Insecticides
4.5 The Target and Transfer of Insecticide
4.5.3 Factors influencing the target and pick-up
An insecticide can only control an insect pest if it is applied in such a way that it reaches its intended insect target. This tar- get can be the eggs, nymphs, larvae, pupae or adults, depending on which are the most harmful stages and the ease with which they can be targeted. The different life cycle stages of an insect vary in behaviour and habitat use which affects their ‘trans- parency’ to the insecticide, that is, their presence at a site that can be usefully treated with an insecticide. For instance, boring stages of an insect will be more diffi-
cult to target with an insecticide than foliar feeders. Thus, there is a need to understand the basic biology, resource use and behav- iour of a pest insect before an insecticide can be applied effectively (Fig. 4.8).
Insecticides can be transferred to their target insects by direct interception, by gaseous or contact transfer or through ingestion of treated material. Direct inter- ception of droplets by a target insect can occur by either impaction or sedimentation and is dependent on the insect’s size and shape as well as droplet size and density.
Different parts of an insect will be more likely to intercept droplets than others, so that the surface distribution of droplets will depend on the morphology of the insect. In flying locusts the area on which sedimentation and impaction occurred was calculated using a measurement referred to as the horizontal equivalent area (HEA;
Wootten and Sawyer, 1954). When only large droplets and the process of sedimen- tation on the horizontal plane of the insect is involved, then the HEA was defined as the horizontal plane area which, when passed through a curtain of droplets of a given diameter at the same velocity of a locust, collected the same number of droplets as the locust. However, droplets are also transferred by impaction as the locust flies into aspray and hence the HEA by definition is the horizontal area which collects by sedimentation alone the same number of droplets as the locust collects by sedimentation and impaction. An approxi- mate allowance for impaction was made by adding to the area of the locust, the pro- jected horizontal area of the surfaces on which impaction takes place, and then treating the problem as one of sedimenta- tion only (e.g. Fig. 4.9; Wootten and Sawyer, 1954). The HEA was greater for small droplets than for large and was also influenced by the air speed of the locust (Fig. 4.10). A change in air speed from 3 to 5 m s21(Fig. 4.10) added about 10% to the HEA for larger droplets and about 80% for smaller ones; that is, as the locust increases its air speed the importance of pick-up by impaction of small droplets increases dra- Fig. 4.6. The relationship between toxicity (1, 10,
100, 1000; LD50ng per insect), droplet diameter and the concentration of a.i. for one droplet to contain an LD50dose (Johnstone, 1973).
Fig. 4.7. The relationship between the LD50(4 days post-treatment) of dicofol and the concentration applied against the red spider mite Tetranychus urticae on the adaxial surface of beans, for a range of droplet sizes (after Munthali and Scopes, 1982).
matically. The impaction of droplets occurred mainly on the wings of the locust and on the head region; this in association with a more even distribution of deposit was thought to be responsible for the observed toxicity response.
Insects such as locusts will readily intercept droplets because they are rela- tively large and mobile, so that if they are within a spray area their chances of inter- cepting an insecticide will be increased the more they fly around. The situation with small, sessile life stages of insects is com-
pletely different. In such cases the proba- bility of interception is entirely dependent on the location of the insect and its size in relation to the size and density of droplets.
When difocol was applied to the adaxial leaf surface of bean plants with an in-flight diameter of 53 µm and density of 300 drops cm22, only 10% of eggs of the red spider mite were directly hit by spray droplets (Munthali and Scopes, 1982).
Small insects such as aphids, mites or whitefly may be less susceptible to direct interception of insecticide droplets than
larger insects but because of their small size they may be more susceptible to a gaseous form of insecticide transfer.
Insecticides that have a high vapour pres- sure (>1022Pa), such as demeton-S-methyl, dichlorvos and phorate may, in conditions of still air, saturate the air of the boundary layer with toxic vapours. Small insects that are within the boundary layer may receive a lethal dose of an insecticide by gaseous transfer, while larger insects that have a significant portion of their bodies above the boundary layer are less likely to succumb.
Ford and Salt (1987) considered the effec- tiveness of gaseous transfer to be depen- dent on the insect body size, its location and behaviour (sessile insects would be most susceptible), the air speed at the plant surface, vapour pressure and the toxicity of the active ingredient. However, there are
few insecticides which have high enough vapour pressures at appropriate tempera- tures for this method of transfer to be gen- erally applicable; the main processes of transfer of insecticides involve direct con- tact and ingestion by insects.
