Vetiver grass (Vetiveria zizanioides (L.) Nash) as trap plant for Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) and Busseola fusca (Fuller) (Lepidoptera: Noctuidae)

The preference of lepidopterous stem borer moths to oviposit on certain wild host plants can be exploited in habitat management systems by using those hosts as trap crops. Vetiver grass (Vetiveria zizanioides (L.) Nash) was evaluated for its attractiveness and suitability to the pyralid Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) and the noctuid Busseola fusca (Fuller) (Lepidoptera: Noctuidae). Two choice tests were conducted in the laboratory and in the greenhouse to determine oviposition choice of C. partellus for maize, Vetiver and rice (Oryza sativa L.), and of B. fusca for Vetiver and maize. C. partellus larval survival was evaluated in green house studies. Results indicated that C. partellus chose Vetiver grass over maize though larval survival on Vetiver was extremely low. B. fusca did not show any host preference.     

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.     

Assessment of Napier millet (Pennisetum purpureumx P. glaucum) and sorghum (Sorghum bicolor) trap crops for the management of Chilo partellus on maize

Two Napier millet (Pennisetum purpureumxP. glaucum) hybrids, namely PBN 83 and PBN 233 and one sorghum (Sorghum bicolor) variety, SL 44, were assessed for their potential role as a trap crop in the management of the stem borer, Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) on maize. Oviposition preference and larval survival and development were determined for different test plants under laboratory and screen house conditions. Further, field dispersal of C. partellus larvae was assessed between Napier millet and maize crops. Results from no-choice and dual-choice tests indicated that Napier millet hybrids were preferred for oviposition over maize by C. partellus moths. Sorghum was, however, not preferred over maize in this respect. Napier millet hybrids were poor larval hosts, and a rapid decline in larval numbers was noticed within the first five days after hatching and virtually no larvae survived to pupation. Leaf area eaten by the borer larvae was significantly less on these hybrids than on maize or sorghum. Plant damage was more severe in maize and sorghum than Napier millet hybrids. No appreciable larval shift was noticed from Napier millet hybrids to the adjoining maize crop. The evaluated Napier millet hybrids, therefore, had potential for use as trap crop in C. partellus management. Sorghum, however, did not hold promise in this respect.     

Oviposition acceptance and larval development of Chilo partellus stemborers in drought‐stressed wild and cultivated grasses of East Africa

Maternal host choices during oviposition by herbivorous insects determine the fitness of their offspring and may be influenced by environmental changes that can alter host‐plant quality. This is of particular relevance to ‘push‐pull’ cropping systems where host preferences are exploited to manage insect pest populations. We tested how drought stress in maize and companion plants that are used in these systems affect oviposition preference, larval feeding, and development of the spotted stemborer, Chilo partellus Swinhoe (Lepidoptera: Crambidae). Five host species were tested (all Poaceae): maize (Zea mays L.), Napier grass (Pennisetum purpureum Schumach), signal grass [Brachiaria brizantha (A. Rich) Stapf], Brachiaria cv. ‘Mulato’, and molasses grass [Melinis minutiflora (Beauv.)]. Under drought stress, maize experienced as much oviposition as control unstressed maize in choice and no‐choice experiments. Similarly, larval leaf damage was not significantly different in drought‐stressed vs. unstressed maize. In contrast, oviposition occurred less on drought‐stressed than on unstressed Napier and signal grass. Oviposition acceptance and leaf damage remained low in both drought‐stressed and unstressed molasses grass and Mulato. Larval survival and development remained high in drought‐stressed maize, but not in Napier, signal, and molasses grass and Mulato, where survival and development were low in both drought‐stressed and unstressed plants. Our results indicate that herbivore responses to drought‐stressed plants depend on the plant species and that drought stress can change host preference and acceptance rankings. In particular, trap‐crops such as Napier grass may not divert oviposition from the main maize crop under drought stress conditions.     

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.     

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.     

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.     

