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    Bugs for Growers — Beneficial nematodes

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    Control sod webworms with beneficial nematodes

    Sod webworms: One of the most damaging pests of turfgrass foliage

    How to identify Sod webworms?

    Full grown larvae/caterpillars of sod webworms are easy to identify as they are about 1 inch long having 4 parallel rows of distinct dark spots along the length of their body and coarse hair distributed all over the body. Also, depending upon species, caterpillars can be gray, brown or greenish in color. Adult sod webworm moths are light, brown or dull ash gray in color.

    Life cycle of sod webworms:

    Sod webworms develop through four different stages including egg, caterpillar/larva, pupa and adult moth. Sod webworms over-winter as caterpillar in silk-lined tunnels prepared in thatch and/or soil. In the spring, overwintering caterpillars resume feeding on new turfgrass growth, while feeding molts (shed its cuticle) 5-9 times and go through 6-10 stages (instars) of development. Last instar larva pupates inside the cocoons built from soil particles and plant debris in May through June. After 10-15 days of pupation, adult moths emerge from pupae and begin mating. After mating, female moths generally start laying their eggs individually while they are flying and dropping them randomly into the grass. Each moth lays up to 500 eggs during a life span of usually 10 to 15 days. Under optimal environmental temperatures, eggs hatch within 7-15 days. After hatching from egg, larval stage starts immediately feeding on grass foliage. These second generation caterpillars of sod webworms feed through September then overwinter and life cycle continues.

    Damaging stages of sod webworms:

    All six – ten stages/ instars of sod webworm caterpillars also called larvae cause damage to different grass species or other host plants. Adult moths do not cause any type of damage to any grass species.

    How damage is caused?

    All the stages of sod webworm caterpillars feed on a variety of grasses. Caterpillars generally chew leaves and stems at the crown of turfgrass causing thinning of grass in small areas. As the infestation progresses, these damaged small areas turn into large brown patches.  When infestation of sod webworms is very high, the damage caused by them is easily noticed as large and irregular brown patches are seen throughout lawns or golf course.  Sod webworm caterpillars feed at night but hide in the silk-lined tunnels prepared by them in thatch and/or soil during day time. The presence of irregular brown patches of dead grass caused by feeding of sod webworm in the middle of a lawn or golf course can reduce its aesthetic value.

    Biological control of sod webworms:

    Biological control agents including Bacillus thuringiensis (a bacterium which produces a toxin and paralyzes the gut of the caterpillar) and entomopathogenic nematodes also recognized as beneficial nematodes have a potential to manage sod webworms. However, beneficial nematodes have showed promising results in controlling sod webworms.

    Why we should use beneficial nematodes?

    Beneficial nematodes can kill sod webworm caterpillar with 48 hours after application. They are commercially available and easy to apply. Beneficial nematodes are not harmful to children, dogs, cats, personnel involved in its application and beneficial insects like honeybees. Beneficial nematodes do not need a special permission to apply because they are exempted by EPA.

    Which species of beneficial nematodes are effective against sod webworms (see literature below)?

    • Steinernema carpocapsae
    • Heterorhabdtis bacteriophora

     What stages of Sod webworms can be targeted?

    All the stages of caterpillars/ larvae and pupae can be targeted because both larvae and pupae are susceptible to beneficial nematodes. 

    What is a recommended rate of beneficial nematodes required to control sod webworms?

    For the successful control most of the soil dwelling insect pests, the optimal rate of 1 billion infective juveniles of beneficial nematodes in 100 to 260 gallons of water per acre is generally recommended (See Table for appropriate amounts of nematodes required for different sizes of sod webworm infested areas to be treated).

    Where you can buy beneficial nematodes?

    • Both Steinernema carpocapsae and Heterorhabdtis bacteriophora nematodes are sold in our store.
    • Steinernema carpocapsae nematodes available both in liquid (sponge-water suspension) and granulated formulations
    • Heterorhabdtis bacteriophora nematodes available only in liquid (sponge- water suspension) formulations

    How long it takes to deliver nematodes?

    We can directly deliver beneficial nematodes at your facility in person (service available only in Athens,Georgia) or by UPS throughout US within 3 days after receiving order.

