Relationship between Adult Pheromone Trap Catches and Injury in Soybean Leaves by Young Larvae of Spodoptera litura F. (Lepidoptera: Noctuidae).

Abstract

Relationships between seasonal fluctuations in pheromone trap catches of Spodoptera litura F. (Lepidoptera: Noctuidae) and the number of soybean leaves newly injured by young larvae of this species were examined in soybean fields from 1999 to 2002 in Saga Pref., Japan. In soybean fields sown in July, a small peak of injured leaves occurred in mid to late August and a large peak occurred in mid to late September. In most cases, the pheromone trap catches and occurrence of injured leaves exhibited similar trends with both peaks occurring almost simultaneously (Ϯ3 days) in mid to late September. This suggests that not only a prominent increase in soybean leaves injured by young larvae, but also a prominent increase in pheromone trap catches are useful information to determine a precise timing of control. On the other hand, the timing of control seems to be difficult to predict using the effective accumulative temperature of this species.

Dispersal of the Common Cutworm, Spodoptera litura, Monitored by Searchlight Trap and Relationship with Occurrence of Soybean Leaf Damage

“…Previous studies that investigated the relationship between the male catch of pheromone traps, using a pheromone lure or a virgin female adult, and the number of moth egg masses laid on the leaves of taro ( Colocasia esculenta [L.] Schott) and groundnut ( Arachis hypogaea L.), showed that male catch peaks occur before, during, or after peaks in egg mass number [ 3 , 4 , 5 ]. Similar results have been reported in soybean [ 6 ].…”

“…The occurrence of damage due to S. litura on all leaves of 1000 soybean plants in each survey field was investigated every Monday, Wednesday, and Friday from mid-August to mid-October [ 6 ]. From mid-September, only 500 soybean plants were surveyed, because the number of leaves per plant became rather large; these numbers of leaves were selected on the basis of a previous report [ 6 ], which precisely indicated the occurrence of soybean damage in the monitored field. The survey area was approximately 10 a and contained 1000 plants seeded using a conventional method.…”

“…The 1st- and 2nd-instar larvae eat only soft mesophyll tissues and leave the leaf epidermis, which results in an obvious pattern of fine white dots on green leaves in fields. One study investigated the relationship between the male catch of a pheromone trap and the number of leaves newly damaged by 1st-instar larvae from July to October over four years; the results revealed that the catch peaks mainly coincided with the number of newly damaged leaves, but varied by ±3 days [ 6 ]. In other words, the catch peak occurred before or after leaf damage.…”

“…The notion that the male adult catch increases before damage occurs does not always fit observations; the trap catch can increase even after the leaf damage has occurred in some cases. The soybean study mentioned above also demonstrates the difficultly of using early trap peaks in August and subsequent temperature trends to forecast the bulk of damage occurring in September [ 6 ]. Thus, there is no accurate means of forecasting soybean damage, and whether additional application of insecticide is needed is currently determined on the basis of farmers’ observation of actual damage in fields.…”

“…Spodoptera litura actively disperse in windy weather [ 7 ] and might even migrate long distances overseas to Japan [ 8 ]. Thus, their dispersal from other areas into target fields requires further study [ 6 ]. Furthermore, while monitoring females is essential (because they lay eggs), pheromone traps only capture males.…”