Spatial dynamics of lepidopterous pests on Zea mays (L.) and Pennisetum purpureum (Moench) in the forest zone of Cameroon and their implications for sampling schemes

The most common lepidopterous borers attacking maize and/or the wild host Pennisetum purpureum in the forest zone of Cameroon are the noctuids Busseola fusca (Fuller), Sesamia calamistis (Hampson) and Poeonoma serrata (Hampson), the pyralids Eldana saccharina (Walker) and Mussidia nigrivenella (Ragonot), and Cryptophlebia leucotreta (Tortricidae). The within-plant distribution on maize and elephant grass was studied for the predominant species B. fusca , and on maize only for E. saccharina to determine the basic sampling unit. On both plant species, B. fusca showed a strong oviposition preference for young plant parts. By contrast, E. saccharina larvae and pupae on maize were only found on older plant parts indicating that it does not oviposit on young plants. No part of the plant strata appeared to be a stable sampling unit and it is recommended to carry out whole plant or whole tiller sampling for maize and grass, respectively. For the development of sampling plans, dispersion was described for all species using Taylor's power law and a non-linear model which gives the relationship between the proportion of infested plants [ P ( I )] and mean density ( m ). Busseola fusca egg batches as well as diapausing larvae and pupae on maize showed a random distribution whereas all the other cases were aggregated, with B. fusca egg batches on elephant grass exhibiting the lowest and M. nigrivenella on maize the highest aggregation. Optimal sample size/mean density curves were developed for groups of insects with low and high aggregation.     

Local adaptation in oviposition choice of a specialist herbivore: The cinnabar moth

Specialist herbivores feed on a restricted number of related plant species and may suffer food shortage if overexploitation leads to periodic defoliation of their food plants. The density, size and quality of food plants are important factors that determine the host plant choice of specialist herbivores. To explore how all these factors influence their oviposition behaviour, we used the cinnabar moth Tyria jacobaeae and the hybrids of a cross between Jacobaea vulgaris and J. aquatica as a study system. While defoliation by the cinnabar moth is common in the coastal area of The Netherlands, it is relatively rare in inland ragwort population. Ragworts contain pyrrolizidine alkaloids (PAs) and those that are found in coastal areas are rich in jacobine-like PAs while those that occur inland are rich in erucifoline-like PAs. We tested how the oviposition preference was influenced by plant size, nitrogen and water content and PA composition. We used cinnabar moth populations from a regularly defoliated area, Meijendel, and Bertogne, a rarely defoliated area. Our results revealed no effects of nitrogen or water content on oviposition preference. Moths from both populations laid larger egg batches on the plants rich in jacobine-like PAs. Moths from Meijendel preferred larger plants and spread their eggs over more egg batches that were, on average, smaller than those of Bertogne moths. These results suggest that Meijendel moths adopted a oviposition strategy to cope with potential defoliation.     

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.     

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.     

Maize landraces recruit egg and larval parasitoids in response to egg deposition by a herbivore

Natural enemies respond to herbivore-induced plant volatiles (HIPVs), but an often overlooked aspect is that there may be genotypic variation in these 'indirect' plant defence traits within plant species. We found that egg deposition by stemborer moths (Chilo partellus) on maize landrace varieties caused emission of HIPVs that attract parasitic wasps. Notably, however, the oviposition-induced release of parasitoid attractants was completely absent in commercial hybrid maize varieties. In the landraces, not only were egg parasitoids (Trichogramma bournieri) attracted but also larval parasitoids (Cotesia sesamiae). This implies a sophisticated defence strategy whereby parasitoids are recruited in anticipation of egg hatching. The effect was systemic and caused by an elicitor, which could be extracted from egg materials associated with attachment to leaves. Our findings suggest that indirect plant defence traits may have become lost during crop breeding and could be valuable in new resistance breeding for sustainable agriculture.     

Preference of Bt‐resistant and susceptible Busseola fusca moths and larvae for Bt and non‐Bt maize