    When to apply beneficial nematodes

    • To target sod webworms, beneficial nematodes should be applied starting from early spring through late summer i.e. when young larval (caterpillars) stages (instars) of sod webworms are already hatched from eggs and started feeding on grass leaves.
    • Since nematodes are very sensitive to UV light, they will die within a minute or two when exposed to full sun. Therefore, nematodes should be applied early in the morning or late in the evening to avoid exposure to UV light.
    • Another advantage of applying nematodes late in the evening is that sod webworm caterpillars can be easily targeted because they are generally active and searching for food during night and easily found by Steinernema carpocapsae nematodes that uses sit and wait (ambush) strategy to attack its passing by host.  Since sod webworm caterpillars are moving actively during night in search of food, they can easily come across to Heterorhabdits bacteriophora nematodes that uses cruising strategy to finds its host. Heterorhabdits bacteriophora nematodes can also find caterpillars that are hiding under thatch during day time.

    How beneficial nematodes kill sod webworms?

    After application of either Steinernema spp. or Heterorhabditis spp. on the lawns, their their infective juveniles find sod webworm larva or pupa and enter into its body cavity through natural openings such as mouth, anus and spiracles. Once infective juveniles of both Steinernema spp. and Heterorhabditis spp are in the insect body cavity, they release several cells of symbiotic bacteria, Xenorhabdus spp. and Photorhabdus spp., respectively from their gut via anus in the sod webworm blood, which is conducive for the multiplication of symbiotic bacteria. In the blood, multiplying nematode-bacterium complex causes septicemia and kill sod webworm caterpillar also called larvae usually within 48 h after infection.

    Literature:

    Grewal, P.S. Koppenhofer, A.M. and Choo, H.Y. 2005. Lawn, turfgrass and pasture applications. In: Nematodes As Biocontrol Agents. Grewal, P.S. Ehlers, R.-U., Shapiro-Ilan, D. (eds.). CAB publishing, CAB International, Oxon. Pp 115-146.

    Optimal rates of beneficial nematodes required to treat different sizes of area

    How many nematodes should be applied?

    For the successful control most of the soil dwelling insect pests, the optimal rate of 1 billion infective juvenile nematodes in 100 to 260 gallons of water per acre is generally recommended.

    Table1. Showing the number of nematodes required to treat different sizes [area in sq ft (sq M)] of lawns or fields

    Nematode species

    Area in sq ft (sq meter)

    1 (0.093)

    10.76 (1)

    108 (10)

    1076 (100)

    2500 (233)

    5000 (465)

    10000 (930)

    43560 (4047) (an acre)

    Heterorhabdtis bacteriophora

    22.9 thousand

    229.6 thousand

    2.3 million

    22.6 million 57.4 million 114.8 million 229.6 million 1.0 billion

    Heterorhabdtis indica

    22.9 thousand

    229.6 thousand

    2.3 million

    22.6 million 57.4 million 114.8 million 229.6 million 1.0 billion

    Steinernema carpocapsae

    22.9 thousand

    229.6 thousand

    2.3 million

    22.6 million 57.4 million 114.8 million 229.6 million 1.0 billion

    Steinernema feltiae

    22.9 thousand

    229.6 thousand

    2.3 million

    22.6 million 57.4 million 114.8 million 229.6 million 1.0 billion
     

    Biological control of turfgrass armyworms with beneficial nematodes

    Armyworms: The important insect pest of turfgrass foliage

    How to identify armyworms?

    Armyworms are easy to identify. Adult moths are light reddish brown in color whereas their full grown larvae/caterpillars are brown in color and having several stripes on their body, and a distinctive "Y" shape mark on the head.
    • [caption id="attachment_184" align="alignnone" width="293" caption="Adult armyworm moth"]"Armyworm moth"[/caption]
    [caption id="attachment_217" align="alignnone" width="252" caption="A full grown caterpillar or larva of armyworm"]"Armyworm caterpillar"[/caption]

    Life cycle of armyworms:

    Armyworms develop through four different stages: Moth, egg, caterpillar (larva) and pupa. Adult moths of armyworm begin emerging from over-wintering pupae early June through early August and start mating. After mating, each female moth lays about 500 eggs in a cluster on the lower surface of the leaves. Under optimal environmental conditions, eggs hatch within one week. After hatching from egg, larval stages starts feeding on grass foliage. While feeding, larvae molts (shed its cuticle) five times, and go through six stages (instars) of development. In the late summer mature, larva falls off the leaves and burrow into ground to form pupa, which survive during winter and life cycle continues. 

