boom sprayer, handgun, or trunk sprayer. To control for effects of application method or nematode source, in vivo laboratory-grown S. carpocapsae, applied using a watering can, was also included. Treatment effects were assessed 39 d (2014) or 19 d (2015) later by measuring percentage of trees still infested, and also number of surviving S. exitiosa larvae per tree. Results indicated that S. carpocapsae provided significant curative control (e.g., >80% corrected control for the handgun application). In contrast, chlorpyrifos failed to reduce S. exitiosa infestations or number of surviving larvae. In most comparisons, no effect of nematode application method was detected; in one assessment, only the handgun and watering can methods reduced infestation. In conclusion, our study indicates that S. carpocapsae may be used as an effective curative measure for S. exitiosa infestations.     

A novel approach to biological control with entomopathogenic nematodes: Prophylactic control of the peachtree borer, Synanthedon exitiosa

Generally, microbial control agents such as entomopathogenic nematodes are applied in a curative manner for achieving pest suppression; prophylactic applications are rare. In this study, we determined the ability of two Steinernema carpocapsae strains (All and Hybrid) to prophylactically protect peach trees from damage caused by the peachtree borer, Synanthedon exitiosa, which is a major pest of stone fruit trees in North America. In prior studies, the entomopathogenic nematodes S. carpocapsae and Heterorhabditis bacteriophora caused field suppression when applied in a curative manner to established S. exitiosa populations. In our current study, nematodes were applied three times (at 150,000–300,000 infective juveniles/tree) during September and October of 2005, 2006, and 2007. A control (water only) and a single application of chlorpyrifos (at the labeled rate) were also made each year. The presence of S. exitiosa damage was assessed each year in the spring following the treatment applications. Following applications in 2006, we did not detect any differences among treatments or the control (possibly due to a low and variable S. exitiosa infestation of that orchard). Following applications in 2005 and 2007, however, the nematode and chemical treatments caused significant damage suppression. The percentage of trees with S. exitiosa damage in treated plots ranged from 0% damage in 2005 to 16% in plots treated with S. carpocapsae (Hybrid) in 2007. In control plots damage ranged from 25% (2005) to 41% (2007). Our results indicate that nematodes applied in a preventative manner during S. exitios’s oviposition period can reduce insect damage to levels similar to what is achieved with recommended chemical insecticide treatments.     

Field Application of Entomopathogenic Nematodes for Control of Delia radicum in Collards

Control of Delia radicum (cabbage maggot) in field collards (Brassica oleracea) was compared after one or two applications of entomopathogenic nematodes, Steinernema carpocapsae (All strain) and Heterorhabditis bacterophora (HP88 strain), a single application of granular chlorpyrifos, and a water-only treatment. Nematodes were applied with a sprayer during the egg stage of first-generation D. radicum, and chlorpyrifos was hand placed around collard stems during the same period. A second nematode application was made 10 days later. Chlorpyrifos treatment resulted in fewer puparia per plant, less root damage and higher yield than all other treatments, including the control. Collard yield from nematode-treated beds did not differ from controls. These data indicate that, under these field conditions, the species or strains of entomopathogenic nematodes tested did not reduce the number of active cabbage maggots, nor did they prevent collard root damage.     

Control of the larvae ofConorhynchus mendicus (Col.: curculionidae) bySteinernema carpocapsae andSteinernema feltiae (nematoda,steinernematidae) in the gharb area (Morocco)

Two entomopathogenic nematodes,Steirnernema carpocapsae andS. feltiae were evaluated in the laboratory and under field conditions against larvae ofConorhynchus mendicus, a pest of sugar beet in the Gharb area of Morocco. Nematodes were applied in an artificially infested field using a knapsack sprayer either during the day or in the evening after sunset. The number of insect cadavers recovered from both soil and sugar beet plants was greater in the treated plots compared with the untreated ones. The percent reduction in live larvae was significantly greater in theS. carpocapsae treated plots (89.5% and 59.5% for night an day application respectively) compared with those treated withS. feltiae (68.5% and 28% respectively). The efficacy of entomopathogenic nematodes was therefore increased by application at night particularly in the case ofS. carpocapsae where control was maintained for 45 days.     