The sustainability of genetically engineered insecticidal Bacillus thuringiensis Berliner (Bt) maize, Zea mays L. (Poaceae), is threatened by the evolution of resistance by target pest species. Several Lepidoptera species have evolved resistance to Cry proteins expressed by Bt maize over the last decade, including the African maize stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae). The insect resistance management (IRM) strategy (i.e., the high‐dose/refuge strategy) deployed to delay resistance evolution is grounded on certain assumptions about the biology and ecology of a pest species, for example, the interactions between the insect pest and crop plants. Should these assumptions be violated, the evolution of resistance within pest populations will be rapid. This study evaluated the assumption that B. fusca adults and larvae select and colonize maize plants at random, and do not show any preference for either Bt or non‐Bt maize. Gravid female B. fusca moths of a resistant and susceptible population were subjected to two‐choice oviposition preference tests using stems of Bt and non‐Bt maize plants. Both the number of egg batches as well as the total number of eggs laid on each stem were recorded. The feeding preference of Bt‐resistant and susceptible neonate B. fusca larvae were evaluated in choice test bioassays with whorl leaf samples of specific maize cultivars. Although no differential oviposition preference was observed for either resistant or susceptible female moths, leaf damage ratings indicated that neonate larvae were able to detect Bt toxins and that they displayed feeding avoidance behaviour on Bt maize leaf samples.     

Responses of ovipositing moths to host plant deprivation: life history aspects and implications for intercropping

1. When considering intercropping as a strategy to reduce pest oviposition, knowledge about the insect’s oviposition behaviour is very important. Physiological effects on the insect because of difficulties in finding a suitable oviposition site may also be important. 2. In the present study, the effects that delays in access to host plants have on lifetime fecundity on diamondback moth and leek moth were examined. The ability to postpone egg laying, fecundity and lifetime oviposition are discussed in relation to intercrop/cover crop as a strategy to reduce oviposition on crop plants. 3. When faced with host plant deprivation, the diamondback moth is relatively more dependent upon host plant stimuli for the onset of egg production. By contrast, leek moth is able to postpone egg production for a longer time. There even appeared to be a tendency for leek moth females to extend their lifetime when faced with host plant deprivation. 4. We conclude that leek moths have the ability to postpone production of eggs and lay them later in life when finally encountering host plants after a period of host plant deprivation. Therefore, the use of intercropping as a strategy to reduce oviposition is questionable. For such an insect, use of a trap crop might be a better option because the female will lay her eggs in the trap crop and not get the opportunity to lay them later in life when finally encountering crop plants.     

Evidence of the establishment of Cotesia flavipes (Hymenoptera: Braconidae), a parasitoid of cereal stemborers,and its host range expansion in Ethiopia

Three lepidopteran cereal stemborers, Chilo partellus (Swinhoe) (Crambidae), Busseola fusca Fuller, and Sesamia calamistis Hampson (Noctuidae) were collected from maize and sorghum in Ethiopia. The noctuid stemborers are indigenous to Africa while C. partellus is an introduced species from Asia. In 1999, the Asian stemborer parasitoid, Cotesia flavipes Cameron (Braconidae) was found to be widespread in Ethiopia, even though it had never been released in the country. In addition to attacking Chilo partellus, Cotesia flavipes was reared from B. fusca and S. calamistis. The origin of C. flavipes in Ethiopia may have been Somalia where it was released in 1997 near the border with eastern Ethiopia. Percent parasitism of borers by C. flavipes was higher in eastern Ethiopia than other surveyed regions, and parasitism was higher in 2000 than 1999. Parasitism was higher when cereals were intercropped with other plants and when wild grass hosts of stemborers were present.     

Evaluation on potential of wild hosts as trap plants for managing gramineous stemborers in maize based-agroecosystem

As part of habitat management system to control cereal stemborers, various wild hosts used as trap plants were studied during the dry season from November 2003 to March 2004 at Melkassa, central Ethiopia. Five wild hosts of the family Poaceae [Pennisetum purpurum (Schumach), Sorghum vulgare variety sudanense (Pers.), Panicum coloratum L., Sorghum arundinaceum Stapf, and Hyperrhania rufa (Nees)] were evaluated as trap plants in maize, Zea mays L.,-based agroecosystem. The results of the study showed that maize plots surrounded by all tested wild hosts had significantly lower mean percentage of foliage damage and stemborer density than maize monocrop plots 15 m away from the treatment blocks. Interestingly, mean foliar damage and stemborer density between maize plots surrounded by wild hosts and maize monocrop plots within the treatment blocks was not significant. Percentage of tunneled stalks was significantly greater in maize monocrop plots 15 m away from the treatment blocks than in maize plots surrounded by all tested wild host plant species. Moreover, the highest mean percentage of parasitism (62%) of Chilo partellus (Swinhoe) by Cotesia flavipes (Cameron) was recorded in maize plots surrounded by P. purpureum. Therefore, the findings revealed that these wild hosts have considerable merit to be used as trap plants in the development of strategies for managing cereal stemborers in maize crops.     

