The number of probing marks made by the adult female BPH in the selected N22 mutants ranged from 12.2 to 45.4 (Table 1, Plate 1, Fig 1). The highest number of feeding punctures were reported in the mutant NH 4535 (45.4) which was on par with the resistant check PTB 33 (45.8) and is notably distinct from other entries. NH 663 and NH 4632 (12.2) recorded lowest number of probing marks. The susceptible check TN1 recorded 10.0 probing marks, MO1 and parent line Nagina22 recorded 37.0 and 19.2 probing marks respectively. The resistant entries (24.9 No. of probing marks) differed significantly from susceptible entries (22.1 No. of probing marks) with regard to number of probing marks by adults.
In case of BPH nymphs, the number of feeding marks ranged from 15.6 to 34.2 (Table 1, Plate 1, Fig 1). Highest number of feeding punctures were found in the entry, NH 663 (34.2) which was comparable with PTB 33 (33.2). Whereas the lowest number of probing marks were observed in NH 4856 and NH 4614 (15.6). Susceptible check TN 1 recorded 8.0 probing marks only. In resistant entries, number of probing marks were greater (23.2) than that in the susceptible entries (17.7). The BPH adults probed more number of times (24.4) compared to nymphs (22.2).
Phloem sap is the main food source for brown planthopper, a vascular feeder. When the stylets are inserted and the labial tip is pressed against the plant epidermis, the BPH secretes a little amount of coagulable saliva. They form a strong bond as a result, leaving distinct circular imprints where the stylet is inserted. A salivary deposit on the plant epidermis is referred to as a “feeding mark”. The frequency of probing over time can be calculated by counting the number of feeding marks on various plant materials
(Sogawa, 1982).
More feeding punctures in the resistant entries may be because these resistant entries didn’t sustain prolonged feeding because of specific feeding deterrents, hazardous substances, or a lack of feeding stimulants present. Hence, the insect had to probe more on the resistant genotypes to locate feeding sites
(Sogawa, 1982). Our results corroborate with the findings of several workers
i.e.,
Reddy et al., (2016) and
Anupama et al., (2018) who reported that resistant varieties received a greater number of probing marks than susceptible ones.
Ponnada et al., (2011) also reported that average probing marks on resistant plants ranged between 30.4 to 42.9 whereas resistant and susceptible checks have recorded 22.1 and 6.7 probing marks, respectively. Resistant cultures like MTU 1075, MTU IJ 206-7-4-1 and MTU PLA 99-1-3-1-2 have received more number of feeding marks which were 128.1, 112.8 and 110.2, respectively. The total honeydew excreted area by the adult BPH females ranged from 31.3 mm
2 to 113.3 mm
2 (Table 1, Plate 2, Fig 2). The total honeydew excreted area (mm
2) was highest in the mutant NH 4613 (113.3 mm
2) which was on par with N22 (140.0 mm
2) while the lowest area was recorded in NH 4536 (31.3 mm
2). The susceptible check TN 1, MO 1 and resistant check PTB 33 recorded 209.0 mm
2, 74.3 mm2 and 14.7 mm
2 honeydew excretion area respectively. The honeydew excretion was less in resistant entries (65.3 mm
2) compared to that on susceptible entries (141.1 mm
2).
The total honeydew area by BPH nymphs ranged from 30.3 mm
2 to 148.0 mm2 (Table 1, Plate 2, Fig 2). NH 663 showed the highest honeydew excretion area (148.0 mm
2). The lowest area was reported in NH 4536 (30.3 mm
2). TN 1, N22, MO 1 and PTB 33 reported honeydew excretion area of 83.3 mm
2, 88.7 mm
2, 41.7 mm
2 and 7.0 mm
2 respectively. The recorded honeydew excretion area by the resistant entries (56.2 mm
2) was lower than that of susceptible entries (71.2 mm
2). In general, the BPH adults fed more and excreted more honeydew (79.5 mm
2) compared to the nymphs (59.0 mm
2).
As sap feeders, homopteran insects absorb the highly water-containing plant sap. The filter chamber mechanism expels extra water to prevent the digestive enzymes from becoming diluted. This excretory product, known as honeydew, is rich in sugars, amino acids, waxes, lipids and other nutrients. The plant sap that BPH consumes directly correlates with the honeydew quantity it excretes.
The amount of honeydew excretion by female adults, when fed on resistant cultivars was less whereas the amount of honeydew excreted was greater when the insects fed on susceptible than on resistant varieties
(Reddy et al., 2016). These differences in the amount of honeydew excretion indicate differences in the relative amount of sap intake. In this study, resistant entries showed significantly less amount of honeydew excretion than that of susceptible check TN1. The feeding rate of the brown planthopper was attributed as the capability to differentiate between susceptible and resistant entries.
The correlation analysis among the damage score, probing marks and honeydew excretion (Table 2) indicated a negative correlation between damage score and probing marks of adults (-0.207) and nymphs (-0.411) eventhoug it is non-significant. The brown planthopper probed more number of times on the entries with less damage score (resistant) and vice versa. There was a significant positive correlation between damage score and adult total honeydew excretion (0.547) and nymphal total honeydew excretion (0.200) which is non-significant.
When the data were subjected to linear regression analysis (Table 3, Fig 3 and 4), a negative relationship was observed between damage score and nymphal and adult probing marks while it was positive between damage score and honeydew excretion area by nymphs and adults. In the adults, probing marks is able to explain 4.27 per cent of the variation in damage score and for each unit increase in the probing marks, the damage score is decreased by 0.97 units. In the nymphs, probing marks showed 16.88 per cent of the variation in damage score and for each unit increase in probing marks, the damage score is decreased by 1.34 units. In the adults, total honeydew describes 55.97 percent of the variation in damage score and for each unit increase in the total honeydew, the damage score is increased by 14.92 units. Whereas in case of nymphs, total honeydew is able to explain 4 per cent of the variation in damage score and for each unit increase in the total honeydew, the damage score is increased by 2.99 units.
Ramesh et al., (2014) and
Anupama et al., (2022) also suggested a positive correlation between damage score and honeydew excretion area and a negative relation between damage score and probing marks made by planthoppers in the mapping population of TN1 X Sinasivappu and rice germplasm accessions.