Hostplant,larval age,and feeding behavior influence midgut pH in the gypsy moth (Lymantria dispar)

Summary The midgut pH of late instar gypsy moth (Lymantria dispar L.) larvae is strongly alkaline, and varies with diet, larval stadium, and time since feeding. Midgut pH rises with time since feeding, and does so more quickly, reaching greater maximum values, on some diets than others. Leaf tissues of 23 tree species resist increases in alkalinity differentially; this trait and differing initial leaf pH may explain the impact of diet on gut pH. Third instar larvae may have gut conditions favorable for tannin-protein binding shortly after ingesting certain foods, but with time midgut alkalinity becomes great enough to dissociate tannin-protein complexes. Older instars rarely exhibit gut pHs low enough to permit tannin activity. Alkaline gut conditions may explain the gypsy moth's ability to feed on many tanniniferous plant species, especially in later instars. Consequences for pathogen effectiveness are discussed.     

单宁酸对棉铃虫谷胱甘肽S转移酶的影响

通过培养基混药法,研究了植物次生物质单宁酸对棉铃虫Helicoverpa armigera（Hübner）谷胱甘肽S-转移酶活性的影响。谷胱甘肽S-转移酶活性随棉铃虫发育期的进程而变化,在卵期最低,5龄、6龄幼虫和成虫期最高。用含0.005%单宁酸的饲料饲喂棉铃虫后,5龄和6龄幼虫的谷胱甘肽S-转移酶活性明显降低,分别为对照的59%和67%。单宁酸低剂量、短时间处理棉铃虫幼虫,可诱导中肠和脂肪体中的谷胱甘肽S-转移酶的活性增加,高剂量或低剂量长时间处理没有诱导增加作用,甚至还有抑制作用。单宁酸连续处理4代,对棉铃虫6龄幼虫中肠谷胱甘肽S 转移酶均有抑制作用,对脂肪体谷胱甘肽S-转移酶活性无明显影响或有抑制作用。单宁酸处理的第4代幼虫对溴氰菊酯的敏感度有增加的趋势,对甲基对硫磷的敏感度没有明显改变。     

Effects of potassium levels over the range occurring in host plants on the growth of an insect herbivore (Manduca sexta)

Third instar tobacco hornworms (Manduca sexta L.: Sphingidae) on low dietary potassium had a lower relative growth rate than individuals on diets with potassium concentrations reflecting those in host-plants, due to decreased consumption rate, lower efficiencies of conversion of ingested and digested food (ECI and ECD), and a prolonged growth/feeding phase. Furthermore, these larvae, when placed on a diet with a moderate potassium concentration through the fourth stadium, ended up being smaller due to lower ECI and less biomass gained, and had a prolonged growth phase, which suggest an irreversible cost of the previous low potassium diet. Third instar hornworms on high potassium diets had lower ECI and ECD, and they had a prolonged growth phase. These individuals, when placed on a moderate potassium diet in the fourth stadium, gained less biomass, than those previously offered hostplant-like-potassium diets. Body potassium concentrations (% dw) at the end of the third stadium were similar among treatment groups. With increasing potassium concentrations in the diet, utilization efficiencies of potassium decreased and potassium concentrations in the frass increased. Correspondingly, water content (% fw) of the newly-molted fourth instar larvae declined with increasing potassium, indicating a passive loss of water during potassium excretion. Low and high dietary potassium reduced survivorship of third instar larvae; fourth instar caterpillars previously fed the low potassium diet also had poor survivorship. We conclude that, within the normal range of potassium concentrations in the hostplants, caterpillar performance is largely unaffected by potassium concentration, but that potassium-poor and potassium-rich diets, such as those hornworms may sometimes experience, can reduce growth and survivorship.     

Formation of insoluble and colloidally dispersed tannic acid complexes in the midgut fluid of Manduca sexta (Lepidoptera: Sphingidae): An explanation for the failure of tannic acid to cross the peritrophic envelopes of lepidopteran larvae

Magnesium and calcium ions, in concentrations comparable to those reported in the midgut fluids of lepidopteran larvae, bring about the precipitation of most of the tannic acid present in simple solutions buffered at pH 8.0 and 10.0, but not at pH 6.5. In contrast, when tannic acid is added to Manduca sexta midgut fluid, less than 31% of the tannic acid added to the gut fluid is converted to a form that can be centrifuged into a pellet. The rest remains in the supernatant solution in the form of a colloidal suspension. Very little of the tannic acid, if any, remains in true solution. We suggest that the tannic acid-containing phase that is produced when tannic acid is added to midgut fluid is a complex multi-molecular aggregate of indefinite chemical composition, incorporating varying amounts of tannic acid, surface-active phospholipids, proteins, and polyvalent metal ions. On the basis of this study, we further suggest that the failure of tannins to diffuse across the peritrophic envelopes of lepidopteran larvae is a result of the capacity of the peritrophic envelope to act as a physical barrier to insoluble and colloidally dispersed particles, not the presence of substances in the matrix that strongly adsorb polyphenols or the presence of an extensive network of fixed anionic sites in the matrix that acts as an electrostatic barrier to the passage of polyphenolate anions. Arch. Insect Biochem. Physiol. 39:109–117, 1998.© 1998 Wiley-Liss, Inc.     

