This study investigated whether prior experience influences the plant food preference of Trigonotylus caelestialium and Stenotus rubrovittatus which cause pecky rice grain, by using rice plants and 2 poaceous weeds. In a choice experiment between Digitaria ciliaris and the rice plants, both T. caelestialium and S. rubrovittatus adults that were reared on D. ciliaris plants showed significant initial preference for D. ciliaris over rice. In a choice experiment between Echinochloa crus-galli var. aristata and rice plants, T. caelestialium adults reared on E. crus-galli var. aristata strongly preferred E. crus-galli var. aristata over rice throughout the experiment. However, at and after 24 h, T. caelestialium adults reared on rice showed no food preference, although T. caelestialium initially preferred E. crus-galli var. aristata. In contrast, S. rubrovittatus adults reared on rice showed no preference between E. crus-galli var. aristata and rice. However, S. rubrovittatus reared on E. crus-galli var. aristata initially preferred E. crus-galli var. aristata to rice, with this preference waning with time. The same results were obtained for both sexes. Although the effect of experience differed with food source and the species of mirid bug, prior experience initially had a noticeable effect, which disappeared with time (1 d).
Phytophagous insects feed on multiple plant species, whereas monophagous insects exhibit high host specificity. Phytophagous insect preferences are influenced by various factors, especially previous experience [
The rice leaf bug, Trigonotylus caelestialium (Kirkaldy), and the sorghum plant bug, Stenotus rubrovittatus (Matsumura) are major rice pests in Japan. Although the nymphs of these bugs grow well on hulled rice, rice plant leaves are not suitable food for their growth. Thus, adults are assumed to cause damage to rice grains when they move to rice plants at the heading stage after growing on other poaceous plants, along with nymphs that emerge from eggs deposited by the adults on rice. Therefore, it is important to identify host plants that are suitable for nymph growth. Field sampling and observations have identified Italian ryegrass, Lolium multiflorum; annual bluegrass, Poa annua; and southern crabgrass, Digitaria ciliaris as important food sources for rice bugs [
It has been reported that rice leaf bugs and sorghum plant bugs are attracted to plants at specific developmental stages [
To study whether bug development on poaceous weeds influences their preference for rice plants, the relative preference between poaceous weeds and rice plants was investigated for T. caelestialium and S. rubrovittatus adults that had had prior experience on different plants. In this study, Digitaria ciliaris and Echinochloa crus- galli var. aristata were used as poaceous weeds because the ears of these plants emerge during the same season as those of rice plants, and because rice bugs grow relatively better on these plants than on other poaceous weeds that are present during the season [
Trigonotylus caelestialium and Stenotus rubrovittatus were collected from fields at the Hokuriku Research Center, National Agricultural Research Center, Niigata Prefecture, Japan. These insects had been reared on young wheat seedlings under laboratory conditions (16:8 L:D photoperiod, 25˚C) for several years as described previously [
Digitaria ciliaris and Echinochloa crus-galli var. aristata that were growing wild, and rice, Oryza sativa, that was cultivated in the paddy fields, were collected from the fields at the Hokuriku Research Center. The flowering stages of these plants were used for tests.
Within 24 h of emergence, 50 - 150 T. caelestialium or S. rubrovittatus adults were released in a plastic rearing cage (34 × 25 × 34 cm). Digitaria ciliaris, E. crus-galli var. aristata, and rice with flowering ears were placed in conical flasks (100 ml) filled with water. Each plant was made available to the adults for 4 d. The plants were renewed daily or every other day. This treatment was conducted under laboratory conditions (16:8 L:D photoperiod, 25˚C).
