Host plant selection behaviour of Chilo partellus and its implication for effectiveness of a trap crop

Female lepidopterans can display a hierarchy of preference among potential host species, a trait thought to arise from the balance between attractants and deterrents to which the insects respond. Host plant ranking by moths and larvae of Chilo partellus Swinhoe (Lepidoptera: Crambidae), an important pest of cereals in Africa, was investigated, and whether eggs deposited on specific host plants yield larvae of particular host preferences. Trap plants are used in management of this pest. However, any ‘disagreement’ in host ranking between moths and larvae could potentially reduce effectiveness of trap crops as larvae emigrate to the main crop from the parent’s preferred trap plant. We also investigated whether host plant preference is influenced by the diet upon which larvae fed as part of an integrated assessment of the relationship between host plant selection and learning in C. partellus. Five host plants (all Poaceae) were used: maize (Zea mays L.), sorghum (Sorghum bicolor Moench), Napier grass (Pennisetum purpureum Schumach), and two varieties of signal grass [Brachiaria brizantha (A. Rich.) Stapf], viz., local (henceforth signal grass) and improved (‘Mulato’). In multiple choice tests, C. partellus female moths preferentially oviposited on Napier grass, followed by sorghum, maize, and signal grass, and least preferred ‘Mulato’. Larvae however equally orientated and settled on leaf cuts of maize, sorghum, signal grass, and Napier grass, but least preferred ‘Mulato’. Moreover, eggs from specific host plants did not yield larvae of particular host preferences. Furthermore, oviposition preference was not altered by the larval food. These results imply only a slight ‘disagreement’ in host ranking behaviour between moths and larvae, which is beneficial for trap cropping as larvae would not ‘reject’ the trap plant and appreciably disperse to the neighboring plants. Moreover, absence of larval learning behaviour indicates that regardless of the larval food C. partellus moths would still be attracted to the selected trap plant.     

Diapause initiation and incidence in the millet stem borer, Coniesta ignefusalis (Lepidoptera: Pyralidae): the role of the host plant

The role of the host plant in the development of larval diapause in the millet stem borer, Coniesta ignefusalis (Hampson) was investigated in northern Ghana in 1996 and 1997. Surveys conducted in farmers' fields in the Guinea and Sudan savannah revealed that of all the upland cereals grown, the insect survived the dry season only in stalks and stubble of pearl millet, Pennisetum glaucum and late sorghum, Sorghum bicolor. This observation was confirmed by results from field trials conducted at the Manga Research Station. In these studies, C. ignefusalis larvae entered diapause only in late millet and late sorghum, with a higher incidence in the former. The insect neither attacked nor entered diapause in maize planted during the same period as the other crops. Results from controlled experiments showed that diapause incidence in the preferred host, millet, was higher in older than in younger plants, suggesting that host plant maturation is a key factor influencing the development of larval diapause in C. ignefusalis.     

The effect of nitrogen fertilizer application to maize and sorghum on the bionomics of Chilo partellus (Lepidoptera: Crambidae) and the performance of its larval parasitoid Cotesia flavipes (Hymenoptera: Braconidae)

