Flight duration of the brown planthopper, Nilaparvata lugens (Homoptera: Delphacidae)

ABSTRACT.

• 1 The brown planthopper, Nilaparvata lugens (Stål), is a major pest of rice in Asia. It is known to make wind-assisted migratory flights each year to colonize the summer rice growing areas of China, Japan and Korea.
• 2 Modelling windborne displacements between rice growing areas in Asia requires migratory behaviour and flight duration to be established for this insect.
• 3 Field and laboratory observations suggest that N. lugens take-off at dusk and that some continue flying for up to 24–26 h if the temperature is ≥ 17°C.
• 4 Trajectories for 10 m above ground level and 1.5 km above mean sea level are used to identify possible sources and, hence, to estimate the flight times of N.lugens caught in nets on ships on the East China Sea in 1973 and 1981.
• 5 Estimated flight times between the sources and the ships ranged from about 9 to 30 h.
• 6 Results suggest that long-distance migration can occur in surface winds, when they are strong, but that long-distance migration is more likely at 1.5 km.
• 7 When simulating windborne displacements of N.lugens, it can be assumed that in areas and at heights where the temperature is ≥ 17°C, some migrants will fly downwind for up to 30 h after a dusk take-off. Others will fly for shorter periods, giving the population as a whole the opportunity to colonize all the rice crops flown over.

     

Bacterial Symbionts of the Brown Planthopper,Nilaparvata lugens (Homoptera: Delphacidae)

The brown planthopper (Nilaparvata lugens Stål), the most destructive pest of rice, has been identified, including biotypes with high virulence towards previously resistant rice varieties. There have also been many reports of a yeast-like symbiont of N. lugens, but little is known about the bacterial microbes. In this study, we examined the bacterial microbes in N. lugens and identified a total of 18 operational taxonomic units (OTUs) representing four phyla (Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes) by sequencing and analyzing 16S rRNA gene libraries obtained from three populations of N. lugens, which were maintained on the rice varieties TN1, Mudgo, and ASD7. Several of the OTUs were similar to previously reported secondary symbionts of other insects, including an endosymbiont of the psyllid Glycapsis brimblecombei, an Asaia sp. found in the mosquito Anopheles stephensi, and Wolbachia, found in the mite Metaseiulus occidentalis. However, the species and numbers of the detected OTUs differed substantially among the N. lugens populations. Further, in situ hybridization analysis using digoxigenin-labeled probes indicated that OTU 1 was located in hypogastrium tissues near the ovipositor and ovary in biotype 1 insects, while OTU 2 was located in the front of the ovipositor sheath in biotype 2 insects. In addition, masses of bacterium-like organisms were observed in the tubes of salivary sheaths in rice plant tissues that the insects had fed upon. The results provide indications of the diversity of the bacterial microbes harbored by the brown planthopper and of possible associations between specific bacterial microbes and biotypes of N. lugens.Close associations between insects and the microbes they harbor appear to be common. Symbionts have been found to contribute to the nutrition, development, reproduction, speciation, and defense against natural enemies of their host insects (1, 11, 18, 30, 39). The small brown planthopper (Laodelphax striatellus) and the white-backed planthopper (Sogatella furcifrea) also reportedly harbor an alphaproteobacterial Wolbachia symbiont (29) that can be transferred horizontally between different insect species and that affects its hosts'' sexual reproduction, cytoplasmic incompatibility, and immune responses (21, 38, 39).The brown planthopper, Nilaparvata lugens Stål (Homoptera: Delphacidae), is a monophagous insect herbivore of rice (13) that feeds on rice phloem and causes serious damage to rice crops. N. lugens reportedly harbors an intracellular, eukaryotic “yeast-like symbiont” (YLS) in the fat body, which plays a key role in recycling uric acid (3, 33). However, little is known about bacterial symbionts in N. lugens.It has been well recognized that diversity exists within insect species and that “biotypes” or populations that are adapted to or that prefer a particular host can frequently develop (10, 12). The behavioral and physiological responses during insect establishment on plants are feeding, metabolism of ingested food, growth, adult survival, egg production, and oviposition (34). In N. lugens, the biotype is assigned to a population with the ability to damage varieties of rice that carry resistance genes and that were previously resistant to it (5). It has been claimed that some biotypes of N. lugens differ in small morphological features, isozymes, and DNA polymorphisms (6, 25, 36). However, the precise nature of the virulence-conferring mechanisms in N. lugens biotypes (and their modes and stability of inheritance) is not clear. It is interesting to survey symbionts in different biotype populations of N. lugens.Generally, the 16S rRNA gene has been used as a molecular marker enabling the detection of as-yet-uncultured microbes, and it facilitates a profound investigation of microbial diversity (2, 22, 44). We initiated a study using molecular methods to investigate the bacterial symbionts of N. lugens. The major objective of this study was to identify bacterial microbes in N. lugens. The identified bacterial microbes appeared to be associated with different populations of N. lugens and in some cases were located in specific tissues, according to in situ hybridization (ISH) analyses.     