Control of the flat-headed root borer <Emphasis Type="Italic">Capnodis tenebrionis</Emphasis> (Linné) (Coleoptera: Buprestidae) with the entomopathogenic nematode <Emphasis Type="Italic">Steinernema carpocapsae</Emphasis> (Weiser) (Nematoda: Steinernematidae) in a chitosan formulation in apricot orchards

Entomopathogenic nematodes Steinernema carpocapsae were applied in a chitosan formulation (Biorend R^®) to control the flat-headed rootborer Capnodis tenebrionis (Coleoptera: Buprestidae) in 5 field trials in the province of Valencia, Spain. Application was performed during spring and summer into the soil around apricot trees at densities of 1 and 1.5 million infective dauer juveniles per tree through drip irrigation, injection or by a drench. For evaluation of the control effect the roots of 106 trees were excavated, the cortex removed and living and dead insects were sampled. Dead larvae were dissected and checked for nematode infestation. Control of C. tenebrionis larvae was between 75% and 90%. No influence of the application method, nematode density or time of application on the control effect was recorded. Recovery of the infested trees was observed already in the season following nematode application. Due to the two year life cycle of the insect with egg laying from May to August, applications during spring and autumn are recommended.     

Evaluation of hydraulic,pneumatic and mechanical agitation for the spray application of Steinernema carpocapsae (Rhabditida: Steinernematidae)

The application of entomopathogenic nematodes (EPN) is generally done using standard spray application techniques. However, in contrast to chemical pesticides, these biological antagonists must remain viable during and after the application process. For the application of EPN, a good agitation system is indispensable as the nematodes tend to sediment fast in a spray tank without agitation. Three agitation systems, viz. mechanical, pneumatic and hydraulic agitation were tested for their ability to keep Steinernema carpocapsae (Rhabditida: Steinernematidae) suspended in an undamaged way. Hydraulic agitation was tested using a centrifugal and a diaphragm pump. Nematode damage was quantified based on viability and infectivity of the EPN. The ability of the agitation system to keep the nematodes in suspension was examined by comparing the nematode concentration observed in the samples taken at different agitation times. Only the hydraulic agitation using the centrifugal pump damaged the nematodes. After 120 min of recirculation, only 19.3% of the nematodes survived. Infectivity was even reduced to 0%. An additional experiment revealed that the temperature rise, from 21.7 to 45.4°C, was responsible for the observed nematode damage. The concentration measurements showed that the pneumatic agitation was unstable. Agitation during 120 min using the other agitation systems resulted in a significant loss of nematodes at 15 cm above the spray tank bottom. In conclusion, mechanical and hydraulic agitation using a diaphragm pump can be recommended when S. carpocapsae is applied, although attention should be paid to possible nematode loss during application.     

Field evaluation of entomopathogenic nematodes for biological control of striped flea beetle, Phyllotreta striolata (Coleoptera: Chrysomelidae)

The striped flea beetle, Phyllotreta striolata (F.) (Coleoptera: Chrysomelidae) is a key pest of crucifer vegetables in Southern China. The use of entomopathogenic nematodes (EPNs) within an integrated pest management approach may offer an effective and environmentally safe strategy to suppress outbreaks of this pest. In the present study, the efficacy of Steinernema carpocapsae All and Heterorhabditis indica LN2 for the control of P. striolata in the field was evaluated, as well as the combined application of EPNs and azadirachtin against the pest. Both nematode species were capable of reducing populations of the soil-dwelling stages of P. striolata, thus leading to a reduction of the adult populations and the associated shot-hole damage on the leaves. Nematode treatments also increased cabbage yields as compared to the control and azadirachtin treatments alone. Azadirachtin alone was not effective to prevent damage by P. striolata, but it could enhance the control effect of S. carpocapsae shortly after application. Osmotically treated infective juveniles (IJs) of S. carpocapsae All performed as well as untreated IJs.     