Toxicity and disruption of midgut physiology in larvae of the European corn borer,Ostrinia nubilalis,by anion transporter blockers

In this study, four blockers of anion transporters (ATs) belonging to four different classes of organic acids, including DIDS (4, 4'‐diisothiocyanatostilbene‐2, 2'‐ disulfonic acid; a stilbene disulfonic acid), NPPB [(5‐nitro‐2‐(3‐phenylpropylamino) benzoic acid; an anthranilic acid)], 9‐AC (anthracene‐9‐carboxylic acid; an aromatic carboxylic acid), and IAA‐94 (indanyloxy acetic acid; an indanyloxy alkanoic acid), were tested for their toxicity against the European corn borer (ECB), Ostrinia nubilalis. All the AT blockers inhibited the growth of larvae, increased the developmental time, and decreased survival compared to controls, when second‐instar ECB larvae were fed for seven days on treated diet. In general, DIDS and NPPB were the most active compounds, with the rank order of activity being DIDS>NPPB>IAA‐94>9‐AC. All the AT blockers decreased the midgut alkalinity in fifth‐instar larvae when fed for 3 h on treated diet. Effective concentrations required for 50% decrease in midgut alkalinity (EC₅₀) ranged between 29.1 and 41.2 ppm and the rank order of activity was NPPB>DIDS>IAA‐94>9‐AC. Similarly, all the tested AT blockers inhibited ³⁶Cl^? uptake from the midgut lumen in fifth‐instar larvae when fed for 3 h on treated diet. Concentrations required for 50% inhibition of ³⁶Cl^? uptake (IC₅₀) ranged between 7.4 and 11.0 ppm and the rank order of activity was DIDS>NPPB>9‐AC >IAA‐94. Modest to highly strong positive correlations observed among growth, midgut alkalinity, and midgut Cl^? ion transport in AT blocker–fed larvae suggested that these effects are causally related to each other. Finally, AT blockers have the potential to become good candidates for development of insecticides with a unique mode of action. © 2009 Wiley Periodicals, Inc.     

Differential effect of tannic acid on two tree-feeding Lepidoptera: implications for theories of plant anti-herbivore chemistry

Summary Feeding efficiencies of ultimate instar larvae of two polyphagous tree-feeding Lepidoptera, Malacosoma disstria (Lasiocampidae) and Orgyia leucostigma (Liparidae), were measured on artificial diets containing from 0% to 8% tannic acid. Relative growth rate (RGR) of O. leucostigma was not affected by up to 8% tannic acid, suggesting that O. leucostigma has evolved an effective counteradaptation to hydrolyzable tannins. In contrast, as little as 0.5% tannic acid caused a significant reduction in RGR of M. disstria, due both to reduced efficiency of conversion of digested food (ECD) and reduced relative consumption rate (RCR), and caused a significant increase in mortality during the pupal stage. Moreover, when reared from hatching on tannin-containing diets, no M. disstria larvae survived past the fourth instar.Although tannins are commonly referred to as digestibility-reducing substances, tannic acid did not reduce the ability of M. disstria or O. leucostigma larvae to digest either the whole diet or nitrogen contained in the diet. For M. disstria, tannic acid acts as a toxin and a feeding deterrent, but not as a digestibility-reducing substance. Growing evidence that tannins commonly act as toxins warrants a reassessment of their role in anti-herbivore chemistry.     

The effect of GNA lectin on the α-amylase activity of the beet armyworm,Spodoptera exigua Hb. (Lepidoptera: Noctuidae)

Beet armyworm (Spodoptera exigua Hb.) (Lepidoptera: Noctuidae) is the major pest of sugar beet (Beta vulgaris). Pesticide applications are the main method of the insect control. So, alternative method/s is/are needed to control this insect species. So, in the current study, the effect of Galanthus nivalis agglutinin (GNA) (snowdrop lectin) on beet armyworm α-amylase was studied. Measurement of the amylase activity of the larval midgut fed on artificial diet and sugar beet leaves showed that the enzyme activity was higher when the larvae fed on artificial diet. However, in both cases, the fourth instar larvae had the greatest amylase activity. Thus, fourth instar larvae were offered artificial diet containing 1 and 2% GNA. Both treatments of the lectin significantly reduced the α-amylase activity of the insect. For example, amylase activity of the fourth instar larvae in the control (fed only on artificial diet) was 2.62 Uml^?1 whilst the activity of the enzyme in the two treatments including diet containing 2 and 1% lectin was 1.45 and 1.75 Uml^?1, respectively. The achieved data showed that lectin, in addition to have toxic effect on the larval growth and development, affects the α-amylase activity of the insect gut.     