    Damaging stages of armyworms:

    • All the six stages/ instars of caterpillars/larvae of armyworms cause damage to grass or other host plants.
    • Adult moths do not cause any type of damage to grasses or other plants.
      [caption id="attachment_213" align="alignnone" width="300" caption="Armyworm caterpillars or larvae cause a serious damage to the foliage of several grass species"]"Armyworm caterpillar"[/caption]

    How damage is caused?

    All the larval stages cause damage by chewing margins of leaves of all kinds of grasses during night but hide under thatch during day time. Under sever infestation, caterpillars can skeletonize turf plant and reduce the aesthetic value of turfgrass.

    Biological control of armyworms:

    • Biological control agents including Nucleopolyhedrovirus, parasitoids (Braconid wasps, Apanteles spp. and Tachinid flies) and entomopathogenic nematodes have a potential to manage armyworms.

    Why we should use entomopathoegnic nematodes?

    • They can kill armyworm larvae with 48 hours after application.
    • They are commercially available and easy to apply.
    • They are not harmful to children, dogs, cats, personnel involved in its application and beneficial insects like honeybees.
    • They do not need a special permission to apply because they are exempted by EPA.

    Which species of entomopathogenic nematodes are effective against Armyworms (see literature below)?

    • Steinernema carpocapsae
    • Heterorhabdtis indica
    • Heterorhabdtis bacteriophora

     What stages of armyworms can be targeted?

    All the six stages of caterpillars/ larvae are susceptible to entomopathogenic nematodes. Pupae are also susceptible to entomopathogenic nematodes.

    What is a recommended dosage of entomopathogenic nematodes required to control armyworms?

    •  For the successful control of most of the soil dwelling insect pests, the optimal rate of 1 billion infective juvenile nematodes in 100 to 260 gallons of water per acre is generally recommended (See Table1).

    Where you can buy entomopathogenic nematodes?

    • All three entomopathogenic nematodes including Steinernema carpocapsae, Heterorhabdtis bacteriophora and Heterorhabdtis indica are sold in our store.
    • Steinernema carpocapsae nematodes available both in liquid (sponge-water suspension) and granulated formulations.
    • Heterorhabdtis indica and Heterorhabdtis bacteriophora nematodes available only in liquid (sponge- water suspension) formulations.

    How long it takes to deliver nematodes?

    We can directly deliver entomopathogenic nematodes at your facility in person (service available only in Athens,Georgia) or by UPS throughout US within 3 days after receiving order.

    When to apply nematodes

    • To target armyworms caterpillars, entomopathogenic nematodes should be applied starting from late June through late August i.e. when young larval (caterpillars) stages (instars) of armyworms are already hatched from eggs (laid in early June) and started feeding on grass leaves.
    • To target especially armyworm pupae (to suppress the armyworm moth emergence in the next spring), entomopathogenic nematodes should be applied in late summer when caterpillars starting to fall off from leaves on the ground for pupation.
    • Since nematodes are very sensitive to UV light, they will die within a minute or two when exposed to full sun. Therefore, nematodes should be applied early in the morning or late in the evening to avoid exposure to UV light.
    • Another advantage of applying nematodes late in the evening is that armyworm caterpillars can be easily targeted because they are generally active and searching for food during night and easily found by entomopathogenic nematodes like Steinernema carpocapsae that uses sit and wait (ambush) strategy to attack its passing by host.  Since armyworm caterpillars are moving actively during night in search of food, they can easily come across to entomopathogenic nematodes like Heterorhabdits bacteriophora and Heterorhabdits indica that uses cruising strategy to finds its host. Because of cruising strategy, both Heterorhabdits bacteriophora and Heterorhabdits indica nematodes can also find caterpillars that are hiding under thatch during day time.

    How entomopathogenic nematodes kill armyworms?

    After application of either Steinernema spp. or Heterorhabditis spp. on the lawns, their their infective juveniles find armyworm larva or pupa and enter into its body cavity through natural openings such as mouth, anus and spiracles. Once infective juveniles of both Steinernema spp. and Heterorhabditis spp are in the insect body cavity, they release several cells of symbiotic bacteria, Xenorhabdus spp. and Photorhabdus spp., respectively from their gut via anus in the insect blood. Insect blood is conducive for the multiplication of symbiotic bacteria. In the blood, multiplying nematode-bacterium complex causes septicemia and kill their insect host usually within 48 h after infection.

