Abstract

Yellow sticky traps may be an alternative monitoring tool to monitoring plants for assessing Plutella xylostella (Lepidoptera: Plutellidae) in Brassica vegetable crops (Sivapragasam & Saito 1986, Hallett et al. 1993). Previous research has shown a significant relationship between the number of moths on sticky traps and total larvae or pupae in the crop two weeks after the trap catch. A trial was designed to test if commercial yellow sticky traps could be used as an indicator for the early instars of P. xylostella in the Brassica vegetable crops in Queensland. Predicting hatching eggs and small larvae enables effective targeting of pesticide applications. Results indicate some relationships exist, however practical use may be considered costly.

Material and methods

Successive crops of broccoli were planted monthly, for 12 months. Each planting was 600 m² and divided into six plots. A yellow sticky trap was placed in the middle of each plot, making a trapping density of one trap/100 m². Traps were positioned just above crop height and adjusted as the crop grew.

The number of P. xylostella moths per trap was recorded weekly. Twice weekly, five plants were randomly chosen from within each plot and the number of eggs, each larval instar and pupae were recorded.

For analyses, trap and plant monitoring data were bulked across the plant ages thereby treating the area as a single population sample for any one date. The relationship between same day trap catches and plant monitoring events were investigated, as well as with subsequent plant monitoring events. Data were transformed using sqrt(x+0.5). Spearman’s ranked correlation test was used for the analysis.

For comparing the costs of each style of monitoring, trap monitoring was based on 30 seconds to count moths on traps and replace the trap and the commercial cost of $1 per trap was used. For plant monitoring commercial consultant rates of $45/hour were used.

Results and discussion

Similar to Hallett et al. (1993), a significant relationship existed between the number of moths on traps and larvae in the crop, as well as eggs. However by comparison, the strongest relationship with eggs and small larvae was found 0-3 days after the trap count and up to 6-8 days for larger larvae (P<0.05) (Figures 1 and 2). These relationships were found at a lower trapping density of 100 traps/ha (1/100 m²), compared with 400 traps/ha (1/25 m²) used by Hallett et al. (1993). Further analyses showed no significant relationship at a trap density lower than 100 traps/ha.

a)b)

Figure 1. Significant relationships between P. xylostella adults on traps and a) eggs and b) II instar larvae 0-3 days after trap catches. Data transformed using sqrt(x+0.5).

This relationship can be taken a step further to consider the implications of using traps and their effect on correct pest management decisions. It assumes that current practices based on using an action threshold of 1.1 larvae per plant (Figure 2) for plant monitoring, provide acceptable yields. Back calculating, this gives a moth count of 1.3 moths per trap as the trap action threshold. Using these thresholds, sticky trap and plant monitoring result in the same spray decision 57% of the time. On 11% of monitoring occasions however, the sticky traps suggest no spray action while plant monitoring suggests spraying; whilst 14% of occasions the reverse was true (Figure 2).

Figure 2. Significant relationships between P. xylostella adults on traps and III to IV instar larvae 6-8 days after trap catches. Data transformed using sqrt(x+0.5).

The other practical consideration for growers is the cost of sticky trap monitoring compared with plant monitoring. Current commercial cost of traps and plant monitoring in Australia (Table 1), mean traps may only be considered for use at certain times in the cropping period or be more suitable for small cropping units. However, using the same trap for two consecutive weeks can create savings. Commercial sticky traps have two sticky sides. This means one side can be uncovered in the first week and then in the second week, after the number of moths is recorded on the first side, the trap can be turned over and the other side be uncovered and used to catch moths in the second week. By using this method, the cost comes down closer to the price of not monitoring at all, $90/ha. Given the level of accuracy in the trap action thresholds discussed previously, this cost saving may not be significant for a grower choosing to establish traps rather than not monitoring at all.

Table 1. Cost per monitoring event of different monitoring methods for DBM

a Current commercial practice of 10-20 plants inspected per planting
b Assumes 1-2 extra sprays/ha used

At this stage, a relationship between sticky trap catches and actual plant counts has only been investigated for P. xylostella. This means in locations where there is a pest complex, for example where other Lepidopteran pests or aphids and thrips need to be considered, it is more cost effective for a grower to use plant monitoring than trapping.

Despite the apparent shortfalls of yellow sticky traps, they are still used at a low trapping density by crop consultants. They serve as a tool that provides complementary information to plant monitoring, indicating the pattern of pest abundance relative to the previous week as well as a measure of some beneficial insect activity.

Acknowledgements

I would like to acknowledge the invaluable assistance of Janet Giles, biometrician, for her advice on data analysis.

References