Contact insecticides are only appropri- ate against insects that are sufficiently mobile to ensure they come into contact with the insecticide deposit. Contact with the insecticide deposit will depend on droplet size and density. In addition, the proportion of the insect that comes into contact with the deposit and the extent of movement over treated areas will also influence insecticide transfer.
The size and nature (e.g. oils) of the deposit will affect the transfer of insecti- cide to the insect. While large particles may be more readily dislodged from an Fig. 4.8. Factors that could influence the location of insects on their hosts.
Fig. 4.9. The estimation of the horizontal equivalent (HE) area (HEA) of a locust: (a) variation of HE of a vertical measurement projected at various angles of approach of droplets; (b) horizontally projected area; (c) vertical areas presented to droplets (after Wootten and Sawyer, 1954).
insect cuticle once picked up, small parti- cles which can penetrate the insect cuticle more effectively tend to adhere to a surface making transfer to the insect more difficult (Ford and Salt, 1987). The pick-up and retention of DDT crystals was described for the tsetse fly, Glossina palpalis, by Hadaway and Barlow (1950). Particles were picked up on the legs and ventral surface of the abdomen. The small particles were transferred from the legs to the antennae, head and other body parts as the flies cleaned themselves. The few larger crystals picked up by the insects were easily removed from the body during cleaning and other movements. Particles were picked up on the legs and ventral aspect of the abdomen as these were the parts of the insect that came into contact with the treated surface. Insects that are not capable of flight or larvae that have a large sur- face/contact area and spend a large amount
of time crawling or walking across treated surfaces are most susceptible to this form of insecticide transfer, e.g. lepidopterous larvae. The behaviour of the insect will also affect the likelihood of contact with insecticides. However, insect behaviour may be influenced by the presence of insecticide deposits (Head et al., 1995).
Spodoptera littoralismoves 45 times faster when walking than when feeding (Salt and Ford, 1984) and this difference could account for the proportion of contacted deposit that is transferred to the insect (Ford and Salt, 1987).
The feeding behaviour of the insect may affect the uptake of a stomach poison applied to the food of the pest insect. The insecticide must be applied to a site on which the insect feeds, the coverage must be sufficient for a lethal dose to be ingested, repellancy and vomiting of the insecticide must be avoided, and the deposit size should be small enough to be eaten by the insect. Wettable powder deposits tend to remain proud of the leaf surfaces and hence are less likely to be ingested than emulsifiable concentrates that sometimes mix with leaf epicuticular waxes (Ford and Salt, 1987).
The behaviour of target insects must always be studied, in some instances it reveals unexpected results and invaluable information. A study of particle size and bait position on the effectiveness of boric acid and sodium borate to control the American cockroach, Periplaneta ameri- cana, (Scriven and Meloan, 1986) provided a possible explanation for variable results found previously by other workers. The cockroaches do not intentionally feed on the boric acid or borate but if it is provided at an appropriate particle size (< mesh size 80) and placed in positions where normal behavioural patterns take the cockroaches (such as along edges and corners) then the fine powder is picked up by their bodies.
However, the insecticidal effect only occurs when the cockroaches ingest the particles as they clean themselves. The results of experiments depend on where the compounds are placed and the particle Fig. 4.10. The horizontal equivalent area (HEA) as a
function of droplet size at locust flight speeds of 3 m s21(– – – –) and 5 m s21(––––) (after Wootten and Sawyer, 1954).
size distribution of the powder (Scriven and Meloan, 1986).
A knowledge of insect behaviour is imperative if insecticide targeting is to be improved (Fluckiger, 1989; Adams and Hall, 1990). This knowledge combined with optimal placement, density and droplet size of insecticides provides a means by which more effective transfer can be achieved (Adams and Hall, 1989). An understanding of these interactions can then be used to determine the most effec- tive formulations and to modify spray char- acteristics and application procedures (Fig.
4.11). This approach is likely to gain favour over empirical observation made during field trials as new techniques lead to a bet- ter understanding of processes underlying the field performance of insecticides (Ford and Salt, 1987).