Two beneficial entomopathogenic nematodes for cucurbit fly control / by Ganpati Jagdale

Two beneficial entomopathogenic nematodes for the control of cucurbit fly, Dacus ciliatus

Two beneficial entomopathogenic nematodes including Heterorhabditis bacteriophora (Fig.1) and Steinernema carpocapsae (Fig. 2) have showed a potential to control cucurbit flies, Dacus ciliatus (Kamali et al., 2013). These nematodes are considered as beneficial nematodes because they have been used as biological control agents to control insects that are damaging to crops and harmful to animals. Also, they are known as entomopathogenic nematodes because of the symbiotic bacteria carried by their infective juveniles are pathogenic to their insect hosts.  The bacteria called Photorhabdus luminescens and Xenorhabdus nematophila are carried byinfective juveniles of Heterorhabditis bacteriophora and Steinernema carpocapsae nematodes, respectively.

Heterorhabditis bacteriophora infective juvenile
Heterorhabditis bacteriophora infective juvenile
Infective juvenile of Steinernema carpocapsae
Infective juvenile of Steinernema carpocapsae

What are cucurbit flies?

Cucurbit flies are scientifically known as Dacus ciliatus and belongs to the family Tephritidae in the order, Diptera. These flies are also recognized with other common names as lesser pumpkin flies and Ethiopian fruit flies. The cucurbit flies are true flies because they have one pair of well developed forewings that are used for flight and a pair of rudimentary hind wings called halters. As name implies, cucurbit flies mainly damage cucurbitaceous crops including, cucumbers, melons, pumpkins and squashes. Adult flies are small about 5-7 mm long with dark brown colored body and presence of yellow markings on their thorax. Females lay 8-10 shiny-white eggs below the skin of host fruit. These eggs hatch within a couple of days into small larvae called maggots. These maggots then start feeding inside the fruit and mature within 5-6 days. The matured larvae/ maggots move in the soil under host the plants for pupation. Adult flies emerges from pupae within 2-3 weeks and life cycle continues.

How Dacus ciliatus cause damage to cucurbit fruits?

During egg laying activity (oviposition) cucurbit flies injures fruits. The eggs laid inside the fruit hatch into small larvae also called maggots, which move inside the fruit for feeding. These oviposition and feeding injuries cause fruits to deform. In addition, cucurbit fruits may rot due to entry by either pathogenic or saprophytic fungi and/or bacteria through injuries.

Biological control of Cucurbit flies.

Several chemical pesticides may be effective in controlling cucurbit flies, but their use on cucurbitaceous crops is restricted due their detrimental effects on human health and the environment.  Two beneficial entomopathogenic nematodes including Heterorhabditis bacteriophora and Steinernema carpocapsae can be used as biological control agents to manage populations of cucurbit flies.

What are Heterorhabditis bacteriophora nematodes?

Beneficial entomopathogenic Heterorhabditis bacteriophora nematodes are microscopic thread like organisms that are known for their ability to parasitize and kill many unwanted insect pests that are damaging to many crops including cucurbits (Fig. 1). Beneficial entomopathogenic Heterorhabditis bacteriophora nematodes belong to family Heterorhabditidae and complete their most of the life cycle inside the insect cadavers (that are infected and killed by them). However, the infective juvenile stage of this nematodes is the only stage that found in the soil and tolerant to many harsh environmental factors including extreme hot and cold temperatures. These infective juveniles always carry symbiotic bacteria called Photorhabdus luminescens in their guts and use as weapons to kill their insect hosts. The infective juveniles of these nematodes actively move in search of both immobile and mobile insects throughout the soil profile. This behaviour of searching of their insect hosts is called as  “cruise foraging” strategy in which infective juveniles use carbon dioxide released by insects as cues. Beneficial entomopathogenic Heterorhabditis bacteriophora nematodes are considered as warm temperature adapted nematodes because they are more effective in killing insect pests in the field when temperature is above 20oC (68oF).

What are Steinernema carpocapsae nematodes?

Beneficial entomopathogenic Steinernema carpocapsae nematodes are also microscopic thread like organisms that are known for their ability to parasitize and kill many unwanted insect pests that are damaging to many crops including cucurbits (Fig. 2). Steinernema carpocapsae nematodes are belong to family Steinernematidae and also complete their most of the life cycle in the insect cadavers (that are infected and killed by them). However, the infective juvenile stage of this nematode is the only stage that found in the soil and tolerant to many harsh environmental factors including extreme hot and cold temperatures. These infective juveniles always carry symbiotic bacteria called Xenorhabdus nematophila in their guts and use as weapons to kill their insect hosts.  Unlike Heterorhabditis bacteriophora nematodes, Steinernema carpocapsae nematodes uses “ambush foraging” strategy in which the infective juveniles sit-and-wait for attacking and infecting passing by insect hosts. The infective juveniles of Beneficial entomopathogenic Steinernema carpocapsae nematodes are known to “nictate” meaning they can stand on their tail in an upright position and attack highly mobile insects like billbugs, sod webworms, cutworms and armyworms. This nematode can also find and kill its host beneath soil surface. Steinernema carpocapsae nematodes are active at many temperatures but most effective against many insect pests at temperatures ranging from 22 to 28°C (72°F - 82°F).

How Heterorhabditis bacteriophora and Steinernema carpocapsae nematodes can control cucurbit flies?

Since mature larvae of cucurbit flies move in the soil for pupation both mature larvae and pupae can be targeted with beneficial entomopathogenic nematodes. When the infective juveniles of both Heterorhabditis bacteriophora and Steinernema carpocapsae are applied to the soil surface in the fields, they start searching for the appropriate stage of their insect hosts such as the larvae/ maggots and pupae of cucurbit flies. Once maggots and/or pupae are located, the beneficial nematode infective juveniles penetrates into the larval or pupal body cavity via natural openings such as mouth, anus and spiracles. In the body cavity, infective juveniles release symbiotic bacteria (see above) from their gut in insect blood where multiplying nematode-bacterium complex can cause septicemia and kill larvae and/or pupae of cucurbit flies usually within 24-48 hours after infection.

Recently, Kamali et al (2013) demonstrated that both Heterorhabditis bacteriophora and Steinernema carpocapsae nematodes were very effective in killing and larvae and adults of cucurbit flies but comparatively less effective in killing their pupae.  In another study, Hussein et al (2006) showed that the Heterorhabditis bacteriophora nematodes were effective against both larvae and pupae of cucurbit flies.

Read following research papers for interaction between beneficial nematodes and cucurbit flies.

  1. Hussein, M.A., El-Wakeil, N. and El-Sebai, T. 2006. Susceptibility of melon fruit fly, Dacus ciliatus, to entomopathogenic nematodes (Rhabditida) and to insecticides.  The International journal of Nematology 16:13-18.
  2. Kamali, S., Karimi, J., Hosseini, M. Campos-Herrera, R.  and Duncan, L.W.  2013. Biocontrol potential of the entomopathogenic nematodes Heterorhabditis bacteriophora and Steinernema carpocapsae on cucurbit fly, Dacus ciliatus (Diptera: Tephritidae). Biocontrol Science and Technology 23: 1307-1323.