Agriculture Reference
In-Depth Information
length. The fully grown larvae migrate to a depth of about 6 cm in the soil,
where they develop into brown pupae. Under both low winter temperatures and
high summer temperatures the insect can enter a state of developmental arrest,
termed diapause, in which it is able survive extremely low or high temperatures.
Low temperatures and short photoperiods or high temperatures and long
photoperiods, respectively, initiate the sequence of physiological changes that
lead to winter or summer diapause (Bin Chen
et al
., 2005). The pupae remain in
diapause during winter or during hot summer periods, following which
development resumes and adults emerge to mate and lay the eggs of the next
generation.
In spring, the year's first-generation adults emerge in fields that grew the
previous season's allium crops, and they must locate the new season's allium
fields to lay their eggs (oviposit) near host plants suitable for larval develop-
ment. Initial dispersal appears to be random in direction but, after a period of
reproductive maturation, the flies become responsive to volatile chemicals
characteristic of alliums (see Chapter 8). At this stage the flies are highly
attracted to dipropyl-disulfide (Pr
2
S
2
), one of the volatiles that onions can
produce, and they will fly upwind towards traps baited with Pr
2
S
2
. Electrical
signals detectable from onion fly antennae when exposed to Pr
2
S
2
indicate that
it is a strong olfactory stimulant (Gouinguene
et al
., 2005).
Having located the source area for the odour they use visual cues to land
preferentially on vertical, green, cylindrical surfaces that mimic the form of
allium plants. These responses suggest that ovipositional host-finding by female
onion flies is dominated by olfactory responses at long range and by visual cues
at short range (c. 1 m) (Judd and Borden, 1991). Having alighted, females
exhibit different behaviours on different-shaped surfaces. On narrow (8 mm
diameter) vertical cylinders they show a characteristic pattern of movement and
frequently go on to oviposit near the base of the cylinder. The number of eggs laid
near the base diminishes with increase or decrease in cylinder diameter and
when the cylinder/substrate angle deviates from 90° (Harris and Miller, 1984).
Female flies oviposit maximally in a substrate at about 20°C, a temperature that
also favours egg survival and development (Keller and Miller, 1990).
Insecticides applied in the furrow when seeds are sown can give very effective
control of onion fly. Ritcey
et al
. (1991) showed that an in-furrow active ingredient
rate of 2.2 kg/ha of organophosphorus insecticides - among them chlorpyrifos
and fonofos - was sufficient to control the first- and second-generation onion
maggots on organic soils in Ontario, Canada. The insecticides were absorbed by
onion seedlings but were concentrated in the roots and skin and, by the time of
bulb harvest, insecticide residues in the bulbs were undetectable.
Similar levels of control have been achieved with even less active ingredient
(AI) per ha by film-coating seeds with insecticide before sowing. Experiments
on organic soils in New York State, USA showed that film-coating onion seeds
with cyromazine at a rate of 50 g AI/kg seed reduced seedling losses from onion
fly to negligible levels (Taylor
et al
., 2001). By applying the insecticide as a