Laboratory and field trials were conducted to evaluate the effect of plant species (maize, sorghum), plant age (young, middle, old) and four different nitrogen fertilization levels (N0-N3) on the bionomics of the invasive crambid Chilo partellus and the performance of its braconid larval parasitoid Cotesia flavipes. Plant N varied significantly between N0 and N1-N3, but the differences among the latter were not significant. Intrinsic rates of increase and net-reproductive rates of C. partellus followed the same trends: they were lowest with N0 and similar among the other treatments. On maize only, mortality of C. partellus and parasitism by C. flavipes tended to decrease with age of the plant while the percentage of borers reaching adulthood (i.e. pupation) increased. Borer mortality and parasitism was lower and pupation higher on sorghum than on maize. On both host plants, percent dry matter content of frass, which could affect ingress of the parasitoid into the borer tunnel, did not vary with nitrogen level but varied with age of the host plants: on maize, it was highest on young plants and on sorghum on old plants. Tunnels were shorter on young maize and sorghum plants; longer tunnels on older plants indicated compensatory feeding by the larva as a result of lower nutritive value of the food source. Consequently, larval weight was lower on older than younger plants. The level of nitrogen fertilization had no effect on food conversion efficiency of C. partellus. Nitrogen did not affect number of C. flavipes progeny while egg load of progeny increased significantly with nitrogen level, on both plant species. Differences in egg load between sorghum and maize were mostly not significant. It was concluded that on depleted soils only, an increase in nitrogen via mulching, rotation with a leguminous crop or fertilization would increase survival of C. partellus on both maize and sorghum and an increase in acreage of maize and in application of nitrogen fertilizer in an area would also increase the parasitism of C. flavipes.     

Assessment of the potential of Napier grass (Pennisetum purpureum) varieties as trap plants for management of Chilo partellus

Ten Napier grass [Pennisetum purpureum Schumach (Poaceae)] varieties, used in various parts of Kenya as animal fodder, were tested for their potential role as a trap crop in the management of the gramineous spotted stemborer, Chilo partellus (Swinhoe) (Lepidoptera: Crambidae), through habitat manipulation. Oviposition preference and larval survival and development were determined for each of these varieties under laboratory and screen‐house conditions. Two‐choice tests revealed that seven of the varieties tested were preferentially chosen by gravid female moths for oviposition over a susceptible maize cv. Inbred A. Larval survival was significantly lower on the majority of the Napier grass varieties (about 10%) than on maize cv. Inbred A (about 60%). Similarly, larval development was about 2 weeks longer on the majority of the Napier grass varieties. It was concluded that four of the varieties tested (Bana, Ex‐Machakos, Gold Coast, and Ex‐Nyanza‐2) had potential for use as trap crops in C. partellus management because they were more preferred by the borer moths for oviposition and allowed minimal survival of the larvae. This minimal larval survival is desirable in the conservation of C. partellus natural enemies when the cultivated host plant is out of season.     

Effect of learning on the oviposition preference of field-collected and laboratory-reared Chilo partellus (Lepidoptera: Crambidae) populations

Recent studies show that Vetiver grass, (Vetiveria zizanioides (L.) Nash), may have potential as a dead-end trap crop in an overall habitat management strategy for the spotted stem borer, Chilo partellus (Swinhoe) (Lepidoptera: Crambidae). Vetiver grass is highly preferred for oviposition, in spite of the fact that larval survival is extremely low on this grass. The oviposition behaviour of female Chilo partellus moths was investigated by determining the amount and size of egg batches allocated to maize and Vetiver plants and studying the effect of rearing conditions and oviposition experience on host plant selection. Two-choice preference tests were used to examine the effect of experience of maize (a suitable host plant) and Vetiver plants on the oviposition choice of C. partellus. For both field-collected and laboratory-reared moths, no significant differences were found in the preference distributions between the experienced groups. It is concluded that females do not learn, i.e. that they do not change their preference for Vetiver grass after having experienced oviposition on either maize or this grass, which supports the idea that trap cropping could have potential as a control method for C. partellus. Differences observed between field-collected and laboratory-reared moths in the amount and size of egg batches laid on maize and Vetiver grass indicate that data obtained from experiments with laboratory-reared insects should be treated with caution.     

