Paraponyx stagnalis Zeller

Kallyadan, Sreejina Sreedharan & M., Nasser, 2024, Making shelter for enemies: parasitoid-induced shelter-building behaviour in Paraponyx stagnalis, Journal of Natural History 58 (21 - 24), pp. 658-673 : 659-660

Paraponyx stagnalis Zeller

( Lepidoptera : Crambidae )

Paraponyx stagnalis (Zeller, 1852) ( Lepidoptera : Pyralidae ) (= Nymphula depunctalis (Guenée)) , commonly known as the rice case worm, poses a significant threat to rice plants in Kerala ( Santhosh and Karthikeyan 2017). These larvae are particularly damaging during the vegetative stage of wetland rice in Asia, defoliating rice plants ( Figure 6 (a)) ( Litsinger et al. 1994). The destructive activity of the larvae of case worms stops after the maximum tillering stage. The damage caused by them is identified by their distinctive leaf cases and the removal of green tissue. These larvae scrape the leaf tissues, leaving only the papery upper epidermis ( Figure 3 ). To differentiate case worm damage from that caused by other pests, one needs to look for the ladder-like pattern on infested leaf tissue, which results from the larval head’s back-and-forth motion during feeding. Infested plants tend to show stunted growth, as several larvae attack the same rice plant, cutting off most leaf tips to construct their larval cases and scraping the remaining leaves ( Dale 1994).

Distinctive features of the rice case worm larvae include their transparent green bodies with a vibrant brownish-orange head ( Figure 6 (e)). To protect themselves from parasitoids and predators, the larvae construct tubular cases by rolling leaves ( Figures 2 and 4 (b)). These cases serve as a protective shelter, lined with silk on the interior and containing a small layer of water essential for breathing ( Figure 6 (g)). The larval period has five instars (eg Surbala et al. 2016).

Rice case worm larvae thrive in submerged plants, exhibiting semi-aquatic behaviour by climbing plants using their legs to feed on leaves ( Figure 3 ). While consumption is constrained in dry conditions, water droplets on leaves enhance larval feeding. Caterpillars navigate water-logged fields during the day by swimming or following water or wind currents to reach young rice plants. The larval stage lasts between 15 and 30 days, with final-instar larvae reaching the maximum length of 20 mm. Upon completing larval development, the fifth-instar larva seals both ends of its leaf case, pupates within it ( Dale 1994) and stays there for 5–11 days ( Viraktamath et al. 1974). Pupae often adhere to rice paddy saplings or wild grasses above water surfaces but may also be immersed in water ( Grist and Lever 1969). The pupa, about 5.5 mm in length and 1.5 mm in width, is initially cream in colour but soon turns silvery white before eclosion. The pupal stage ( Figure 4 (c,d)) lasts approximately a week. The adult usually emerges through a hole in the upper end of the case ( Figure 4 (e)). The life cycle of P. stagnalis is completed within a month ( Pathak and Khan 1994).

Figures

Figure 2. Pictorial representation of the process of formation of a normal leaf shelter: (a) first leaf slit made by the first-instar rice case worm larva; (b) making of the second slit; (c) removal of the leaf tip, and rolling of the leaf portion between the first and second slits; (d) detachment of the tubular case from the remaining leaf. Tthe types of parasitised larval leaf shelter: Type 1 = folding of the leaf portion above the first leaf slit; Type 2 = folding of the leaf portion between the first and second cut; and Type 3 = removal of the leaf portion above the first cut and folding the leaf between the two leaf slits.

Figure 3. A pictorial representation of the Paraponyx stagnalis life cycle.(a) Eggs are laid on the abaxial side of the leaf. (b) Newly emerged first-instar larvae are feeding on the leaf tissue. (c,d) Larvae making the leaf shelter. (e) Folded leaf case in the rice leaf. (f) Tubular cases fall off the rice plant, floating on the water. (g) The rice case worm uses its legs to climb up adjacent plants. (h) Caterpillars feed on leaves. (i) Pupae adhere to the culms of rice paddy saplings via the lateral border of the cases, slightly above the water surface.

Figure 4. Biology of Paraponyx stagnalis. (a) Unparasitised late-instar P. stagnalis larva in a cut-open case. (b) Normal tubular case of P. stagnalis. (c) Newly formed P. stagnalis pupa in a cut-open case. (d) P. stagnalis pupa within the silken cocoon. (d) Adult P. stagnalis.

Figure 6. Leaf shelter of Paraponyx stagnalis. (a) Tubular case of late-instar larva. (b,c) Tubular cases of early-instar larvae. (d) Caterpillar making leaf shelter. (e,f) Caterpillars’ bright brownish-orange head protruding from the tubular case. (g) Silk covers the case’s interior, storing a small layer of water.