One ear each of D. ciliaris, E. crus-galli var. aristata, and rice was placed in sample tubes filled with water. Two tubes with D. ciliaris and rice or E. crus-galli var. aristata and rice were placed diagonally at the corner of a plastic rearing cage (34 × 25 × 34 cm). Ten adults reared on these plants as described above were released in a plastic cup (top diameter: 8 cm, bottom diameter: 6 cm, height: 4 cm). The plastic cup was placed at the center of the plastic rearing cage and then the cap was removed to allow the adults to disperse. Adults reared on D. ciliaris were released in D. ciliaris vs. rice cages. Those on E. crus-galli var. aristata were released in E. crus- galli var. aristata vs. rice cages. Those on rice were released in D. ciliaris vs. rice cages or E. crus-galli var. aristata vs. rice cages. The number of adults on the plants was counted at 3, 6, 24, 27, and 30 h after starting the test. The test was conducted separately for males and females to prevent interaction between the sexes from influencing food preference. Twelve cages (i.e., 120 individuals) were used for each test, except for T. caelestialium males reared on E. crus-galli var. aristata (7 cages). These tests were conducted under rearing conditions (16:8 L:D photoperiod, 25˚C).
The numbers of adults on rice plants and other poaceous plants were compared at each time point using binomial test. The ratios of the adults present on the plant species on which they had been reared versus those present on the plant species on which they had not been reared were compared between rice and each poaceous species at each time point using Fisher’s exact test. For example, at 3 h, the total number of T. caelestialium females reared on D. ciliaris that were present on D. ciliaris and rice was 64 and 25 individuals, respectively (
After rearing of females on D. ciliaris, a significantly higher number of females was found on D. ciliaris than on rice at 3, 6, 24, and 27 h (number of individuals on D. ciliaris/rice = 64/25, 77/20, 64/29, and 54/34 respectively) (binomial test, p < 0.05;
After rearing of males on D. ciliaris, a significantly higher number of adult males was present on D. ciliaris than on rice at 3, 6, and 27 h (number of individuals on D. ciliaris/rice = 46/27, 64/15, and 42/22 respectively) (binomial test, p < 0.05;
After rearing of females on D. ciliaris, a significantly higher number of females was present on D. ciliaris versus rice at all of the time points (number of individuals on D. ciliaris/rice at 3, 6, 24, 27, and 30 h = 54/34, 66/33, 81/34, 86/27, and 80/29 respectively) (binomial test, p < 0.05;
After rearing of males on D. ciliaris, a significantly higher number of males was present on D. ciliaris than on rice at all of the time points (number of individuals on D. ciliaris/rice at 3, 6, 24, 27, and 30 h = 67/25, 74/20, 54/15, 72/12, and 70/16 respectively) (binomial test, p < 0.05;
After rearing of females on E. crus-galli var. aristata, a significantly higher number of females was present on E. crus-galli var. aristatathan on rice at all of the time points (number of individuals on E. crus-galli var. aristata/rice at 3, 6, 24, 27, and 30 h = 71/8, 58/8, 47/8, 41/1, and 42/10 respectively) (binomial test, p < 0.05;
After rearing of males on E. crus-galli var. aristata, a significantly higher number of males was present on E. crus-gallivar. aristata than on rice at 3 - 27 h (number of individuals on E. crus-gallivar. aristata/rice at 3, 6, 24, and 27 h = 28/8, 30/6, 17/1, and 21/4 respectively) (binomial test, p < 0.05;
After rearing of females on E. crus-galli var. aristata, a significantly higher number of females was present on E. crus-galli var. aristata than on rice at 3 and 6 h (number of individuals on E. crus-galli var. aristata/rice = 60/13 and 70/17 respectively) (binomial test, p < 0.05;
respectively) (binomial test, p > 0.05;
After rearing males on E. crus-galli var. aristata, a significantly higher number of males was present on E. crus-galli var. aristata than on rice at 3 and 6 h (number of individuals on E. crus-galli var. aristata/rice = 76/15 and 74/13 respectively) (binomial test, p < 0.05;
Stenotus rubrovittatus exhibited different preferences for the two tested poaceous plants (D. ciliaris and E. crus-galli var. aristata) versus rice. In the D. ciliaris-rice combination, after rearing of individuals on D. ciliaris, more S. rubrovittatus adults preferred D. ciliaris to rice. In contrast, after rearing of individuals on rice, no preference between rice and D. ciliaris was initially detected, although more adults frequented D. ciliaris over time (about 1 d). In contrast, for the E. crus-galli var. aristata-rice combination, after rearing of S. rubrovittatus on rice, adults exhibited no preference between E. crus-galli var. aristata and rice at any of the time points (except females at 3 h). However, after rearing of individuals on E. crus-galli var. aristata, more adults initially preferred E. crus-galli var. aristata over rice. This observation contrasted with the results obtained for D. ciliaris, wherein the difference between D. ciliaris and rice disappeared with time. Significant differences in the ratios were initially obtained between plants used for rearing for both the D. ciliaris-rice combination and the E. crus- galli var. aristata-rice combination, with this difference disappearing over time (about 1 d). These results indicate that rearing plant experience influenced food preference initially, but this effect disappeared over time (about 1 d). Variation in the preference of rice bugs for rice and the two weed species (D. ciliaris and E. crus- galli var. aristata) may thus be influenced by innate preferences.