Field boundaries of Panicum maximum as a reservoir for predators and a sink for Chilo partellus

Abstract:  The biological-control function of field boundaries of Guinea grass, Panicum maximum Jacq. on the spotted stem borer, Chilo partellus Swinhoe was examined as a reservoir for arthropod predators and as a trap plant for the pest. Field border vegetation and predator density were manipulated to determine the effect of the grass border on the abundance of stem borers and their predators in maize fields, and the effect of predators on the stem borer population. The strip of Guinea grass supported an abundance of earwigs and spiders, the potential predators of stem borer eggs and larvae. Density of C. partellus larvae in the Guinea grass strips was low throughout the season and only young larvae were collected, suggesting the inferiority of the grass stand as a habitat for stem borer larvae. These results indicate that Guinea grass is a good agent of habitat management to selectively enhance arthropod predators of stem borers and act as a sink for the pest. Predator removal resulted in a higher density of C. partellus than control in maize-bordered plots. On the other hand, no difference was found in the stem borer density between predator treatments in grass-bordered plots, probably because of insufficient predator reduction in removal plots. These results suggest that the predator assemblage found in the study site has, if sufficiently abundant, potential to limit the C. partellus population in maize fields. Even though the Guinea grass stand harboured an abundant number of predators, the grass boundaries around maize fields did not enhance predator populations within the crop field. Furthermore, field boundaries of Guinea grass had no measurable effect on the within-field density of C. partellus as a trap crop. Creating a polyculture within the crop and early planting of the grass could further enhance the biological-control function of Guinea grass boundaries.     

Evaluation of Napier grass (Pennisetum purpureum) varieties for use as trap plants for the management of African stemborer (Busseola fusca) in a push–pull strategy

We evaluated eight Napier grass [Pennisetum purpureum Schumach (Poaceae)] varieties, used in various parts of eastern Africa as fodder, for their potential role as trap plants in the management of the African stemborer, Busseola fusca Füller (Lepidoptera: Noctuidae) through a push–pull strategy. Oviposition preference, larval orientation, settling, arrest and dispersal, feeding, mortality and survival, and development were determined for each of these varieties under laboratory and screen house conditions. Two‐choice tests showed that only two of the varieties tested (cv. Bana and cv. Uganda Hairless) were preferentially chosen by gravid female moths for oviposition over a susceptible maize variety, cv. Western Hybrid 502. Larval preference was, however, highly variable. Larval feeding by first instars on the maize leaves was more intense and significantly more than on leaves of all the Napier grass varieties evaluated. Food consumed and amounts assimilated by the third instars over a 24‐h period were not different among larvae fed on stems of maize and those fed on stems of the various Napier grass varieties. Larval survival was significantly lower on all the Napier grass varieties (below 3%) than on maize (about 44%). Similarly, larval development was about 2–3 weeks longer on majority of the Napier grass varieties. It was concluded that cv. Bana had potential for use as a trap plant in the management of B. fusca because it was more preferred by the moths for oviposition, equally preferred as maize by the larvae for orientation, settling, and arrest, and allowed minimal survival of the larvae. It can thus be used with such ‘push’ plants as Desmodium spp. (Fabaceae) in a ‘push–pull’ strategy, but the effectiveness of such a strategy would strictly depend on proper establishment and management of these companion plants.     

Some aspects of feeding responses in the stem borer Chilo partellus swinhoe (Lepidoptera: Pyralidae).

Feeding responses of Chilo partellus larvae to different food sources, were studied. Maize and sorghum were more preferred to artificial diet by the larvae irrespective of their rearing media. Larvae fed on maize and sorghum leaves as well as maize and sorghum stems. The rearing medium did not have any effect on the feeding preference; it could only improve but not induce the feeding behaviour.     

Establishment of Cotesia flavipes (Hymenoptera: Braconidae) in sugarcane fields of Ethiopia and origin of founding population

