3.1. Crop Loss Assessment Techniques
There are several approaches for estimating damage and yield losses in millets. One method, which has been successfully used in West Africa, involves the caging of individual panicles or whole plant stands into which a known number of insects are introduced (Krall et al., 1995). Apart from providing direct quantitative information on pest damage, this method can also be used to study pest activity. However, field trials and extensive surveys are necessary to obtain data on the extent of crop damage and yield losses. The crop loss assessment techniques can be classified as follows (Nwanze, 1988, quoted by Krall et al., 1995):
(a) Incidence ratio
(b) Visual score paired analysis
(c) Damage density loss ratio
(d) Quantitative assessment (insecticide trials).
Grain size for many millets is relatively very small compared to other grains, and this tends to reduce susceptibility to infestation by storage insects, but to increase the likelihood of loss by spillage and seepage from sacks, and also by contamination by sand or other foreign matter. Therefore, there are possibilities of difficulties of assessing percentage grain damage and consequent weight loss (McFarlane, 1995). Grain cleaning by sieving and winnowing may also be less efficient and more wasteful.
Damage to millet heads caused by the developing head miner larvae is very typical and is characterized by the presence of spiral miners developed by larvae during feeding. In mines caused by full grown larvae, there is a typical presence of white faecal pellets. Damage due to young larvae results from their feeding in the florets (floral glumes) initially, and this type of damage in not easy to detect unless the developing panicle is very carefully examined. The damage due to late instars can be easily detected because they feed on the base of the flowers or florets, which fall out, leaving the mines open. The early phase of this damage ca be detected by the uneven surface of the millet head as the developing larvae push florets/seeds from the rachis. (Youm, 1995).
In contrast, the bio-ecology of the scarab beetle Rhinyptia infuscata until recently is usually poorly documented due to the nocturnal feeding habits of this insect. Although Rhinyptia infuscata has been reported to attack millet in Niger (Guevremont, 1981 quoted by Youm, 1995) and in Senegal (Gahukar and Pierrard, 1983, quoted by Youm, 1995), it was not, until recently, that its pest status was recognized. Its population density has been reported to be about 500 000 adults per ha. The damage due to R. infuscata is more difficult to describe due to its nocturnal feeding habits and similarities with damages caused by other panicle feeding insects. The beetle is reported to feed on the stigmas of millet, resulting in empty spikelet (ICRISAT, 1990, quoted by Youm, 1995). It also feeds on flowers, often resulting in empty glumes.