Efficacy of soil treatments of entomopathogenic nematodes against the larvae, pupae and adults of Tuta absoluta and their interaction with the insecticides used against this insect

The efficacy of soil treatments of three native entomopathogenic nematodes (Steinernema carpocapsae, S. feltiae and Heterorhabditis bacteriophora) against Tuta absoluta larvae, pupae and adults was determined under laboratory conditions. The effect of three insecticides commonly used against T. absoluta, in the survival, infectivity and reproduction of these nematode strains was also evaluated. When dropped into soil to pupate, soil application of nematodes resulted in a high mortality of larvae: 100, 52.3 and 96.7 % efficacy for S. carpocapsae, S. feltiae and H. bacteriophora respectively. No mortality of pupae was observed and mortality of adults emerging from soil was 79.1 % for S. carpocapsae and 0.5 % for S. feltiae. The insecticides tested had a negligible effect on nematode survival, infectivity and reproduction. No sublethal effects were observed. Infective juveniles that survived to insecticide exposition were able to infect Galleria larvae with no significant differences from the control. The Galleria larvae affected by the three insecticides tested served as suitable hosts for the infection and reproduction of the nematodes. These results suggest that larvae of T. absoluta, falling from leaves following insecticide application, could be suitable hosts for nematodes, thereby increasing their concentration and persistence in the soil.     

Entomopathogenic nematodes control Plum Sawflies (Hoplocampa minuta and H. flava)

Plum sawflies are among the most damaging pests of European plum. Current control strategy implies insecticide application. Three species of entomopathogenic nematodes (EPN), Steinernema feltiae Filipjev, S. carpocapsae Weiser and Heterorhabditis bacteriophora Poinar were tested under laboratory and field conditions to assess effectiveness against larval and adult stages. Laboratory tests resulted in up to 100% mortality of last instar larvae before construction of a cocoon. However, the nematodes were not able to penetrate the cocoon. Foliar application did not result in plum sawflies larvae infestation by EPNs. Under field conditions, the nematodes reduced the number of emerging adults by application against sawfly larvae in the previous year before migration into the soil for overwintering by 62%–92%. Application of the nematodes against adults just before their anticipated emergence resulted in reduction of fruit infestation up to 100%. Mean results of 5 trials using caged trees were 47.8% with S. feltiae, 56.3% with S. carpocapsae and 62.9% with H. bacteriophora. In open field trails, control of adults obtained with S. feltiae at 0.5 million nematodes/m² was 98.2 and 67.8% and at 0.25 million m⁻² 41.7 and 41.2%. Forecasting adult emergence and optimal soil moisture conditions are essential for success of the nematode application.     

Effect of Soil Moisture and a Surfactant on Entomopathogenic Nematode Suppression of the Pecan Weevil, Curculio caryae

Our overall goal was to investigate several aspects of pecan weevil, Curculio caryae, suppression with entomopathogenic nematodes. Specifically, our objectives were to: 1) determine optimum moisture levels for larval suppression, 2) determine suppression of adult C. caryae under field conditions, and 3) measure the effects of a surfactant on nematode efficacy. In the laboratory, virulence of Heterorhabditis megidis (UK211) and Steinernema carpocapsae (All) were tested in a loamy sand at gravimetric water contents of negative 0.01, 0.06, 0.3, 1.0, and 15 bars. Curculio caryae larval survival decreased as moisture levels increased. The nematode effect was most pronounced at –0.06 bars. At –0.01 bars, larval survival was ≤5% regardless of nematode presence, thus indicating that intense irrigation alone might reduce C. caryae populations. Overall, our results indicated no effect of a surfactant (Kinetic) on C. caryae suppression with entomopathogenic nematodes. In a greenhouse test, C. caryae larval survival was lower in all nematode treatments compared with the control, yet survival was lower in S. carpocapsae (Italian) and S. riobrave (7–12) treatments than in S. carpocapsae (Agriotos), S. carpocapsae (Mexican), and S. riobrave (355) treatments (survival was reduced to approximately 20% in the S. riobrave [7–12] treatment). A mixture of S. riobrave strains resulted in intermediate larval survival. In field experiments conducted over two consecutive years, S. riobrave (7–12) applications resulted in no observable control, and, although S. carpocapsae (Italian) provided some suppression, treatment effects were generally only detectable one day after treatment. Nematode strains possessing both high levels of virulence and a greater ability to withstand environmental conditions in the field need to be developed and tested.     

