Key words: barnyard grass: Echinochloa: E. crus-galli var. crus-galli: E. oryzicola:
It has been suggested that Echinochloa weeds are no longer a problem in Japan. However, if herbicides cease to be applied to paddy fields these soon become covered with Echinochloa (Iwasaki, 1985). Hence, Echinochloa weeds may only be temporarily suppressed by herbicides. Not only herbicides but also various integrated control methods such as biological control should be used to regulate these weeds.
The starting point for the biological control of a weed in paddy fields is to find a selective biological agent against it which leaves the rice crop intact. So far, with regard to biological agents for Echinochloa weeds, it is known that grass carp ( Ctenophryngodon idella) and red tilapia ( Tilapiamossambica XT. nilotica-anrea) (Itoh et al. 1991b) and a planthopper, Sagatodes pusanus (Mohamed et al. 1986), all have the potential to control weeds. However, they are not selective. E. leucotaeniella is a moth ( Fig. 1), the larva of which is a selective stemborer which infests only Echinochloa (Goto 1977). It was discovered in 1974 in the stem of barnyard grass growing in a rice nursery in Shonai District, Yamagata Prefecture, in northern Japan. Before E. leucotaeniella can be used as a biological control agent of Echinochloa, many problems have to be solved. In the present paper, the relationship between Echinochloa and E. leucotaeniella is described. In addition, the potential of E. leucotaeniella as a biological control agent is discussed.
Relationship between E. Leucotaeniella and Echinochloa
A survey was carried out of three crops, rice ( Oryza sativa), Japanese barnyard millet ( Echinochloa utilis) and corn ( Zea mays), with regard to infestation by 43 weed species belonging to 16 families. Infestation with stemborer was found only in Echinochloa, namely E. utilis, E. oryzicola and E. crus-galli var. crus-galli (Goto 1977). The relationship between E. leucotaeniella and Echinochloa is presented in Fig. 2. Echinochloa species are characterized by four main features namely;
- Numerous narrow stems,
- The stem is enclosed by a hard outer layer,
- Leaf development is divided into two phases, and
- They have a submerged habitat.
These characters are closely related to the life cycle of E. leucotaeniella.
Numerous Narrow Stems
The spatial distribution of E. leucotaeniella larvae per plant and per stem were investigated in E. oryzicola growing in open paddy fields. The data were analyzed using the m-m method (Iwao 1968). The results showed that most infestation consisted of one larva per stem, equivalent to a uniform distribution pattern (Goto 1985c). Dispersed hatching behavior may be a cause of this uniform distribution, which may be an advantage in that each larva can develop without competition. On the other hand, the distribution pattern of larvae in individual plants was recognized as aggregative, which indicated that the larvae might have begun as a clump. This aggregative nature may come from egg mass deposition on the plant, as is the characteristic mode of moth reproduction. The egg mass size of E. leucotaeniella was small, with an average of only 13 eggs (Goto 1978). The size of the egg mass is closely related to the number of Echinochloa stems. Hence, the survival rate of eggs might be regulated by the number of stems, in that each hatching larva needs one stem in order to survive.
Habituation in the Stem
E. leucotaeniella larvae entered the stem immediately after hatching and developed into pupae, with a high survival rate. Hence, the inside of the stem may be a safe haven for E. leucotaeniella.
On the other hand, the behavior of hatching, eclosion, oviposition and copulation occurs outside the stem, and circadian rhythms can be observed in these behavior ( Fig. 1; see also Goto 1979, Goto et al. 1989). Each peak of the circadian rhythms occurred at different times and under different conditions, according to the growing stage of the larvae. E. leucotaeniella may in this way be able to avoid various unfavorable environmental conditions occurring outside the stem.