The ratio of the number of S. rubrovittatus adults on D. ciliaris versus rice and E. crus-galli var. aristata versus rice increased with time, possibly because S. rubrovittatus had a stronger preference for D. ciliaris than for E. crus-galli var. aristata. In comparison, the ratio of T. caelestialium adult numbers on D. ciliaris versus rice decreased with time, whereas it remained consistently higher for E. crus-galli var. aristata versus rice, possibly because T. caelestialium has a stronger preference for E. crus-galli var. aristata than for D. ciliaris. If prior experience affects preference, after rearing of T. caelestialium adults on rice, more individuals would be expected to prefer rice over E. crus-galli var. aristata initially, and then this preference would subsequently decrease. However, the opposite results were obtained. It is possible that the quality of plants in the flasks deteriorated with time. Nevertheless, preference should be influenced by prior experience, because significantly different adult ratios were obtained between the two weeds and rice, depending on the plants used for rearing. Because T. caelestialium had a strong preference for E. crus-galli var. aristata, adults reared on E. crus-galli var. aristata might have retained this preference, even when the test plants deteriorated. In contrast, T. caelestialium adults reared on rice might have selected E. crus-galli var. aristata when the plants were in good condition, switching to rice plants when E. crus-galli var. aristata deteriorated.
Prior experience influenced plant preference behavior of T. caelestialium and S. rubrovittatus. However, the effect of experience disappeared after just 1 d. Experience during specific periods (egg-young larvae) has been shown to influence adult oviposition in a parasitic wasp [
It remains unclear what regulates the relationship between preference and prior experience. Both rice bug species are attracted by odors released from host plants [
Trigonotylus caelestialium and S. rubrovittatus females release sex attractants to attract males [
In the current study, adult bugs tended to select plants that they had previously experienced. If the effect of experience in nymphs is sustained to the adult stage, adults may select the same plant species as that on which the nymphs grew. Therefore, if these two rice bug species developed on weeds, they would probably select the same plant species as adults. However, if the weeds deteriorate and disappear, the bugs may transfer and adjust to rice plants in a relatively short period. The results of this study showed that the influence of prior experience declined with time, leading to a situation where the adult ratio on rice plants versus weeds became equal. Thus, these ratios indicate innate preference. Consequently, even though rice bugs grow on weeds, they are expected to adjust to rice plants with time, and spread over rice paddies.
In this study, it was expected that the mirid bugs that developed on poaceous weeds would be inhibited from moving to rice plants because of the effect of food experience, leading to food preference. Although the food preferences of the bugs were influenced by prior experience, this effect was unlikely to be strong. The adult mirid bugs changed their preference for rice plants in a short time period (1 d). In this study, only 2 species of poaceous weeds were used for the tests; however, the two bugs showed different responses to these weeds. Hence, it is important to investigate the effect of other plants. Previous studies have already demonstrated how the oviposition [
I am grateful to Dr. H. Higuchi, Ryukoku University, and Dr. A. Takahashi, NARO Tohoku Agricultural Research Center, for their support and suggestions.
Atsuhiko Nagasawa, (2016) Food Preference of Pecky Rice Bugs Is Influenced by Experience. Advances in Entomology,04,104-113. doi: 10.4236/ae.2016.42012