Biological Control of Soil Pests by Mixed Application of Entomopathogenic and Fungivorous Nematodes

In greenhouse experiments, massive application of the fungivorous nematode, Aphelenchus avenae, in summer at 26-33 C (1 x l0⁵ nematodes/500 cm³ autoclaved soil) or in autumn at 18-23 C (5 x 10⁴ nematodes/500 cm³ autoclaved soil) suppressed pre-emergence damping-off of cucumber seedlings due to Rhizoctonia solani AG-4 by 67% or 87%, respectively. Application of 2 x l0⁵ A. avenae to sterilized soil infested with R. solani caused leafminer-like symptom on the cotyledons, which did not occur in mixed inoculations with the entomopathogenic nematode, Steinernema carpocapsae. When 1 x 10⁶ A. avenae were applied 3 days before inoculation with 100 Meloidogyne incognita juveniles, gall numbers on tomato roots were reduced to 50% of controls. Gall numbers also were suppressed by S. carpocapsae (str. All). Reduction in gall numbers was no greater with mixed application of A. avenae and S. carpocapsae than with application of single species, even though twice the number of nematodes were added in the former case. These nematodes were positively attracted to tomato root tips. Aphelenchus avenae suppressed infection of the turnip moth, Agrotis segetum, but not the common cutworm, Spodoptera litura, by S. carpocapsae.     

Susceptibility of the Plum Curculio,Conotrachelus nenuphar,to Entomopathogenic Nematodes

The plum curculio, Conotrachelus nenuphar, is a major pest of pome and stone fruit. Our objective was to determine virulence and reproductive potential of six commercially available nematode species in C. nenuphar larvae and adults. Nematodes tested were Heterorhabditis bacteriophora (Hb strain), H. marelatus (Point Reyes strains), H. megidis (UK211 strain), Steinernema riobrave (355 strain), S. carpocapsae (All strain), and S. feltiae (SN strain). Survival of C. nenuphar larvae treated with S. feltiae and S. riobrave, and survival of adults treated with S. carpocapsae and S. riobrave, was reduced relative to non-treated insects. Other nematode treatments were not different from the control. Conotrachelus nenuphar larvae were more susceptible to S. feltiae infection than were adults, but for other nematode species there was no significant insect-stage effect. Reproduction in C. nenuphar was greatest for H. marelatus, which produced approximately 10,000 nematodes in larvae and 5,500 in adults. Other nematodes produced approximately 1,000 to 3,700 infective juveniles per C. nenuphar with no significant differences among nematode species or insect stages. We conclude that S. carpocapsae or S. riobrave appears to have the most potential for controlling adults, whereas S. feltiae or S. riobrave appears to have the most potential for larval control.     

Nonsusceptibility of the Honey Bee,Apis mellifera (Hymenoptera: Apidae), to Steinernematid and Heterorhabditid Nematodes

We exposed honey bee workers and brood to four entomopathogenic nematode species under conditions normally encountered in the hive by spraying nematodes onto combs. Mortality of adult bees exposed to any of the nematode species was less than 10%, and there was no evidence of nematode infection when dead adults were dissected. To assess the impact of nematodes on brood, we used smaller-size honey combs placed in the second story (super) of a hive and large brood combs placed in the main section of the hive. Our results were inconsistent between these two experimental designs. The smaller honey combs sprayed with Steinernema carpocapsae contained the largest number of uncapped ceils, those sprayed with Heterorhabditis baeteriophora or S. riobravis contained an intermediate number of uncapped cells, and control combs and those sprayed with S. glaseri contained the fewest nmnber of uncapped cells. Large combs sprayed with S. riobravis contained more uncapped ceils than controls or those sprayed with S. carpocapsae, although the differences were not significant. Our results do not support the hypothesis that high-temperature-tolerant species of nematodes are necessarily more infective to honey bees.     

