Pheromone-based monitoring system to manage the millet stem borer Coniesta ignefusalis (Lepidoptera: Pyralidae)

Introduction

The millet stem borer, Coniesta ignefusalis (Hampson), is a damaging pest of pearl millet [Pennisetum glaucum (L) R. Brown] in the Sahelian and subSahelian zones of Africa from Senegal to Sudan. There are usually three generations of the pest in a year in the wetter area (eg. Nigeria), and two, occasionally three in the drier regions (eg. Niger) in Africa.

The damage is caused by the larvae feeding on the stem (Figure 1).

Figure 1. Active full-grown millet stem borer larva (feeding and damage) in millet stem.

Towards the end of the rainy season, the larvae enter into diapause (Figure 2) in the stems and stubbles and survive until the following rainy season.

Figure 2. Diapause larva in dry millet stem.

The ICRISAT information Bulletin "Coniesta ignefusalis: a hand book of information" by O.Youm, K M Harris, and K F Nwanze published in 1996 provides a thorough review of the biology, distribution, damage potential of the millet stem borer, and measures for controlling it as well as an annotated bibliography on the species and its control.


"What are pheromones?

Pheromones are chemical messages produced by one member of a species, which produce a specific response in another member of the same species. In mammals they may be used for territorial marking, eg. when a cat rubs the corner of its mouth against its owner's leg, it is marking him/her as part of its "territory" with secretions from a scent gland.

There are also sex pheromones, which have attracted the most interest especially in Lepidoptera (Butterflies and moths).

Sex pheromones are emitted by an individual that is ready to mate (usually the female) to attract members of opposite sex.

Millet Stem Borer Pheromone: Identification and Field Trials

Collaborative work between scientists of ICRISAT-Niamey and the Natural Resources Institute (UK) found five biologically active compounds in hexane washings of female millet stem borer pheromone glands. A blend of three of these-two alcohols and aldehyde-was shown to be more attractive than virgin female moths in field tests in Niger. Further work showed that the most attractive blend contained the three compounds 100:5:3.3 ratio. (Figure 3) Addition of either or both of other two compounds reduced the attractiveness. Click here for Technical Details of Pheromone and Lure Production

Figure 3. Chemical structure of components of the millet stem borer pheromone.

Pheromone Traps

Experiments have been conducted at ICRISAT-Niamey, Niger, to compare the effect of various trap designs and trapping surfaces on the capture of millet stem borer moths with artificial pheromone lures. A water-based trap has been proved to be the most efficient and cost-effective (Figure 4).

Figure 4. Pheromone trap with catches set up in a millet field.

The measurements given below provided optimal trapping efficiency. Deviating significantly from this design (even in such details as the proportion of the water surface covered by the trap lid) is likely to result in reduced catches between different areas. The trap system described below was tested through the West and Central African Millet Research Network in Benin, Burkina Faso, Gambia, Ghana, Mali, Niger, Nigeria and Senegal, and Togo. It has worked well in all the areas in which it has been tested, suggesting that there are unlikely to be separate geographic races of millet stem borer, which would have required the development of different pheromone blends.



How to make traps?

The trap consists of an aluminum tray 32 cm in diameter, 3-4 cm depth, and filled with water of depth of 2 cm. Four holes of 3 mm of diameter (too small to allow the passage of trapped moths) are drilled in the side of the tray, 1 cm below the lip, to allow rainwater to drain away without losing the captured moths from the trap. Suspended 2-5 cm above the tray is a lid (21 cm in diameter), which acts to shade the pheromone lure and water surface to reduce evaporation (see figure 4). It also increases the likelihood of moths approaching the lure and contacting the water surface. Larger lids appear to inhibit moth entry into the trap, and significantly smaller ones allow the moths to avoid contacting the water.

The addition of 10-20 mL of standard motor oil (new or used) to the water improves the catch by reducing the surface tension of the water and also serve to reduce evaporation. If motor oil is not available, a few milliliters of detergent or a bar of domestic soap agitated in the water for about 1min has the same effect on moth captures.
Vegetable oil is not effective. Plastic water trays have a limited life in the high light intensity and high temperatures of the Sahelian zone.

A stable base greatly improves the reliability of the trap. A wooden shelf to hold the water tray, supported on a 5 cm x 2 cm wooden stake driven into the soil is ideal. The trap lid is suspended by wire from an arm at the top of the stake and anchored to the trap tray by wire drawn through three holes drilled around the tray.


How to make Lures?

The technical details of the pheromone and the procedure for making lures are described in Youm et al. (1998)

The artificial pheromone attractant is a mixture of three components plus an antioxidant, which protects the pheromone from being degraded by heat and light. It has been shown that 0.5mg of the active pheromone ingredient per lure with 0.5mg of the antioxidant is optimal. As with most lepidopterian pheromones, the precise proportion of each of the three components in the mixture is important for optimal attractiveness. In the case of millet stem borer, the optimal ratio is 100:5:3.3. A solution of active pheromone ingredient and the antioxidant is injected into a polythene dispenser where it is absorbed into the walls. This lure is then hung in trap to attract male stem borers.


How to Use Lures?

The lure is suspended from a piece of wire directly under the center of the trap lid and above the water. Lures can be stored in glass and metal containers (including aluminum foil) in a domestic refrigerator (4°C) for several months or in a freezer indefinitely. There is no health hazard involved in storing lures.

Where and when to Place Traps?