Changes of zeatin riboside content in rice plants due to infestation by Nilaparvata lugens (Homoptera: Delphacidae)

The effect of Nilaparvata lugens (St?l) (Homoptera: Delphacidae), infestation on the content of zeatin ribosides (ZR) in rice plants was investigated with enzyme-linked immunosorbent assay. Hydroponics experiments were conducted on 'Zhendao 2' rice, in which plants were subjected to N. lugens infestation at three nonhopperburn-causing densities (15, 30, and 60 nymphs per hill) for 2, 4, 6, and 8 d and at one hopperburn-causing density (240 nymphs per hill) for 2, 4, and 6 d, respectively. When rice plants were infested at the nonhopperburn-causing densities, ZR content in leaves varied significantly with the infestation density. Compared with the control plants, ZR content in rice leaves decreased significantly after infestation by 60 nymphs per hill for 2 d, but it tended to increase due to prolonged infestation at all the nonhopperburn-causing densities. In contrast, ZR content in rice roots significantly reduced after the plants being infested at the density of 15 nymphs for 2 d and at all densities for prolonged duration, except for the plants infested by 60 nymphs for 6 and 8 d, in which the ZR content increased or did not change significantly. However, infestation at the hopperburn-causing density caused significant reduction in ZR content in rice roots, regardless of infestation duration, and in rice leaves from the plants subjected to 2-d infestation. These results are discussed in relation to the possible physiological reaction of rice plants to N. lugens infestation and the resultant severe damage or hopperburn.     

Imidacloprid susceptibility survey and selection risk assessment in field populations of Nilaparvata lugens (Homoptera: Delphacidae)

Imidacloprid has been used for many years to control planthopper Nilaparvata lugens (St?l) (Homoptera: Delphacidae) in China. To provide resistance assessment for the national insecticide resistance management program, we collected a total of 42 samples of the planthoppers from 27 locations covering eight provinces to monitor their dose responses and susceptibility changes to imidacloprid over an 11-yr period (1996-2006). Results showed that most field populations maintained susceptibility from 1996 to 2003 except for a population from Guilin, Guangxi, in 1997, which showed a low level of resistance to imidacloprid. However, surveys conducted in 2005 indicated that 16 populations from six provinces quickly developed resistance with resistance ratios ranging from 79 to 811. The data collected in 2006 revealed that the resistance levels in 12 populations collected from seven different provinces decreased slightly (RR = 107-316), except the Tongzhou population (Jiangsu Province), which developed 625-fold resistance. Dominant and intensive use of imidacloprid in a wide range of rice, Oryza savita L., growing areas might be a driving force for the resistance development. Migration of the insect also significantly boosted the resistance levels due to extensive and intensive use of imidacloprid in emigrating areas and continuous postmigration sprays of the chemical. In addition, laboratory resistance selection using imidacloprid showed that resistance ratio increased to 14-fold after 27 generations, suggesting that quick resistance development might be associated with more frequent applications of the insecticide in recent years.     

Repeated copulation in the brown planthopper, Nilaparvata lugens Stál (Homoptera; Delphacidae)

Abstract. 1. Mating of the brown planthopper, Nilaparvata lugens , was investigated in relation to oviposition. Females became unreceptive immediately after mating and showed various types of repelling behaviour to courting males. As a result, females usually did not mate repeatedly in quick succession, but after ceasing to lay fertilized eggs they behaved as virgins and mated again before producing more fertilized eggs.
2. Copulation, followed by deposition of fertilized eggs, occurred twice, or in a few cases three times, throughout the adult stage. Copulation lasted about 2 min at the first mating and about 1 min at the second or third mating.
3. When the number of fertilized eggs began to decrease rapidly, oviposition rate also decreased, but it increased again immediately after re-mating. Repeated copulation was also related to the potential rate of population increase. In the present experiments, the estimates of net reproductive rate ( R ₀) in brachypterous females was 287.7 (first mating), 88.3 (second mating) and 7.2 (third mating) and in macropterous females 286.8, 120.6 and 3.6 respectively.
4. In a patchy environment, repeated copulation following an invasion may contribute to the increase of a population.     