Infectivity of Steinernema carpocapsae and S. feltiae to Larvae and Adults of the Hazelnut Weevil,Curculio nucum: Differential Virulence and Entry Routes

We investigated the existing susceptibility differences of the hazelnut weevil, Curculio nucum L. (Coleoptera:, Curculionidae) to entomopathogenic nematodes by assessing the main route of entry of the nematodes, Steinernema carpocapsae strain B14 and S. feltiae strain D114, into larvae and adult insects, as well as host immune response. Our results suggested that S. carpocapsae B14 and S. feltiae D114 primarily entered adult insects and larvae through the anus. Larvae were more susceptible to S. feltiae D114 than S. carpocapsae B14 and adults were highly susceptible to S. carpocapsae B14 but displayed low susceptibility to S. feltiae D114. Penetration rate correlated with nematode virulence. We observed little evidence that hazelnut weevils mounted any cellular immune response toward S. carpocapsae B14 or S. feltiae D114. We conclude the differential susceptibility of hazelnut weevil larvae and adults to S. carpocapsae B14 and S. feltiae D114 primarily reflected differences in the ability of these two nematodes to penetrate the host.     

Biological control of the black vine weevil, Otiorhynchus sulcatus (Coleoptera: Curculionidae) with entomopathogenic nematodes (Nematoda: Rhabditida)

Trials conducted under glasshouse conditions showed that control of Otiorhynchus sulcatus larvae in strawberry plants can be effective using Steinernema carpocapsae and Heterorhabditis megidis, given that temperature and moisture extremes are avoided. In field experiments, the double line T-Tape® drip irrigation system performed better than the single line T-Tape® system, effectively distributing the nematodes along and across strawberry raised beds, and placing them close to the root zone where O. sulcatus larvae feed. As soil temperatures are satisfactory for nematode infectivity from late spring to early autumn, nematode applications were aimed at late instar larvae during spring, and early instar larvae during summer. Late summer field treatment with S. carpocapsae induced 49.5% reduction of the early instar larvae, and field application of the same nematode species in late spring resulted in 65% control of late instar larvae. In the same trial, spring application of H. megidis caused 26% mortality of late instar larvae of O. sulcatus.     

The nematode Neoaplectana carpocapsae and the beet armyworm Spodoptera exigua: Infectivity of prepupae and pupae in soil and of adults during emergence from soil

The nematode, Neoaplectana carpocapsae, infected >90% of the prepupae of Spodoptera exigua in soil even at concentrations as low as five nematodes/cm² of soil surface. Pupae were less susceptible to nematode infection in soil than prepupae, with mortality ranging from 10 to 24% and 10 to 83% for pupae exposed 3–5 days and 6–8 days to the nematode, respectively. Longer exposure (6–8 days) of the pupae to the nematode resulted in higher mortality with a positive relationship with increasing concentrations. Adults of S. exigua were susceptible to nematode infections as they emerged from the soil. The higher nematode concentrations (25 and 50 nematodes/cm²) resulted in higher adult mortality. The majority of nematode-induced mortality occurred within 24 hr after emergence. The susceptibility of emerging S. exigua adults to N. carpocapsae offers a new dimension for insect control.     

Control of Codling Moth,Cydia pomonella (Lepidoptera: Tortricidae), with Steinernema carpocapsae: Effects of Supplemental Wetting and Pupation Site on Infection Rate

Infection of cocooned codling moth (cydia pomonella) larvae by the entomopathogenic nematode Steinernema carpocapsae was studied in three field experiments. Factors that varied within or between experiments included method of application, type of substrate containing cocooned larvae, time when nematodes were applied, seasonal effects, and supplemental wetting before or after nematode application. Conventional air-blast sprayer applications of 0.5–5.0 million infective juveniles (IJs)/tree in fall resulted in ca. 30% mortality of larvae in cardboard trap bands, whereas hand-gun application (2 million IJs/tree) produced mortality of ca. 70%. Application in the evening caused higher larval mortality than application in the morning when no supplemental wetting was used after treatments. Morning and evening applications caused equivalent larval mortality when a postwetting treatment was included. In a trial conducted in midsummer, supplemental wetting, either before or after hand-gun application of 1 million IJs/tree, enhanced nematode-produced mortality. Mortality approached 100% if both pre- and postwetting was used. Larvae in exposed cocoons on apple wood were infected at a higher rate (86%) than those on wood in less exposed positions (73%) or in nonperforated cardboard (72%). Mortality rates for larvae in perforated cardboard were intermediate (77%). Application volumes used to deliver nematodes slightly enhanced infection rate of larvae in some substrates but not others. In one trial, parasitism of codling moth by the wasp Mastrus ridibundus (Ichneumonidae) was negatively correlated with nematode infection of codling moth larvae. Dissections showed that ca. 10% of larvae infected by nematodes had been attacked by the wasp.     

