Spodoptera frugiperda egg mass scale thickness modulates Trichogramma parasitoid performance
The fall armyworm (FAW) Spodoptera frugiperda (Smith) is a major agricultural pest in Americas, Africa and Asia. Egg masses are covered with scale layers and the various scale layer thicknesses of these masses can affect parasitoids efficiency. The present study aimed to determine dynamics of the scale thickness on egg masses and its effect on Trichogramma parasitoid performance. The scale thickness ranged from 0μm to 400 μm and can be graded in three levels. Level I was the naked egg masses without or covered with a thin scale, and the thickness was below 80 μm. Level II was medium covered, with scales where 20%–80% eggs could be seen; the thickness was between 80 μm to 180 μm. Level III was fully-covered with scales and the thickness was above 180 μm (up to 400 μm). The egg mass scale thickness decreased with increasing age of egg laying FAW females; the proportion at level I increased during female aging, while proportion of levels II and III decreased during oviposition period. During FAW female lifetime, the level I showed the highest proportion (51.9%) while the level III showed the lowest (9.9%). The parasitism rate of FAW eggs by Trichogramma dendrolimi varied according to scale thickness, with higher parasitism on eggs and egg masses at level I (31.6%, 78.3%, respectively) and lowest parasitism on level III (eggs: 1.9%; egg masses: 23.1%). We documented factors modulating parasitism effectiveness on FAW and we suggested that timely parasitoid releases targeting egg masses at Level I scale thickness could enable maximizing biocontrol service provided by Trichogramma on FAW.