    Literature:

    • Andalo, V., Santos, V., Moreira, G.F., Moreira, C., Freire, M. and Moino, A. 2012.   Movement of Heterorhabditis amazonensis and Steinernema arenarium in search of corn fall armyworm larvae in artificial conditions.  Scientia Agricola 69: 226-230.
    • Ansari, M.A., Waeyenberge, L. and Moens, M. 2007.  Natural occurrence of Steinernema carpocapsae, Weiser, 1955 (Rhabditida: Steinernematidae) in Belgian turf and its virulence to Spodoptera exigua (Lepidoptera: Noctuidae). Russian Journal of Nematology 15: 21-24.
    • Kim, J. and Kim, Y. 2011.  Three metabolites from an entomopathogenic bacterium, Xenorhabdus nematophila, inhibit larval development of Spodoptera exigua (Lepidoptera: Noctuidae) by inhibiting a digestive enzyme, phospholipase A (2). Insect Science 18: 282-288.
    • Negrisoli, A.S., Garcia, M.S., Negrisoli, C.R.C.B., Bernardi, D. and da Silva, A. 2010.  Efficacy of entomopathogenic nematodes (Nematoda: Rhabditida) and insecticide mixtures to control Spodoptera frugiperda (Smith, 1797) (Lepidoptera: Noctuidae) in corn crops. Crop Protection 29: 677-683.

    How to deploy your nematode army and kill insect pests

    Nematodes require special care and there are several methods of applications. The process depends on how the Nematodes come when you purchase them. It is also important to know how many nematodes need to be applied to be effective (See Table 1). This post is an overview or what is required for effective use of nematodes.

    What kind of equipment is used for application of entomopathogenic nematodes?

    • To apply liquid (Sponge) formulated Steinernema carpocapsae or Heterorhabdtis bacteriophora nematodes on a large area (>5000 sq ft) you can use traditional Knapsack/backpack sprayers or any other sprayers with up to 300 PSI pressures that are generally used for application of chemical pesticides.
    • To apply liquid formulated Steinernema carpocapsae or Heterorhabdtis bacteriophora nematodes on a small areas (<5000 sq ft) such as vegetable and ornamental gardens you can use simple watering cans (shown in photo below).
    • To apply granule formulated Steinernema carpocapsae nematodes on a small (<5000 sq ft) or large area (>5000 sq ft) you can use traditional spreaders that are used for application of granular or pallet pesticides or synthetic fertilizers.

    How are nematodes applied?

    • As stated above, entomopathogenic nematodes that you receive in sponge as liquid formulation are thoroughly mixed in water and can be easily sprayed on grass foliage with the traditional Knapsack/backpack sprayers or watering cans.
    • However, at time of spraying care should be taken that the nematodes should not be allowed to settle at the bottom of sprayer or watering can to avoid their uneven distribution.
    • To avoid the settling of nematodes at the bottom of sprayer or watering can, nematode suspension should be constantly agitated.
    • However, nematodes will be easily damaged, if they are agitated through excessive recirculation of spray mix or if the temperature in the tank increases beyond 86oF.
    • Nematodes can also be applied through different types of irrigation systems but pumps should have proper pressure to avoid damage to nematodes and screen sizes should be larger than 50 mesh so that nematodes will pass through them live.
    • Nematodes received in granular formulation can be directly applied using traditional spreaders that are used for application of granular or pallet pesticides or synthetic fertilizers. [caption id="attachment_80" align="alignnone" width="300" caption="Watering can for application of entomopathogenic nematodes on a small area"]Entomopathogenic nematodes can applied with a watering can[/caption]

    When to apply nematodes

    • To target any soil dwelling stages of insect pests, entomopathogenic nematodes should be applied when young larval (caterpillars)/ nymphal stages (instars) of insects are already hatched from eggs and started feeding on grass leaves or any other crops.
    • Since nematodes are very sensitive to UV light, they will die within a minute or two when exposed to full sun. Therefore, nematodes should be applied early in the morning or late in the evening to avoid exposure to UV light.
    • Another advantage of applying nematodes late in the evening is that caterpillars/larvae of most of the insect pests can be easily targeted because they are generally active and searching for food during night and easily found by entomopathogenic nematodes like Steinernema carpocapsae that uses sit and wait (ambush) strategy to attack its passing by host.  Since caterpillars of many insects are moving actively during night in search of food or larvae of those insects remaining at one place and continuously feeding plant roots, can easily come across to entomopathogenic nematodes like Heterorhabdits bacteriophora or other species of Heterorhabditis that uses cruising strategy to finds its host. Heterorhabditis nematodes can also find caterpillars that are hiding under thatch during day time.