Population forecasting model of Nilaparvata lugens and Sogatella furcifera (Homoptera: Delphacidae) based on Markov chain theory

Nilaparvata lugens (St?l) and Sogatella furcifera (Horváth) are the two most important long-distance migratory insect pests that cause great yield losses to rice in China. Accurate long-term population forecast is needed to implement effective management strategies for these two rice pests. In this paper, a transition probability matrix of 5-yr steps of Markov chain theory was constructed based on 31-yr light-trapping data of the two pests from 1977 to 2007 in Jiangkou County, Guizhou, China. The weight of each step for the transition probability matrix was calculated according to its prediction accuracy. Insect occurrence levels in the sixth year were predicted based on the occurrences of the previous 5 yr. Nonparametric Wilcoxon paired sample tests showed that there were no significant differences between the actual and predicted occurrences for both N. lugens and S. furcifera. In addition, the models accurately forecasted field occurrence in 2008 in Jinangkou County for both species. The results showed that the Markov models developed in this study offer an effective method for long-term population forecasting of N. lugens and S. furcifera and thus provide plant protection agencies and organizations with valuable information in implementing appropriate management strategies for these two devastating rice pests in Jiangkou and neighboring areas.     

The variability of mating signals and preferences in the brown planthopper,Nilaparvata lugens (Homoptera: Delphacidae)

Male and female brown planthoppers, Nilaparvata lugens(Stål) (Homoptera: Delphacidae), exchange substrate-transmitted signals prior to mating. The pulse repetition frequency of the male song is known to be involved in mate recognition and also to vary among geographical populations. Here the variability of male signals, female signals, and female preferences has been examined within a population. Female preference variation has been partitioned into variation in mean preference and variation in the window of preference of individuals. The genetic component of variation has been examined using isofemale lines. Male signal variation was limited (CV=8%) and was mainly within individuals. Female signal variation was greater (CV=15%). Female mean preference varied little (CV=10%) and was closely matched to the male signal mean, but the preference window was wide (> 4 male signal standard deviations on average) and variable (CV=56%). There was evidence for genetic variation only for preference window. These results are discussed in relation to theories of signal system evolution.     

In vivo passages of heterologous Beauveria bassiana isolates improve conidial surface properties and pathogenicity to Nilaparvata lugens (Homoptera: Delphacidae)

In entomopathogenic hyphomycetes, desired candidates against the brown planthopper, Nilaparvata lugens (a sap-sucking rice pest in Asia), are lacking. In this study, 21 Beauveria bassiana isolates from heterologous host insects showed low pathogenicity to third-instar nymphs sprayed at the high concentration of ∼1000 conidia/mm², causing only 2-23% mortalities. Of those, three isolates killed significantly more nymphs (up to 45-62%) after two in vivo passages but no more after further passage. Conidial hydrophobicity rates (H_r), zeta potentials (P_z), and subtilisin-like protease (Pr1) activities (A_p) of these isolates showed the same trends in the three host passages (N: 0-3). In multivariate correlation, the variables N, H_r and P_z were found contributing 89% to the mortality variation (r² = 0.89). Significant positive correlations were also found between H_r and N (r² = 0.64), P_z and N (r² = 0.52), A_p and N (r² = 0.51), H_r and A_p (r² = 0.45), and P_z and A_p (r² = 0.57), respectively. However, irregular changes of H_r and P_z occurred in four other isolates, whose pathogenicity to N. lugens was not enhanced by repeated host passages, resulting in no correlation between the variables. Our data indicate that the conidial surface properties H_r and P_z associated with cuticle adhesion reflect the heterologous host-induced adaptation and help to select fungal candidates against N. lugens from repeated in vivo passages.     

Histological observations on transovarial transmission of a yeast-like symbiote in Nilaparvata lugens Stal (Homoptera, Delphacidae)

Transovarial transmission of a yeast-like symbiote (YLS) in the brown planthopper, Nilaparvata lugens Stal, was observed with light and electron microscopy. Light micrographs showed that there was no YLS in testes and spermathecae of the mated females, indicating that sperm is not involved in the transovarial transmission of the symbiote. Both light and electron micrographs showed the processes of YLS transmission from fat body to the oocyte. In females, the symbiotes in mycetocytes moved out of the syncytium, which is formed from a layer of fat body cells, by exocytosis, and released into hemocoel. Then, the free YLS in hemolymph approached to the ovarioles near pedicel and were enclosed by follicle cells. They entered the follicle cells around the primary oocyte by endocytosis at epithelial plug of the ovariole. The YLS aggregated at the posterior end of the mature egg after entering, and finally formed a symbiote ball.     