Cotesia flavipes (Cameron) (Hymenoptera: Braconidae) is used as a classical biological control agent against Chilo partellus (Swinhoe) (Lepidoptera: Crambidae), a serious exotic pest of cereal crops in eastern and southern Africa. This parasitoid has been introduced into several African countries for the control of C. partellus in maize, Zea mays L., and sorghum, Sorghum bicolor (L.), but it has never been released in Ethiopia. It is hypothesized that it spread into Ethiopia from populations released in Kenya and Somalia to become the predominant parasitoid of C. partellus in maize and sorghum fields of the country. In recent surveys conducted in Ethiopia, C. flavipes was recovered from C. partellus in sugarcane, Saccharum L. spp. hybrids, at a site >2,000 km from the nearest known release sites in Kenya and Somalia. These findings question published hypotheses that estimate the dispersal rate of C. flavipes to be 60 km per year in Africa, and they suggest that since its release in Africa this parasitoid has developed strains adapted to searching particular host plants infested by particular stem borers. The anomalies between our results and previous reports evoked the hypothesis that C. flavipes in Ethiopian sugarcane might be a different strain. To test this hypothesis, we compared partial COI gene sequences of C. flavipes collected from sugarcane in Ethiopia and those of specimens from other African countries to determine the origin of the Ethiopian population. In addition, COI sequences were obtained for C. flavipes from other continents. The C. flavipes population established in Ethiopian sugarcane is most closely related to the populations released against C. partellus in maize in other parts of Africa, which were derived from the original population imported from Pakistan. The dispersal rate of the parasitoid was estimated to be >200 km per year.     

Population dynamics of caterpillars on three cover crops before sowing cotton in Mato Grosso (Brazil)

Direct seeding mulch-based cropping systems under a preliminary cover crop such as millet are common in some areas of Brazil. Lepidopteran pests that damage cotton, soybean and maize crops can proliferate on cover crops, so preventive chemical treatments are necessary. Very little data is available on these pests on cover crops. This paper presents the dynamics of Spodoptera frugiperda, S. eridania, Mocis latipes and Diatraea saccharalis caterpillars monitored at Primavera do Leste, Mato Grosso state (Brazil) during the of 2005/2006 and 2006/2007 cropping seasons on four cover crops, i.e. finger millet (Eleusine coracana), pearl millet (Pennisetum glaucum), sorghum (Sorghum bicolor) and ruzigrass (Brachiaria ruziziensis). The pests were visually counted on plants within a 1 m2 transect (wooden frame). Caterpillars were reared to facilitate identification of collected species and parasitoids. Many S. frugiperda caterpillars were observed on millet in 2005, with a maximum of 37 caterpillars/m2. On sorghum, we found 30 caterpillars/m2, or 0.83 caterpillars/plant. The Diatraea borer attacked sorghum later than the other pests. M. latipes was also observed on millet. The millet cover crop had to be dried for at least 1 month before direct drilling the main cotton crop in order to impede S. frugiperda infestations on cotton plantlets, thus avoiding the need for substantial resowing. The comparative methodological aspects are discussed.     

Distribution and relative importance of cereal stem borers and their natural enemies in the semi-arid and cool-wet ecozones of the Amhara State of Ethiopia

The distribution and relative importance of lepidopteran and coleopteran stem borers and their natural enemies on maize and sorghum were studied in cereal growing zones of the Amhara State of Ethiopia from 2003 to 2004. Sorghum is the major crop in semi-arid eastern and maize in the cool-wet western zones of the Amhara state. Four administrative zones, 10 districts and 88 localities in the semi-arid ecozone (SAE) and four zones, 19 districts and 71 localities in the cool-wet ecozone (CWE) were chosen for the study. In SAE, the species composition was 91% Chilo partellus (Swinhoe) (Lepidoptera: Crambidae), 8% Busseola fusca (Fuller) (Lepidoptera: Noctuidae) and 1% Sesamia calamistis Hampson (Lepidoptera: Noctuidae). In the CWE, maize and sorghum are grown in different ecozones and thus B. fusca was the dominant species on sorghum, whereas 61% B. fusca and 39% S. calamistis were recorded on maize. Borer density generally increased with crop growth stage. C. partellus parasitism by C. flavipes Cameron (Hymenoptera: Braconidae), which occurred only in SAE, varied among districts ranging from 5% to 39%. In the CWE, unidentified nematodes parasitized medium-sized B. fusca larvae during the wet months. Population of native parasitoids was very low. The coleopteran borer, Rhynchaenus niger (Horn) (Coleoptera: Rhynchophoridae), attacked sorghum plants in both regions. Sorghum yields were negatively related to plant damage variables and positively to larval parasitism and plant growth variables. On maize, plant damage was too low to affect yields. Taylor’s power law indicated aggregated distribution for C. partellus and B. fusca larvae and pupae combined.     