Two-Phase Leaf Development
The leaf development of Echinochloa can be divided into two phases, vegetative and reproductive. The stem during the vegetative phase is short and weak, but during the reproductive phase it becomes long and robust. The diapause rate of E. leucotaeniella increases as Echinochloa passes into the reproduction phase ( Fig. 3). In plants at the same leaf stage, the diapause rate was higher among larvae in E. crus-galli var. crus-galli than among those in E. oryzicola. During the vegetative phase, the entire stem of Echinochloa is eaten by the larvae, so that the stem soon dies. During the reproductive stage, the stem survives because it is only partially eaten.
Submerged Habitat of Echinochloa
Echinochloa is a well-known weed in rice fields (Yubuno 1975) or in swamps and other aquatic habitats (Holm et al . 1977). E. leucotaeniella exhibits moisture tolerance throughout its various stages of development ( Table 1). The egg mass is oviposited at the leaf tip, and hatching normally occurs even if the leaf is submerged. After hatching, the larvae immediately disperse and enter the leaf sheath. As the larva eats the leaf tissue it moves down to the base of the leaf, which it soon enters. In moving through the stem, the larva does not make any hole to the outside, so water around the plant does not infiltrate through the stem. Before pupation, E. leucotaeniella larvae make a hole in the stem from which the moths later emerge. However, the hole is still covered by the epidermal layer of the stem, so that water does not infiltrate through the hole. When stems infested with pupae were put under water, eclosion occurred normally and the moths flew out from the stem through the water into the air. Thus, E. leucotaeniella can be seen to exhibit high moisture tolerance from the egg to the moth stage. E. leucotaeniella may in fact be the only stemborer which can use submerged Echinochloa stems.
Relationship between E. Leucotaeniella and Echinochloa Species
The ecological and physiological properties of the two species E. crus-galli var. crus-galli and E. oryzicola are presented in Table 1 (Goto 1985b). Weeds can be defined as organisms adapted to habitats frequently disturbed by man (Oka 1983). Ehrendorpher (1965) has pointed out several features common to weeds, such as the prolonged period before seed emergence, rapid plant development, a wide range of phenotypic plasticity and wide seed dispersal. E. crus-galli var. crus-galli possesses at least two features characteristic of weeds, i.e. fast development at the early stage of plant growth, and conversion from the vegetative to the reproductive phase at an early stage, resulting in early maturity. The plants are tall and robust, producing large numbers of small, long-awned seeds. E. oryzicola, on the other hand, has several characteristic features which mimic rice (Morinaga et al. 1942), i.e., prolonged seed germination, slow development at the early growth stages, belated phase conversion, and a weak, narrow stem.
In order to assess the potential of E. leucotaeniella in the biological control of Echinochloa species, newly hatched larvae of E. leucotaeniella were inoculated in Echinochloa species. The results are presented in Fig. 4. There was no significant difference in the dry weight of shoots and seeds between inoculated and control (non-inoculated) at any stage of E. crus-galli var. crus-galli. However a significant difference was found between inoculated and non-inoculated plants of E. oryzicola at the 6th and 10th leaf stages. The rate at which stems were eaten by the larvae, and the occurrence of leaves with signs of feeding, are presented in Fig. 5. The feeding pattern is marked only as present or absent. When the two Echinochloa species were compared, 100% of plants at the vegetative stage showed signs of having been eaten. Damage to E. oryzicola was very marked because the entire stem had been eaten. Stem elongation began after phase conversion, which occurred at about the 11th stage in E. oryzicola and the ninth stage in E. crus-galli var. crus-galli. E. oryzicola has a prolonged vegetative period, so that it is exposed to infestation over a long period. E. crus-galli var. crus-galli, on the other hand, developed rapidly at the early growth stages, so it was able to recover from any serious damage received during its vegetative phase.