Magnetic and electric fields induce directional responses in Steinernema carpocapsae

Entomopathogenic nematode species respond directionally to various cues including electrical stimuli. For example, in prior research Steinernema carpocapsae was shown to be attracted to an electrical current that was applied to an agar dish. Thus, we hypothesised that these nematodes may use electromagnetic reception to assist in navigating through the soil and finding a host. In this study we discovered that S. carpocapsae also responds to electrical fields (without current) and to magnetic fields; to our knowledge this is the first report of nematode directional movement in response to a magnetic field. Our research expands on the range of known stimuli that entomopathogenic nematodes respond to. The findings may have implications for foraging behavior.     

European earwig (Forficula auricularia) as a novel host for the entomopathogenic nematode Steinernema carpocapsae

The natural history of many entomopathogenic nematode species remains unknown, despite their wide commercial availability as biological control agents. The ambushing entomopathogenic nematode, Steinernema carpocapsae, and the introduced European earwig, Forficula auricularia, forage on the soil surface. Since they likely encounter one another in nature, we hypothesized that earwigs are susceptible to nematode infection. In the laboratory, the LC₅₀ for F. auricularia was 226 S. carpocapsae/earwig and the reproductive potential was 123.5 infective juvenile nematodes/mg tissue. This susceptibility depended on host body size with significantly higher mortality rates seen in larger earwigs. In a study of host recognition behavior, S. carpocapsae infective juveniles responded to earwig cuticle as strongly as they did to Galleria mellonella cuticle. We also found that earwigs exposed to S. carpocapsae cleaned and scratched their front, middle and back legs significantly more than controls. Coupled with previous field data, these findings lead us to suggest that F. auricularia may be a potential host for S. carpocapsae.     

Effects of a novel entomopathogenic nematode-infected host formulation on cadaver integrity, nematode yield, and suppression of Diaprepes abbreviatus and Aethina tumida

An alternative approach to applying entomopathogenic nematodes entails the distribution of nematodes in their infected insect hosts. Protection of the infected host from rupturing, and improving ease of handling, may be necessary to facilitate application. In this study our objective was to test the potential of a new method of formulating the infected hosts, i.e., enclosing the infected host in masking tape. Tenebrio molitor L. cadavers infected with Heterorhabditis indica Poinar, Karunakar and David or Steinernema carpocapsae (Weiser) were wrapped in tape using an automatic packaging machine; the machine was developed to reduce labor and to standardize the final product. The effects of the tape formulation on the ability to protect the cadavers from mechanical damage, nematode yield, and pest control efficacy were tested. After exposure to mechanical agitation at 7-d-post-infection, S. carpocapsae cadavers in tape were more resistant to rupture than cadavers without tape, yet H. indica cadavers 7-d-post-infection were not affected by mechanical agitation (with or without tape), nor was either nematode affected when 4-d-old cadavers were tested. Experiments indicated that infective juvenile yield was not affected by the tape formulation. Laboratory experiments were conducted measuring survival of the root weevil, Diaprepes abbreviatus (L.), or the small hive beetle, Aethina tumida Murray, after the application of two H. indica-infected hosts with or without tape per 15 cm pot (filled with soil). A greenhouse experiment was also conducted in a similar manner measuring survival of D. abbreviatus. In all experiments, both the tape and no-tape treatments caused significant reductions in insect survival relative to the control, and no differences were detected between the nematode treatments. Fifteen days post-application, the infected host treatments caused up to 78% control in A. tumida, 91% control in D. abbreviatus in the lab, and 75% in the greenhouse. These results indicate potential for using the tape-formulation approach for applying nematode infected hosts.