    What care should be taken before and after application of nematodes?

    • All nematodes require proper soil moisture for their optimal movement and infectivity.
    • The activity and infectivity of nematodes can be enhanced by maintaining optimum moisture levels in the soil before and after their application.
    • In case of nematode application in turf, turf should be irrigated immediately after application with at least 1/2 inch of water to rinse off nematodes from the foliage and move them into the soil and thatch.
    • As nematodes are very sensitive to heat and cold, their infectivity will be reduced if soil temperature is below 4oC (39.2oF) and above 35oC (95.2oF).
    • Soil temperatures between 20 to 30oC (68 to 86oF) are considered favorable for application of majority of nematode species and their virulence. 

    How many nematodes should be applied?

    • For the successful control most of the soil dwelling insect pests, the optimal rate of 1 billion infective juvenile nematodes in 100 to 260 gallons of water per acreis generally recommended (Table 1). Table1. Number of nematodes required to treat different sizes [area in sqft (sq M)] of lawns.

      Nematode species

      Area in sq ft (sq meter)

      1 (0.093)

      10.76 (1)

      108 (10)

      1076 (100)

      2500 (233)

      5000 (465)

      10000 (930)

      43560 (4047) (an acre)

      Heterorhabdtis bacteriophora

      22.9 thousand

      229.6 thousand

      2.3 million

      22.6 million 57.4 million 114.8 million 229.6 million 1.0 billion

      Heterorhabdtis indica

      22.9 thousand

      229.6 thousand

      2.3 million

      22.6 million 57.4 million 114.8 million 229.6 million 1.0 billion

      Steinernema carpocapsae

      22.9 thousand

      229.6 thousand

      2.3 million

      22.6 million 57.4 million 114.8 million 229.6 million 1.0 billion

      Steinernema feltiae

      22.9 thousand

      229.6 thousand

      2.3 million

      22.6 million 57.4 million 114.8 million 229.6 million 1.0 billion
       

    Use entomopathogenic nematodes to control cutworms

    Cutworms: A key pest of turfgrass foliage How to identify cutworms?