Millet stem borer moths fly and mate close to the ground. Although trap height is not a critical factor, a lure height of not more than 0.5 m from the ground appears to be optimal, whatever the height of the surrounding crop. Larvae diapause is broken by the first rains, adequate moisture, and an appropriate day length. Traps should, therefore, be placed in the field no later than 10 days after the sowing rains begin. If traps are placed in the field before the emergence of the young millet plant and cattle are allowed to graze in the field, care should be taken to ensure that they do not drink the water in the traps.
Comparisons of mouth densities at different locations will be more reliable if traps are located within the crop, at least 2 m from the edge of the field. Where traps are to be used only for monitoring, a single trap should be placed within the millet field at least 20 m from the edge.

Servicing Traps

The lure continues to release pheromone for at least 6 weeks, but the attractiveness decreases significantly after 4 weeks. For consistently high attractivity, it is recommended that lures be replaced 2 weeks. The water and soap/oil will need to be replaced more frequently, depending on the evaporation rate at the particular site and the time of year. Traps should be examined at least twice a week and the catch counted. Old lures should either be returned to the supplier for refilling or disposed of as domestic waste. They should not be discarded close to traps where they may interfere with the trapping efficiency.


Identifying Trap Catches

As the pheromone is species-specific, the trap catch should almost consist entirely of millet stem borer. Therefore, sorting the catch is not a problem as it is with light traps or bait traps. Complete illustrated descriptions of all life stage of millet stem borer are given in the NRI publication "Ennemis du mil au Sahel" and the ICRISAT Bulletin "Coniesta ignefusalis: a handbook of information".

The moth (Fig. 5) has a wingspan of 22-30mn and a wing length of 8-15mm. The female is usually larger than the male. The forewings are straw yellow. The upper surface of the hind wings is satin white. Occasionally, a very small number of catches of other species may occur. Among the other species that might be caught are Helicoverpa armigera (Hübner) and Spodoptera littoralis (Boisduval), which are much larger and have dark patterned wings. The millet head miner moth (Heliocheilis albipunctella de Joannis) is of a size and color similar to Coniesta ignefusalis but the fringed forewings have streaks of white and white and the hindwings are gray rather than white. In addition, the forewings of male have two transparent "windows", absent in C. ignefusalis. In Southern and Eastern Africa confusion with Chilo partellus is also possible; however, there is no evidence of C. ignefusalis occurrence in these regions.

Figure 5. Adult female of the millet stem borer on millet (resting position).


Research in Progress and Future Prospects

Pheromone traps for monitoring the millet stem borer have proved effective in at least 9 West Africa countries in which they have been used to date. This regional monitoring is expected to cover more countries of the West and Central African Millet- Research Network. Trials have also been undertaken to explore the potential for controlling millet stem borer population by mass trapping and by mating disruption.

Traps arrayed around farmers' granaries have shown that damage in terms of infested hills and trapping close to these storage structures can reduce percentage of dead hearts. This reduction in infested hills and dead hearts is possibly due to the reduced population of males through mass trapping. Granaries made of millet stems infested by millet stem borer larvae in diapause are a major source of infestation of young millet of the succeeding season. Further work is in progress to determine whether or not the application of this method is economically viable.

Mating disruption was assessed at sadoré near ICRISAT-Niamey in 1993. Trap catches in treated plots were significantly suppressed, suggesting that sexual communication was being disrupted. Further trials with longer-lasting dispensers were carried out. Though dispensers lasting more than 6 week were developed, the dry Sahelian zones require more stable and longer lasting dispensers for use in mating disruption.

Two of the five components of the millet stem borer pheromone inhibit male approaches to traps. It may be possible in the future to use these "inhibitors" to disrupt natural sexual communication.

The identification and availability of the sex pheromone of the millet stem borer provides a valuable opportunity to monitor insect activity and time control measures to reduce insect populations and damage. In future, it may be possible to use artificial pheromone to control the pest directly in an environmentally desirable way. When integrated with the use of tolerant varieties and cultural control practices such as crop-residue management, this may finally be the opportunity to practice full and effective integrated management against this key pest of pearl millet in the Sahelian and Sudanian zones. On-farm economic benefits for using pheromone-based control and monitoring of the millet stem borer needs to be assessed in the future for wide scale application of the technique.


Acknowledgements:

Funding for this research was partially provided by DFID, AFDB, and IFAD.

References:

Youm O, Harris KM and Nwanze KF. 1996. Coniesta ignefusalis (Hampson), the millet stem borer: a handbook of information. (In En. Summaries in En, Fr, Es.) Information Bulletin, no 46. Patancheru 502324, Andhra Pradesh, India: International Crops Research Institute for the Semi-Arid Tropics. 60 pp. [Part1: Review pp. 1-18; Part 2: Annotated bibliography pp. 19-52] ISBN 92-9066-253-0

Youm O, Russell D and Hall DR. 1998. Use of pheromone traps for monitoring the millet stem borer (Coniesta ignefusalis). (In En. Summaries in En, Fr, Es.) Information Bulletin No. 40. Patancheru 502324, Andhra Pradesh, India: International Crops Research Institute for the Semi-Arid Tropics, and Central Avenue, Chatham Maritime, Kent ME4 4TB, UK: Natural Resources Institute. 20 pp. ISBN 92-9066-290-5. IBE040.

For more information please contact:

Dr Ousmane Youm
Principal Scientist (Entomology),
ICRISAT-Patancheru 502 324.