Petiole and stem borer (Lepidoptera: Noctuidae) in Scrophularia [0497]

OrderLepidoptera
FamilyNoctuidae[M,L]
Lower taxoncf. Hydraecia stramentosa
No. spp. involvedOne confirmed [1-C]
Feeding modeBorer in petiole and stem
Host plantFigwort, Scrophularia sp. (Scrophulariaceae)
Figwort borer larva

This insect was first detected in the present study via the observation of distorted, wilting lower leaves on shoots of the host in early May. Close examination of these leaves revealed caterpillars tunneling in their petioles. These caterpillars were reared nearly to maturity, first in the petioles in which they were found, and later, as they grew, in stems of the host.

The larvae apparently belong to the species Hydraecia stramentosa, based on the findings of A.F. Winn (1916) and his colleagues, who were the first to elucidate parts of the species' life history, including its root-feeding stage. Winn's published account provides useful details about late-stage larvae in roots, the pupation location, timing of adult emergence, and adult behavior including egg laying. Having mainly observed later instar larvae, Winn was under the impression that this species feeds entirely in roots; the current study's finding of early instar larvae feeding in petioles is evidently new information, assuming these petiole-feeding larvae are indeed H. stramentosa. The claim here that larvae also feed in stems in the wild is based on the assumption that they would tunnel through the stems in order to move from the petioles to the roots, but this has not been directly observed. It would be helpful if the petiole-tunneling larvae could be reared all the way to adulthood to confirm their identity as H. stramentosa.

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Inhabited lower leaves from shoots of the host, harvested in early May. Each petiole has been tunneled by a caterpillar. Note the distortion of the petiole and the wilting of the leaf blade.
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Two affected leaves, harvested in early May. The caterpillars' tunneling has distorted the petioles and the leaf blades have wilted as a result.
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Detail of an affected petiole, showing discoloration and distortion due to a larva's internal feeding.
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Inhabited petiole, backlit, showing larva and frass inside.
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Middle-instar larva in mid-May, reared on stem pieces.
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Middle-instar larva in mid-May.
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Dorsal view of larva's head and thorax, showing distinctive patterning.
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Larva in early June, reared on stem pieces.
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Larva in early June.
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Lateral view of larva.

Specimen data for images

Coll. 05/02/20, photos taken 05/02/20 (01-03), 05/04/20 (04), 05/17/20 (05-07), and 06/08/20 (08-10).

References

Winn, A.F. 1916. The home of Gortyna stramentosa. Forty-sixth Annual Report of the Entomological Society of Ontario: pp. 43-48. Retrieved September 16, 2023 from https://www.biodiversitylibrary.org/page/8002018.

Page created 09/16/23. Last update: 11/19/24