Associations between host plant concentrations of selected biochemical nutrients and Mexican rice borer,Eoreuma loftini,infestation

The Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Crambidae), is an economic pest of sugarcane (Saccharum spp. hybrids) and other graminaceous host crops, and it attacks grassy weeds. Oviposition preference has been known to be for plants with leaves that form folds. This study is the first to associate the nutritional quality of crop and forage plant hosts with Mexican rice borer injury. Three experiments were conducted to determine the levels of selected biochemical nutrients, including free amino acids and sugars, in four grass weeds [barnyardgrass, Echinochloa crus‐galli (L.); broadleaf signalgrass, Urochloa platyphylla (Munro ex C. Wright); johnsongrass, Sorghum halepense (L.) Pers.; and Vasey's grass, Paspalum urvillei Steud.], a forage grass [sudangrass, Sorghum bicolor (L.) Moensch spp. drummondi (Nees ex Steud.) de Wet & Harlan], three crop species [maize, Zea mays L.; sorghum, Sorghum bicolor (L.) Moench; and sugarcane, Saccharum spp.], and a sorghum‐sudangrass forage hybrid (all Poaceae). Of 16 free amino acids detected among plants in the first two experiments, only high accumulations of free histidine in sudangrass and maize were associated with increased infestations by Mexican rice borer larvae. In all three experiments, high levels of fructose were associated with heightened Mexican rice borer infestation. Ramifications of these findings on the potential dispersal of this invasive pest in the USA and possible applications of fructose in baits are discussed.     

Suitability of brachiaria grass as a trap crop for management of Chilo partellus

The cereal stemborer Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) is a major insect pest of sorghum (Sorghum bicolor L. Moench) and maize (Zea mays L.) in Africa. Trap cropping systems have been shown to be a valuable tool in management of this pest. To optimize trap cropping strategies, an understanding of host‐plant preference for moth oviposition and host suitability for larval survival on potential trap plants is a prerequisite. Therefore, we assessed seven brachiaria accessions (Poaceae) for preference by C. partellus moths and subsequent larval performance. In two‐choice tests with a local open‐pollinated maize variety (cv. Nyamula), significantly higher numbers of eggs were deposited on brachiaria accessions Marandu, Piata, and Xaraes than on maize, whereas fewer eggs were recorded on plants of Mulato II, Mulato I, and Cayman. There was a significant and negative correlation between the trichome density on plant leaves and C. partellus oviposition preference for brachiaria. In addition to poor larval performance on brachiaria, there was no clear ranking in the accessions regarding larval orientation, settling, arrest, and food ingestion and assimilation. First instars did not consume leaf tissues of brachiaria plants but consumed those of maize, which also suffered more stem damage than brachiaria plants. No larvae survived on brachiaria plant tissue for longer than 5 days, whereas 79.2% of the larvae survived on maize. This study highlights the preferential oviposition of C. partellus on brachiaria plants over maize and the negative effects that these accessions have on subsequent larval survival and development. Our findings support the use of brachiaria as a trap crop for management of C. partellus through a push‐pull technology.     