Determination and distribution of a female-specific protein in the brown planthopper, Nilaparvata lugens Stal (Homoptera: Delphacidae)

Electrophoretic analyses of hemolymph and body or ovary homogenates from reproducing females, males, and 5th instar nymphs of the brown planthopper, Nilaparvata lugens Stal, revealed a protein band of 175 kDa in females. An immunoblot test using antibody against this protein showed a positive reaction with a 175 kDa protein from female body or ovary homogenates. It is likely that this protein in hemolymph is vitellogenin (Vg). Distribution of Vg was determined by immunofluorescence and immunogold labeling techniques. The results showed that the positive immunofluorescence reactions were present in yolk particles, the intercellular space of follicle cells, hemolymph, and the epithelial plug of ovarioles. In addition, the yeast-like symbiotes (YLS) in mycetocytes of adults and various nymphal instars as well as those free in hemolymph or entering oocytes also exhibited a positive reaction. Electron micrographs showed that immunogold particles were found most in yolk mass and YLS over other tissues. Especially the YLS in various developmental stages all contained immunogold particles, implying that the symbiote is somewhat related with production of the female-specific protein.     

Photosynthesis and translocation of assimilates in rice plants following phloem feeding by the planthopper Nilaparvata lugens (Homoptera: Delphacidae)

Experiments were conducted to measure the effect of feeding by the planthopper Nilaparvata lugens (St?l) on photosynthesis and the translocation of assimilates in rice plants, Oryza sativa L. We used mature japonica rice plants and applied the 13CO2 feeding method to evaluate those physiological effects. The photosynthetic rate was suppressed by N. lugens infestation, especially at the lower leaf position, with rates 30% lower than that of control plants at the booting stage. Leaf nitrogen concentration in infested plants was also lower than that in control plants. After flowering, the chlorophyll content and total plant dry weight were reduced by N. lugens. Stem and sheath dry weights were significantly reduced by N. lugens infestation, whereas panicle dry weight was not affected. Little effect was found on disruption in translocation of assimilates, even when 13CO2 was supplied to the infested leaves. Results suggested that removal of assimilates and reduction in photosynthesis by N. lugens have the greatest effect on growth and yield of rice plants as compared with the disruption in the translocation of assimilates. Plant death can occur by N. lugens infestation if the amount of energy supplied is less than that required for tissue maintenance.     

Preference and performance of Anagrus nilaparvatae (Hymenoptera: Mymaridae): effect of infestation duration and density by Nilaparvata lugens (Homoptera: Delphacidae)

The effect of Nilaparvata lugens (St?l) infestation duration and density on the host preference and performance of Anagrus nilaparvatae Pang et Wang, an egg parasitoid of rice planthoppers, was determined. The results showed that the parasitoid preferred N. lugens eggs on the plants infested with 10 gravid N. lugens females for 1 d to those plants infested with 10 gravid females for 2 or 3 d. It was also found to prefer N. lugens eggs on plants infested with 10 or 20 adult females after 24 h of infestation to those plants infested with 5 or 80 females. The parasitoid's offsprings had lower survival rates, fecundities, female ratios, indexes of capacity for population increase, and longer developmental durations on plants when they were infested with high N. lugens density (80 adult females per plant). However, the performance of the parasitoid on plants infested with low N. lugens density (5 female adults per plant) was similar to those on plants with intermediate N. lugens density (10 or 20 adult females per plant). Low preference of the parasitoid for N. lugens eggs on plants with heavy or light infestation levels may be correlated with low host suitability and detectability, respectively. The result implies an important role of herbivore-induced rice volatiles in the host preference of the parasitoid A. nilaparvatae, by which the parasitoid perceives the host and its suitability.     

