Agriculture Reference
In-Depth Information
among its seed stalks. The insect may overwinter as an adult or a pupa, and
control is aided by deep ploughing to bury and destroy the pupae, or by
frequent cultivation to expose them to predators.
Adults become active in egg-laying when mean temperatures exceed
10-12°C. In France, where the moth is a severe pest, up to five generations
occur per year in the south, and systems for integrated control have been
developed. The number of generations per year depends on temperature, with
three being normal in Germany and just two in the UK. Models based on day-
degrees can predict rate of development, and hence when egg-laying adults
will be flying. Adult emergence and numbers are also monitored using traps
baited with (2)-11-hexadacanal. This is a volatile chemical (pheromone)
produced by the female moth that attracts males. The information gathered is
used to time the application of insecticides in relation to when the moths are
laying eggs. In Germany, two applications of deltrathrin, a synthetic pyrethroid
insecticide with some residual activity, applied 3 weeks after the beginning of
egg-laying, gave excellent control (Richter and Hommes, 2003).
Studies of behaviour and the electrical responses of antennae on leek moth
adults exposed to the volatiles derived from the alk(en)yl-cysteine sulfoxide
precursors found in leeks and other alliums (see Chapter 8) have shown that
the moths locate allium plants for their egg-laying using these olfactory signals
(Dugravot and Thibout, 2006). When leeks are attacked by leek moth larvae,
they increase the quantity of the alk(en)yl-cysteine sulfoxide precursors
produced and, as a result, more volatiles are produced by leeks subject to leek
moth attack than by plants that are simply injured mechanically (Dugravot
et
al.
, 2005).
One of these volatiles, dimethyl disulfide (DMDS), has been shown to be a
powerful neurotoxin for non-adapted insects (Dugravot
et al.
, 2003), and the
sulfur volatiles function as a defence against non-adapted plant-feeding insects.
For leek moth larvae, however, the higher level of volatiles in leeks under their
attack is not lethal, but it does lead to a slower development rate in males and
about a 20% reduction in egg-laying potential in females (Dugravot and Thibout,
2006). Furthermore, DMDS is a powerful attractant for the wasp
Diadromus
pulchellus
, which is the main parasitoid of leek moth larvae. DMDS is produced by
particular bacteria in the frass of leek moth larvae (Thibout
et al.
, 1995). As a
consequence, the parasitic wasps are more strongly attracted to leeks damaged
by leek moth feeding than to undamaged or mechanically damaged leeks. These
findings suggest a complex defence system against leek moth involving four
trophic levels: the plant, the pest, the frass bacteria and the parasitoid. The
increase in volatile precursor production following the pest attack directly
reduces the reproductive capacity of the developing larvae, but it also has an
indirect effect in attracting more parasitoid wasps. The combined effect may be to
reduce the numbers of second-generation pests (Dugravot and Thibout, 2006).
The leek moth is one of the 20 species of moths - including several cutworms
- listed as allium pests by Soni and Ellis (1990). In tropical regions the beet army