    • Adult cutworm moths are yellowish brown or dark gray in color with several black dots on the wings. Full grown cutworm caterpillars are reddish or blackish brown color with greasy look (see below photo of a full grown caterpillar, which was found on turfgrass in my yard) and as shown in the picture, they curled into a C-shaped ball when disturbed (see photo below).
    [caption id="attachment_77" align="alignleft" width="300" caption="Full grown cutworm caterpillar"]Cutworm on turfgrass[/caption]
    [caption id="attachment_79" align="alignleft" width="300" caption="C-shaped cutworm larva"]Cutworm on an ornamental plant[/caption]
    Life cycle of Cutworms:
    • Cutworms develop through four different stages: Egg, caterpillar/larva, pupa and adult moth.
    • Adult moths are emerged from over-wintering pupae early in the spring.
    • Adult moths lay singly over 1200 eggs over several days near the tips of the turf grass leaf blades.
    • Under optimal environmental conditions, eggs hatch within 10 days.
    • After hatching from egg, young larval stage immediately starts feeding on grass foliage and molts (shed its cuticle) five times, go through six stages (instars) of development and depending upon length of season they can complete 3 to 6 generations.
    • In the late summer mature, larvae burrow into ground to form pupa, which survive during winter.
    Damaging stages of cutworms:
    • All six stages/ instars of caterpillars/larvae of cutworms cause damage to grass or other host plants.
    • Adult moths do not cause any type of damage to grasses or other plants.
    How is damage caused?
    • As name implies, at the time of feeding, all the six stages of caterpillars of cutworm cut off leaves or stems to the crown of turf grass. Caterpillars generally feed on grass at night but hide under thatch during day time. The damage caused by cutworms is easily identified because of the presence of circular spots of dead grass on lawns/golf courses. The presence of circular spots of dead grass in the middle of a lawn/golf course can reduce its aesthetic value.
    Biological control of cutworms:
    • Both biological control agents including Bacillus thuringiensis var. kurstaki (a bacterium which produces a toxin and paralyzes the gut of the caterpillar) and entomopathogenic nematodes have a potential to manage cutworms.
    • Entomopathogenic nematodes also recognized as beneficial nematodes have showed promising results in controlling cutworms.
    Why we should use entomopathoegnic nematodes?
    • They can kill cutworm larvae with 48 hours after application.
    • They are commercially available and easy to apply.
    • They are not harmful to children, dogs, cats, personnel involved in its application and beneficial insects like honeybees.
    • They do not need a special permission to apply because they are exempted by EPA.
    Which species of entomopathogenic nematodes are effective against cutworms (see literature below)?
    • Steinernema carpocapsae (see picture on the right side about cutworm caterpillar/larva killed by Steinernema carpocapsae nematodes)
    • Heterorhabdtis bacteriophora  [caption id="attachment_82" align="alignright" width="300" caption="Entomopathogenic nematode, Steinernema carpocapsae killed cutworm caterpillar cadaver in a white trap"]"Entomopathogenic nematode Steinernema carpocapsae infected cutworm cadaver"[/caption]
     What stages of cutworms can be targeted?
    • All six stages of caterpillars/ larvae are susceptible to entomopathogenic nematodes.
    • Pupae are also susceptible to entomopathogenic nematodes.
    What is a recommended dosage of entomopathogenic nematodes required to control cutworms?
    •  For the successful control of most of the soil dwelling insect pests, the optimal rate of 1 billion infective juvenile nematodes in 100 to 260 gallons of water per acre is generally recommended (See Table for appropriate amounts of nematodes required for different sizes of areas to be treated).
    When to apply nematodes
    • To target cutworms, entomopathogenic nematodes should be applied starting in late spring i.e. when young larval (caterpillars) stages (instars) of cutworms are already hatched from eggs (laid in early spring) and started feeding on grass leaves.
    • Since nematodes are very sensitive to UV light, they will die within a minute or two when exposed to full sun. Therefore, nematodes should be applied early in the morning or late in the evening to avoid exposure to UV light.
    • Another advantage of applying nematodes late in the evening is that cutworm caterpillars can be easily targeted because they are generally active and searching for food during night and easily found by entomopathogenic nematodes like Steinernema carpocapsae that uses sit and wait (ambush) strategy to attack its passing by host.  Since cutworm caterpillars are moving actively during night in search of food, they can easily come across to entomopathogenic nematodes like Heterorhabdits bacteriophora that uses cruising strategy to finds its host. These nematodes can also find caterpillars that are hiding under thatch during day time.
    How entomopathogenic nematodes kill cutworms?
    • After application of either Steinernema spp. or Heterorhabditis spp. on the lawns, their their infective juveniles find cutworm larva or pupa and enter into its body cavity through natural openings such as mouth, anus and spiracles.
    • Once infective juveniles of both Steinernema spp. and Heterorhabditis spp are in the insect body cavity, they release several cells of symbiotic bacteria, Xenorhabdus spp. and Photorhabdus spp., respectively from their gut via anus in the insect blood.
    • Insect blood is conducive for the multiplication of symbiotic bacteria. In the blood, multiplying nematode-bacterium complex causes septicemia and kill their insect host usually within 48 h after infection.
    Where can you buy entomopathogenic nematodes?
    • Both Steinernema carpocapsae and Heterorhabdtis bacteriophora nematodes are sold in our store.
    • Steinernema carpocapsae nematodes available both in liquid (sponge-water suspension) and granulated formulations.
    • Heterorhabdtis bacteriophora nematodes available only in liquid (sponge- water suspension) formulations.
    How long does it take to deliver nematodes?
    • We can directly deliver entomopathogenic nematodes at your facility in person (this service available only in Athens,Georgia) or by UPS throughout US within 3 days after receiving order.
    Literature:
    • Ebssa, L. and Koppenhofer, A.M.  2011.   Efficacy and persistence of entomopathogenic nematodes for black cutworm control in turfgrass.   Biocontrol Science and Technology 21: 779-796.