Rice can trap and kill Spodoptera frugiperda in its spikelets

Rice plants can trap and kill young caterpillars of Spodoptera frugiperda during the flowering phase. Researchers at the University of Arkansas documented the phenomenon in rice spikelets. The mechanism involves non-glandular trichomes and the natural closure of the spikelet after anthesis. The discovery points to a possible physical defense of the plant against herbivores in reproductive structures.

The study began with an observation made during experiments with rice and Spodoptera frugiperda. Second instar larvae were found dead inside the spikelets. The researchers conducted four experiments with wild rice, Oryza sativa japonica cv. Kitaake. The assays evaluated caterpillar exposure to panicles, bioassays in Petri dishes, choice tests, and analysis of volatile compounds emitted by the panicles. The caterpillars used in the study came from commercial breeding and received a wheat germ-based diet until the second instar.  

First experiment

In the first experiment, scientists placed 30 second-instar larvae in a rice panicle with a flag leaf at the beginning of anthesis. After 48 hours, 50% of the caterpillars were trapped in the panicles. The presence of the flag leaf did not prevent the larvae from migrating to the reproductive structures.

Second experiment

In the second experiment, the authors used panicles with florets in anthesis and panicles in the pasty stage in Petri dishes. Each dish received a second-instar caterpillar, kept there for 24 hours. The result repeated the pattern observed in the plants. Approximately 53% of the larvae became trapped. According to the authors, when the caterpillar attempted to feed on the panicle, approximately half of the individuals died within 48 hours, with or without available leaf material.

Images from the study show the sequence proposed by the authors. The larva enters the spikelet during anthesis, attempts to access the floret, and becomes trapped between the structures covered in trichomes. Then, the spikelet closes naturally after anthesis. This movement traps the caterpillar and leads to its death.

Third experiment

The third experiment evaluated the potential attraction of caterpillars to floral volatiles. The larvae showed a preference for panicles with florets compared to panicles in a pasty stage, although the result was at the statistical limit reported by the authors. The test presented a chi-square of 6,2, two degrees of freedom, and a p-value of 0,05. For the researchers, the result provides additional evidence about the possible role of floral volatiles in guiding caterpillars to the panicle.

Scanning electron microscopy analysis indicated damage to the larva's body after separation from the spikelet. Images show trichomes on the spikelet, the trapped larva, and deformation in the insect's anterior segment. Scientists attributed the damage to the mechanical pressure of the spikelet's closure and the presence of dense trichomes. They also hypothesized that the angle of trichome insertion played a role in the entrapment.

The observation was validated in two other rice lines. The researchers tested the conventional cultivar Diamond, used in the south-central United States, and the GSOR 302684 line, described as rich in trichomes. Both showed a similar effect.

Fourth experiment

In the fourth experiment, researchers collected volatiles from panicles with florets shortly after anthesis and from panicles in the pasty stage, two to three weeks after anthesis. The collection lasted six hours. The analysis was performed by gas chromatography coupled with mass spectrometry. Total emissions and average volatile content were higher in panicles with florets.

 Compounds found only in panicles with florets included dodecane, ethylhexanal, and an unidentified alcohol. Ethylhexanol appeared in both instars, but in twice the quantity in panicles with florets. The authors speculate that these compounds may participate in the attraction of Spodoptera frugiperda for the florets. After approaching, the larva becomes attached by the trichomes.  

According to the researchers, the trichomes present in the spikelets may act as a defense system against insects, especially against floral feeders in early stages. They indicate the need for further assays with trichome-deficient mutants, measurements of trichome density, length and angle, as well as studies on the mechanics of entrapment and the response of different instars.

Further information at doi.org/10.1186/s13717-026-00683-8