The relationship between E. leucotaeniella and Echinochloa weeds is summarized in Table 2. E. oryzicola was attacked by E. leucotaeniella at the vegetative stages. Since the vegetative stage of this weed species is longer than that of E. crus-galli var. crus-galli, there was severe damage due to the long period of infestation. Since E. crus-galli var. crus-galli entered the reproductive stages earlier than E. oryzicola, E. leucotaeniella consequently entered diapause at an early stage. In Shonai District, moths belonging to the 3rd generation sometimes emerged (Goto 1985a). These were probably emerging from E. oryzicola. During the reproductive stage of both Echinochloa species, 4th instar larvae were found to have a high rate of movement in E. oryzicola, and the mortality of late instar larvae was high. The site for making a cocoon for diapause larvae was regularly found at the stem node of E. oryzicola, but there was no regular site for cocoons in E. crus-galli var. crus-galli. Although E. leucotaeniella is well adapted to Echinochloa, it causes so much damage to the weed as to decrease its own survival rate. Probably the special vulnerability of E. oryzicola is due to changes from weedy features to characters mimicking rice. This artificial selection took place during the relatively short period that rice has been cultivated, while Echinochloa and E. leucotaeniella have probably coexisted for a very long time. The changes in the characteristic features of E. oryzicola have made it a weed which is difficult to control, but these may assist the stemborer to control it more easily. In recent years, direct seeding of rice has been replacing direct transplanting of rice in the irrigated fields of Thailand, the Philippines, Sri Lanka, Malaysia and Fiji. As a result, a tremendous increase in the population of grassy weeds, especially Echinochloa, has been noted (Itoh 1991a). Paddy fields with a serious infestation of barnyard grass in Peninsular Malaysia had a yield reduction of as much as 41% (Azmi 1985). There are five species of Echinochloa in the rice fields of Malaysia, namely the E. crus-galli complex, E. oryzicola, E. stagnina, E. picta and E. colona (Itoh 1991b). E. stagnina and E. picta are perennial, while the three other species are annual weeds. A survey of five Echinochloa species in Malaysia found that four of them were infested with E. leucotaeniella. Infestation with E. picta was particularly heavy. However, no infestation was found of E. colona or of rice.
It is important that the stemborer entered into stems, not only of annual but also of perennial species, because new stems from perennial species are present throughout the year to serve as food for the stem borer. This means that a high moth density can be maintained throughout the year.
Intensive programs to use Echinochloa as a biological control agent can be expected in the future. These will both ensure that there is a food supply for the stemborer throughout the year, and synchronize the period of oviposition of E. leucotaeniella with the vegetative stage of Echinochloa plants.
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Dr. Banpot commented that a different species of the same genus Emmalocera* is a stem borer pest of sugarcane in India. He recommended that great care should be taken in introducing it as a weed control agent, to prevent any unwanted effects on non-target species. Dr. Goto was asked whether other species of insect are known to attack barnyard grass in Japan. He replied that there are a number of natural enemies, including the Oriental corn borer, but this also attacks crops.
Dr. Mochida enquired whether Dr. Goto had attempted the mass production of E. leucotaeniella. Dr. Goto replied that some work had been done on this, but that there were still considerable difficulties.
Dr. Kim was interested in the possibility of differences in the response to E. leucotaeniella in different rice varieties. Even if E. leucotaeniella were to give good control of barnyardgrass, he felt there might be a danger that some rice varieties would be susceptible. Dr. Goto agreed that thorough testing would be necessary, and felt that this should cover the feeding behavior of the larvae as well as the ovipositing behavior of the female, to see whether the species is sufficiently closely adapted to Echinochloa. He added that numerous host feeding tests using many different plant species had failed to show that E. leucotaeniella had any other host plant than Echinochloa.
Index of Images
Figure 1 Male (Left) and Female (Right) of E. Leucotaeniella
Figure 2 The Relationship between E. Leucotaeniella and Echinochloa
Figure 3 Relationship between the Leaf Stage and Rate of Diapause
Figure 4 Relationship between Leaf Stage and DRY Weight in Pot
Figure 5 Rate at Which the Stem Is Eaten by E. Leucotaeniella, and Rate of Occurrence of Leaves Used As Food
Table 1 Comparison between the Ecological and Physiological Properties of E. Oryzicola and Those of E. Crus-Galli Var. Crus-Galli
Table 2 Relationship between E. Leucotaeniella and Two Echinochloa Species
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