The results demonstrate that on vegetative structures of soybean plants (leaf and petiole), E. heros nymphs from all instars can feed and all presented a similar feeding behavior, encompassing pathway and ingestion phases. Notably, we observed similarities in waveform appearance between nymphs feeding on leaf surfaces and those feeding on petioles. However, intriguing differences emerged in the duration of the pathway phase. Younger nymphs exhibited longer pathway durations compared to their older counterparts, suggesting potential challenges in penetrating plant tissues.
This difference may be attributed to the developmental stage of the nymph’s mouthparts (stylets) and the type of plant tissue explored. Younger nymphs have less rigid mouthparts, making it more challenging for them to penetrate plant tissues, especially in tougher tissues such as the petiole. In fact, in older nymphs (fourth and fifth instars), it was observed that the duration of the pathway waveform tended to be numerically longer on the petiole compared to the leaf, which has a more easily penetrable structure.
Our findings demonstrate that nymphs provided with vegetative structures exclusively explore xylem vessels as their feeding site. However, this 'water' ingestion serves only for hydration, and not as nutrient source, as vascular tissues, especially xylem, are rich in water and poor in essential nutrients (Taiz and Zeiger 2004); whereas the seed endosperm consists of regions of cells highly concentrated in nutrients (Slansky and Panizzi 1987). Stink bugs are, in the majority, seed feeders, thus, nymphs are unable to survive and complete their development feeding exclusively on vegetative structures of host and non-host plants (e.g., Chocorosqui and Panizzi 2008, Tomacheski et al. 2019). However, they also require water to maintain body hydration, as reported for other sucking insects (Spiller et al. 1990), and/or nutrient balance after ingesting a concentrated food - seed endosperm (Lucini et al. 2016). This 'water' is primarily obtained from vegetative structures of the host plant or other plants, but can also be obtained directly on reproductive structures (Lucini et al. 2018a).
The study by Lucini et al. (2023) revealed that nymphs of E. heros consumed cell contents from xylem while feeding on reproductive structures (immature pods). Interestingly, it was observed that nymphs of all ages generally ingested approximately 2–4 times more xylem sap from pods compared to vegetative structures. This discrepancy might be attributed to the consumption of highly concentrated seed tissue, necessitating an osmotic balance, as previously mentioned. These findings corroborate with those of fifth instar Nezara viridula nymphs, which also consumed from xylem vessels while feeding on soybean vegetative structures (petioles) and immature pods, with a higher time spent in the xylem vessels on pods (Mitchell et al. 2018).
Intriguingly, our analysis of xylem feeding parameters revealed nuanced trends. Nymphs displayed an inclination towards increased xylem feeding activities on petioles compared to leaf surfaces, indicating potential differences in accessibility or tissue composition. Additionally, there was a numerical trend towards heightened xylem feeding activities with nymphal age, suggesting potential physiological changes associated with development.
Moreover, we observed a distinct feeding pattern in second instar nymphs, characterized by a higher intake of cellular content from xylem vessels compared to older instars (third and fourth). During the first instar, the nymphs remain clustered and do not feed (although some findings show that they do, see Rivera and Mitchell [2020]), but once they move into the second instar, they are much more active as they begin to disperse from their egg mass, which could explain why they ingest much more compared to older instars.
Overall, our study provides comprehensive insights into the intricate feeding behavior of E. heros nymphs on vegetative structures of soybean plants, shedding light on their adaptation strategies across different developmental stages. These findings can contribute to development more effective control strategies for managing E. heros on soybean crops.