Motivation models fail to explain oviposition behaviour in the diamondback moth

Abstract. It is widely accepted that previous experience and internal physiological factors (such as egg‐load) affect host‐plant discrimination during oviposition by phytophagous insects. However, there is some debate as to how these factors interact in a mechanistic sense to control acceptance. The role of learning and host deprivation in host acceptance by adult diamondback moths (Plutella xylostella L.) was investigated. In the first experiment, we tested whether experience of a lower ranked host cabbage increased acceptance of a higher ranked host as predicted by a hierarchy threshold model. Moths trained on cabbage were over three times more likely to accept cabbage during testing than untrained moths. There was no effect of cabbage training on acceptance of cress, indicating that the effect of training was species‐specific. In a second experiment, designed to test the prediction of motivation models that insects become less discriminating when deprived of oviposition opportunities, depriving females of host plants for 2 nights significantly increased female egg‐load (×2.3). Host deprivation did not decrease discrimination between the preferred host cress and cabbage. Cabbage and cress plants were equally likely to have been accepted by nondeprived moths after 1 night of exposure, yet moths deprived of hosts for 2 nights strongly preferred cress when tested during the first 20 min of the scotophase. During this 20‐min period, previous host deprivation increased acceptance of host plants generally but did not decrease discrimination between hosts. These data contradict the expectation that there is an inverse relationship between host species discrimination and the failure of an insect to find hosts as found in existing oviposition acceptance models. As an alternative, the Incremental Acceptance Model of host acceptance behaviour is presented, in which responsiveness to a host is a function of the recent encounter rate with host‐specific stimuli, and the oviposition reflex is regulated by nonspecific cues such as egg‐load.     

Feeding Experience of Bemisia tabaci (Hemiptera: Aleyrodidae) Affects Their Performance on Different Host Plants

The sweetpotato whitefly, Bemisia tabaci biotype B is extremely polyphagous with >600 species of host plants. We hypothesized that previous experience of the whitefly on a given host plant affects their host selection and performance on the plants without previous experience. We investigated the host selection for feeding and oviposition of adults and development and survival of immatures of three host-plant-experienced populations of B. tabaci, namely Bemisia-eggplant, Bemisia-tomato and Bemisia-cucumber, on their experienced host plant and each of the three other plant species (eggplant, tomato, cucumber and pepper) without previous experience. We found that the influence of previous experience of the whiteflies varied among the populations. All populations refused pepper for feeding and oviposition, whereas the Bemisia-cucumber and the Bemisia-eggplant strongly preferred cucumber. Bemisia-tomato did not show strong preference to any of the three host palnts. Development time from egg to adult eclosion varied among the populations, being shortest on eggplant, longest on pepper, and intermediate on tomato and cucumber except for the Bemisia-cucumber developed similarly on tomato and pepper. The survivorship from egg to adult eclosion of all populations was highest on eggplant (80-98%), lowest on pepper (0-20%), and intermediate on tomato and cucumber. In conclusion, the effects of previous experience of whiteflies on host selection for feeding and oviposition, development, and survivorship varied depending on host plants, and host plants play a stronger role than previous experience. Preference of feeding and oviposition by adults may not accurately reflect host suitability of immatures. These results provided important information for understanding whitefly population dynamics and dispersal among different crop systems.     

Distribution ofChilo partellus (Lepidoptera: Pyralidae) egg batches on maize

Distribution ofChilo partellus egg masses was studied in field, greenhouse, and laboratory experiments. The eggs were laid in batches mainly on the lower side and the lower leaves of the plant. The egg batch size ranged from 1 to 169 eggs, with a median of 33.5 eggs per batch (average, 40.5). Oviposition ofC. partellus is described at two levels. The first level, choice of oviposition plants, followed a random distribution. The second level, number of egg batches per plant, followed an aggregated distribution in the field, where more than one egg batch was deposited on the same plant by the same female, which was found on 25% of the oviposition plants. A mechanism for egg-layingC. partellus females to perceive preceding oviposition or injured plants could not be detected. Oviposition site choice seemed to be mediated by tactile stimuli.