Ethiprole resistance in Nilaparvata lugens (Hemiptera: Delphacidae): possible mechanisms and cross-resistance

This article reports the current status of ethiprole resistance in Nilaparvata lugens Stål in the central region of Thailand, together with the associated resistance mechanisms. A resistance survey found that a field population had developed 308.5-fold resistance to ethiprole. Further selection with ethiprole for nine generations in the laboratory led to 453.1-fold ethiprole resistance. However, following this selection procedure, the resistance of N. lugens to other insecticides decreased to about one-third of its original resistance. This result implies that there is no cross-resistance between ethiprole and other kinds of insecticides in this pest. In an in vivo study of synergisms, triphenyl phosphate (TPP) exhibited a strong synergism (SR 4.2) with ethiprole in the resistant hoppers, piperonyl butoxide (PBO) also showed significant synergistic effects with ethiprole (1.6), but diethyl maleate (DEM) did not show any obvious synergism with ethiprole (1.2). An in vitro biochemical study indicated that esterase activity increased with ethiprole resistance in N. lugens, that P450 monooxygenase activity also increased significantly with high resistance, but that glutathione S-transferase activity did not. These results reveal that increases in esterase activity and P450 monooxygenase activity cause the ethiprole resistance observed in the field populations of N. lugens. Whether the mechanisms for ethiprole resistance involve target-site sensitivity is not yet known; further molecular analysis is required. However, an analysis of insecticide cross-resistance and the insecticide application history of the resistant populations indicated that target resistance was present and that rotation between insecticides with different modes of action will provide a key countermeasure to maintain the efficacy of ethiprole.     

Interpopulation Crosses, Inheritance Study, and Genetic Variability in the Brown Planthopper Complex, Nilaparvata lugens (Homoptera: Delphacidae)

Studies on hybridization, inheritance, and population genetics of brown planthoppers that infest rice and weeds were undertaken using starch gel electrophoresis to determine whether the weed-infesting population represents a biological race or a species. F₁ and F₂ generations were produced by crosses between parental insects from the two populations with little indication of hybrid sterility. Gpi, Mdh, and Idh loci were inherited in a simple Mendelian fashion in families of two sympatric populations. Sixteen populations of Nilaparvata spp. from eight locations were collected. The Mdh, Idh, Pgm, Gpi, 6Pgd, and Acp loci were polymorphic. The N. lugens of rice with high esterase activity were clustered into a group and characterized by the presence of alleles Gpi ¹¹⁰ and Gpi ¹²⁰ , whereas N. lugens from weeds with low esterase activity were clustered into another group and characterized by Gpi ¹⁰⁰ and Gpi ⁹⁰ . There was a lack of heterozygotes between the common alleles of the two populations. This means that the two groups of individuals belong to different gene pools.     

An orange-eye mutant of the brown planthopper,Nilaparvata lugens (Hemiptera: Delphacidae)

An orange-eye mutant of the brown planthopper (BPH), Nilaparvata lugens (Stål), was found in a green house and has since been maintained together with a normal-eye phenotype of BPH in an insectary. The orange color was expressed in all developmental stages of BPH: the eye spots of eggs and the eyes of nymphs and adults of both sexes and wing forms. Cross-mating results suggested that the inheritance of the orange-eye phenotype is controlled by a single autosomal recessive allele. The gene symbol related to this mutant was designated as “org”. Developmental duration and mortality of nymphal stages were not significantly different between the normal phenotype (homozygous and heterozygous) and the mutant. In addition, reproduction was not significantly different among mating combinations of the three BPH genotypes (+/+, +/org, org/org). The effect of eye color on mating of BPH was insignificant in a mate choice test which consisted of one orange-eye female, one orange-eye male, and one homozygous normal-eye male. Offspring produced by the orange-eye female BPH hatched and developed into adults normally, indicating that the eye color mutant found in this study is different from the red-eye BPH (Mochida, 1970) which showed the egg lethal effect in the red-eye BPH female.     

Differential resistance and cross-resistance to three phenylpyrazole insecticides in the planthopper Nilaparvata lugens (Hemiptera: Delphacidae)

Cross-resistance to two fipronil analogs, butene-fipronil and ethiprole, was detected in fipronil-resistant field populations and a resistant laboratory strain of the planthopper Nilaparvata lugens (St?l) (Hemiptera: Delphacidae), although the two analogs have not been used widely in rice-growing areas in China. The results showed that six field populations with 23.8-43.3-fold resistance to fipronil had reached a higher level of cross-resistance to ethiprole (resistance ratio [RR] = 47.1-100.9-fold) and had a minor level of cross-resistance (RR = 3.4- 8.1-fold) to butene-fipronil. After 10 generations of selection, the RR to fipronil increased from 7.3-fold to 41.3-fold. At the same time, the insect increased cross-RR to ethiprole from 16.3-fold to 65.6-fold, whereas it had only minor increase in cross-resistance to butene-fipronil from 2.8-fold to 4.0-fold. These results confirmed that fipronil-resistant N. lugens could develop a higher level of cross-resistance to ethiprole, although it still maintained a lower level cross-resistance to butene-fipronil. Our data suggest that ethiprole is not a suitable alternative for controlling N. lugens, once the insect has developed a high level resistance to fipronil. Further investigation is necessary to understand the cross-resistance mechanisms in N. lugens.     