Effect of sorghum phenology on the control of Chilo partellus with Nosema marucae (microspora: Nosematidae)

An aqueous suspension of Nosema marucae spores was sprayed on foliage of sorghum plants that had been infested with neonate Chilo partellus larvae at 3, 5, 7 and 9 weeks after (plant) emergence (WAE). These periods corresponded to plants at the tillering, early booting, soft dough and late maturation phenological stages, respectively. The extent of borer infestation and plant damage was monitored until crop harvest, and compared with plants which had been similarly infested but not sprayed with the pathogen, and with plants in insect‐free control plots. When applied early, infestation with neonate borer larvae caused most damage to the plants and the greatest reduction in yield in the untreated plots. Early treatment with N. marucae spores at 3 and 5 WAE resulted in the greatest reduction in damage to plants and the highest improvement in yield of seeds. It is apparent, therefore, that for maximum crop protection the microsporidian needs to be applied when sorghum is in an early phenological stage.     

Effect of sorghum resistant to Stenodiplosis sorghicola (Coquillett) (Diptera: Cecidomyiidae) on oviposition and development of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae)

Abstract Helicoverpa armigera oviposition preference for, and larval development on sorghum hybrids with differing resistance to sorghum midge, Stenodiplosis sorghicola , were investigated. When H. armigera larvae were fed seed of resistant and susceptible hybrids in the laboratory there were no differences in larval and pupal sizes or the rate of development. The same result was recorded when larvae fed on panicles on plants in a glasshouse. On some sampling occasions, significantly more eggs were laid on panicles of resistant hybrids in the field. This occurred when plants were in plots and also in a mixed planting. Midge-resistance status did not affect levels of egg parasitism. In a field study using recombinant inbred lines between a midge-resistant and a midge-susceptible line, no relationship was found between level of resistance and oviposition of H. armigera . We conclude that, although midge-resistant hybrids are sometimes preferred for oviposition by H. armigera, the resistance per se does not determine this preference. Egg survival, larval survival, development and resultant damage are not significantly affected by the midge-resistance status of the host.     

Tolerance of Bt corn (MON 810) to maize stem borer, Chilo partellus (Lepidoptera: Pyralidae)

Transgenic corn (MON 810), expressing the Bacillus thuringiensis (Bt) protein, Cry1Ab, was evaluated under greenhouse conditions for its tolerance to the maize stem borer, Chilo partellus. Bt corn (MON 810) provided effective protection against the stem borer even under a high level of larval infestation in the greenhouse. The observed tolerance is examined and discussed in the light of the susceptibility of C. partellus to the Cry1Ab protein in laboratory bioassays. The implications of the tissue concentrations of Cry1Ab in MON 810, and baseline susceptibility recorded in the current study, for insect-resistance management are discussed.     

Bt-maize as a potential trap crop for management of Eldana saccharina Walker (Lep., Pyralidae) in sugarcane

Abstract:  Notwithstanding the introduction of several pest management tactics, the stalk borer Eldana saccharina Walker (Lep., Pyralidae) remains the most serious pest in South African sugarcane. A novel tactic for managing this pest in sugarcane would be the use of a dead-end trap crop that attracts moths for oviposition and curtails subsequent larval development, thereby reducing pest population size. Glasshouse bioassays, in which moths chose to oviposit on maize producing Bacillus thuringiensis Cry1Ab toxin ( Bt -maize), non- Bt -maize or sugarcane of two cultivars (borer-resistant and -susceptible), showed that E. saccharina laid significantly more eggs and egg batches per dry leaf and unit mass of dry leaf on maize ( Bt or non- Bt ) than on either of the cane cultivars. When moths had a choice of ovipositing on 2-, 3-, 4- or 5-month-old maize ( Bt and non- Bt ), dry leaf number and mass of dry leaf material was significantly correlated with number of eggs and egg batches, indicating that older plants, which carried larger amounts of dry leaf matter, were more attractive for oviposition. Finally, glasshouse assays in which hatching larvae fed on 2.5-, 3.5- and 4.5-month-old Bt and non- Bt -maize plants, showed that the Cry1Ab toxin was effective in killing E. saccharina larvae in all Bt -maize plant growth stages, confirming that Bt -maize fulfilled the third requirement (curtailing larval development) of a dead-end trap crop for this pest. We argue that Bt -maize warrants further testing in the field as a trap crop, both alone and as a component of a 'push–pull' or habitat management system for E. saccharina in sugarcane.     

