Florence Grovida Gardening

Field monitoring

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Regular crop walking and inspection are essential for the successful prediction of pest and pathogen epidemic development. The crop is inspected at regular intervals to determine whether pest and pathogen problems are emerging. Quantitative evaluations of a developing pest or pathogen problem are gained visually through the manual use of pests and disease assessment keys that quantify developing signs and symptoms. Examples are shown in Figs 7.2 and 7.3 for powdery mildew (E. cruciferarum) on the buds and leaves of Brussels sprout (B. oleracea var. gemmifera) (Dixon, 1976), respectively.

Quantifying a developing pest infestation can utilize basic biological processes of pests to advantage for their control. The first such process is that of their ability to detect odours (smelling) (Cobb, 1999). Insects may be drawn to odours emitted by the crops.

The cabbage seed weevil (Ceutorhynchus assimilis), for example, responds positively to extracts of oilseed rape (B. napus) and to the colour of the crop plants. The active attractant molecules are a-farnesene (component of flowers) and 3-butenyl and 4-pentenyl isothiocyanates (analogous to components of foliage) which are components of Brassica (Evans and Allen-Williams, 1998).

The insect sex drive can be utilized in monitoring through attractive chemicals that females emit to attract males which encourages them into what are termed pheromone traps that can be used to assess insect populations and their likely potential to cause damage to crops. Monitoring the size of pest infestations using traps treated with sex pheromone chemicals to attract males is a well established technique in integrated pest management (Ridgway et al., 1990). The timing of subsequent applications of insecticides in order to gain control depends on the establishment of reliable threshold values with which to predict the onset of damage.

There is frequent use of natural pheromones and their synthetic analogues attractive to male diamond back moth (Plutella xylostella). Factors such as type of trap, trap height, duration of pheromone activity and diurnal patterns of attraction need to be optimized by field studies allowing the establishment of ecologically efficient thresholds for integrated pest control. This approach for diamond back moth (P. xylostella) in crops of cabbage (B. oleracea var. capitata), cauliflower (B. oleracea var. botrytis) and knol khol (B. oleracea var. gongylodes) has been compared with visual monitoring where insecticides are applied as soon as a threshold value of one hole per leaf is reached. Reddy and Guerrero (2001) showed that when eight, 12 and 16 male moths were caught per night in cabbage, cauliflower and knol khol, respectively, this was indicative of the optimum time to spray (Table 7.1). This replaced a routine spraying system of making applications every 7-15 days depending on the risk of pest damage. The use of pheromone-mediated monitoring replaces more costly and often more cumbersome sampling techniques. This work has established a good correlation between numbers of insect larvae, levels of infestation and crop yields with the numbers of adult males attracted to and caught by the pheromone traps.

Fig. 7.3. Brussels sprout leaf powdery mildew (Erysiphe cruciferarum) disease assessment key.