Occurrence and short-distance migration of Nilaparvata lugens (Hemiptera: Delphacidae) in the Vietnamese Mekong Delta

In migration analysis of the brown planthopper, Nilaparvata lugens (Stål), in a tropical region, it is difficult to know the arrival timing of the immigrants because rice plants in different growth stages are simultaneously cultivated, and local insects may already occur in destination areas. Due to this analytical difficulty, additional information is necessary to discriminate between immigrants and locally originated individuals in the light trap catch. This study applied rice phenology maps retrieved from satellite images to analysis of N. lugens’s migration in the Vietnamese Mekong Delta. The analysis also employed occurrence data by dense light traps and trajectory analysis. The result showed that the occurrence of N. lugens peaked periodically at an interval of 27.2–30 days, changing in association with the rice cropping area in the harvesting stage. For an N. lugens occurrence in late July 2009 in the delta, the rice phenology suggested catch increases in light traps in a destination area could likely be attributed to immigrants from outside the area. Therefore, the migration analysis suggested that N. lugens that emigrated from paddy fields in An Giang and Kien Giang provinces possibly moved to east-neighboring provinces. The migration distance was estimated to be about 100 km at a maximum.     

Life-table analysis of the performance of Nilaparvata lugens (Hemiptera: Delphacidae) on two wild rice species

Life tables of the planthopper Nilaparvata lugens (St?l) (Hemiptera: Delphacidae) on two wild rice species, Oryza officinalis Wall, ex Watt. and Oryza rufipogon Griff., and on 'Taichung Native 1' (O. sativa indica TN1) were studied at 27 degrees C in the laboratory. The raw data were analyzed based on the age-stage, two-sex life table to take both sexes and the variable developmental rate among individuals and between sexes into consideration. The intrinsic rate of increase (r), the finite rate (lamda), the net reproduction rate (R0) and the mean generation time (T) of N. lugens on O. officinalis were -0.0616 d(-1), 0.9402 d(-1), 0.10 offspring, and 36.7 d, respectively, showing that the species is resistant to N. lugens. The resistance of O. officinalis is expressed in the long developmental time from first to fifth instar of N. lugens, the reduced preadult survival rate, the short adult longevity, and low fecundity. However, the population parameters of the N. lugens on O. rufipogon and TN1 were 0.1096 d(-1), 1.1158 d(-1), 67.82 offspring, and 38.6 d and 0.1340 d(-1), 1.1434 d(-1), 144.77 offspring, and 37.2 d, respectively. These population parameters showed that N. lugens can successfully survive and reproduce on both O. rufipogon and TN1.     

Putative protein digestion in a sap-sucking homopteran plant pest (rice brown plant hopper; Nilaparvata lugens: Delphacidae)--identification of trypsin-like and cathepsin B-like proteases

Sap-sucking phytophagous insect species of the order Hemiptera have been assumed not to carry out digestive proteolysis, but instead to rely on free amino acids in the phloem and xylem saps for their nutritional requirements. Extracts prepared from isolated guts of rice brown planthopper (Nilaparvata lugens), a homopteran crop pest, were shown to contain protease activity, with hydrolysis of both protein and synthetic peptide substrates being observed. Assays with specific inhibitors suggested that a trypsin-like serine protease was responsible for most of hydrolytic activity against synthetic substrates. A cDNA library was prepared from RNA extracted from N. lugens gut tissue, and screened for protease-encoding sequences. cDNAs for a cathepsin B-like protease and a trypsin-like protease were isolated and fully characterised; the latter exhibits a novel C-terminal region and an unusual activation mechanism, and represents a small gene family. Soya bean Kunitz trypsin inhibitor (SKTI) is an effective inhibitor of protein hydrolysis by N. lugens gut extracts in vitro, explaining why transgenic rice plants expressing this protein are partially resistant to the insect (Mol. Breed. 5 (1999) 1). It is suggested that digestive proteolysis may be widespread in sap-sucking homoptera, and can make a significant contribution to nutrition.