Species richness and parasitism in an assemblage of parasitoids attacking maize stem borers in coastal Kenya

Abstract. 1. Parasitoids were reared from four species of lepidopteran stem borer collected in maize in southern coastal Kenya from 1992 to 1999. The stem borers included three native species, Sesamia calamistis Hampson, Busseola fusca Fuller, and Chilo orichalcociliellus (Strand), and one exotic borer, Chilo partellus (Swinhoe). A total of 174 663 caterpillars was collected, of which 12 645 were parasitised.
2. Twenty-six primary parasitoid species were reared from the exotic borer, C. partellus , indicating a rapid accumulation of native parasitoids on the alien borer.
3. The three most abundant parasitoids were the larval parasitoids Cotesia sesamiae Cameron, Cotesia flavipes (Cameron), and the pupal parasitoid Pediobius furvus Gahan. The pupal parasitoid Dentichasmias busseolae Heinrich and the larval parasitoid Goniozus indicus Ashmead were also common. All used an ingress-and-sting method of attack.
4. Cotesia flavipes , introduced into Kenya in 1993, was found in all seasons from 1997 onwards, and has become the most abundant stem borer larval parasitoid in the area. A native congener, Cotesia sesamiae , appeared in all seasons from 1992 to 1999. Together, these two parasitoids accounted for 83.3% of the parasitised borers.
5. Thirty parasitoid species were recovered in Kilifi district, 27 in Kwale, and 15 in Taita Taveta. Parasitism was much greater in Taita Taveta district than in Kilifi or Kwale districts.     

Prairie grasses as hosts of the northern corn rootworm (Coleoptera: Chrysomelidae)

We evaluated 27 prairie grass species thought to be among those dominant 200 yr ago in the northern midwest as larval hosts of the northern corn rootworm, Diabrotica barberi Smith and Lawrence. Maize (Zea mays L.), spring wheat (Triticum aestivum L.), and grain sorghum (Sorghum bicolor L.) were included as controls for a total of 30 species. Twenty pots of each species were planted in a randomized complete block design. Each pot was infested 5 wk later with 20 neonate northern corn rootworm larvae. Two pots within each species and block were assigned an extraction date of 7 or 14 d after infestation. The remaining two pots from each block were used to monitor adult emergence. The percentage of larvae recovered, change in larval head capsule width, and change in average dry weights varied significantly among the grass species. The highest percentage of larvae was recovered from slender wheatgrass, Elymus trachycaulus (Link), and this was significantly greater than the percentage recovered from all other species including maize for the 14-d sample date. Several additional species were also relatively good hosts, in that the percentage of larvae recovered from these species was not significantly different from maize. The average dry weight of larvae recovered was significantly greater for larvae recovered from maize than for larvae recovered from all other species except slender wheatgrass, when the two samples dates were combined. Overall, adults were produced from only 6 of the 28 species evaluated, and no analysis was performed because of the low numbers. The results of this study are discussed in relation to the potential of alternate hosts of northern corn rootworm to serve as a bridge to survival on transgenic maize.     

Nitrate concentration and nitrate reductase activity in the leaves of three legumes and three cereals*

Nitrate concentration and nitrate reductase activity (NRA) were studied in the leaves of soybean (Glycine max), groundnut (Arachis hypogaea and cowpea (Vigna unguiculata) and sorghum (Sorghum bicolor), pearl millet (Pennisetum americanum) and maize (Zea mays) at three nitrogen fertiliser levels in two field experiments. Higher nitrate concentrations were detected in the leaves of groundnut, cowpea and pearl millet than in sorghum and maize. Nitrate content in the leaves and leaf NRA were not related across crop species, nor was a generalised pattern of leaf NRA and leaf nitrate observed within legumes or within cereals. Nitrogen application resulted in higher nitrate availability in the leaves, with varied leaf NRA.