Some observations on the food of larvae of Procladius bellus (Diptera: Chironomidae)

Analysis of the gut contents of Procladius bellus (Loew) larvae collected in field samples indicated that it was a detrivitore‐omnivore. The observed dietary change from first and second instars (detritivore‐herbivores) to third and fourth instars (omnivores) did not suggest a change in feeding behaviour from detritivore to predator, but rather that the animal material was consumed along with larger detrital material. Small detrital material (1–15 μm) was an important dietary component and detritus in general contributed 50–70% of the overall diet. Diatoms and green algae contributed 15–20%, while blue‐green algae contributed about 10%. Animal material contributed about 5% to second and third instar larvae and 20% to fourth instar larvae.     

Penicillin-induced oxidative stress: effects on antioxidative response of midgut tissues in instars of Galleria mellonella

Penicillin and other antibiotics are routinely incorporated in insect culture media. Although culturing insects in the presence of antibiotics is a decades-old practice, antibiotics can exert deleterious influences on insects. In this article, we test the hypothesis that one of the effects of dietary penicillin is to increase oxidative stress on insects. The effects of penicillin on midgut concentrations of the oxidative stress indicator malondialdehyde (MDA) and on midgut antioxidant enzyme (superoxide dismutase [SOD], catalase [CAT], glutathione S-transferase [GST], and glutathione peroxidase [GPx]) and transaminases (alanine aminotransferase and aspartate aminotransferase) activities in greater wax moth, Galleria mellonella (L.), were investigated. The insects were reared from first instars on artificial diets containing 0.001, 0.01, 0.1, or 1.0 g penicillin per 100 g of diets. MDA content was significantly increased in the midgut tissues of each larval instar reared in the presence of high penicillin concentrations. Activities of antioxidant and transaminase enzymes did not show a consistent pattern with respect to penicillin concentrations in diet or age of larvae. Despite the increased penicillin-induced oxidative stress in gut tissue, antioxidant and transaminase enzymes did not correlate with oxidative stress level or between each other in larvae of other age stages except for the seventh instar. We found a significant negative correlation of MDA content with SOD and GST activities in seventh instars. SOD activity was also negatively correlated with CAT activity in seventh instars. These results suggest that exposure to dietary penicillin resulted in impaired enzymatic antioxidant defense capacity and metabolic functions in wax moth larval midgut tissues and that the resulting oxidative stress impacts midgut digestive physiology.     

Protein digestion in larvae of the red oak borer Enaphalodes rufulus

Abstract In the Ozark Mountains of the U.S.A., the red oak borer Enaphalodes rufulus contributes to the destruction of red oaks. To understand nutrient digestion in E. rufulus larvae, digestive proteinases are compared in both larvae fed heartwood phloem and those transferred to artificial diet. The pH of gut extracts is approximately 6.3 in the midgut and foregut and decreases to 5.5 in the hindgut region. The hydrolysis of casein by midgut extracts from E. rufulus larvae fed either artificial diet or phloem from tree sections increases in buffers greater than pH 6.19, with maximum hydrolysis being observed at pH 10.1. Casein zymogram analysis reveals two major proteinase activities in larval midgut extracts of diet‐fed larvae, with molecular masses of approximately 25 and 40–60 kDa, whereas phloem‐fed larvae have proteinase activities corresponding to 40, 45, 60, 80 and >100 kDa. Substrate analysis indicates at least one major trypsin‐like activity in both gut extracts with a molecular mass of >100 kDa, but two chymotrypsin‐like activities of approximately 25 and >200 kDa are found only in diet‐fed larvae. Inhibitors of serine proteinases are most effective in reducing the general proteolytic activity of midgut extracts from larvae fed either food source. The data indicate that serine proteinase inhibitors have the potential to reduce E. rufulus larval damage to oaks. In particular, transgenic technologies incoporating trypsin inhibitors may be effective in reducing protein digestion in phloem‐feeding larvae.     

Antioxidant defenses in caterpillars: role of the ascorbate-recycling system in the midgut lumen

This study demonstrates that an ascorbate-recycling system in the midgut lumen can act as an effective antioxidant defense in caterpillars that feed on prooxidant-rich foods. In tannin-sensitive larvae of the forest tent caterpillar, Malacosoma disstria (Lasiocampidae), ingested tannic acid is oxidized in the midgut lumen, generating significant quantities of peroxides, including hydrogen peroxide, which readily diffuses across cell membranes and is a powerful cytotoxin. By contrast, in the tannin-tolerant larvae of the white-marked tussock moth, Orgyia leucostigma (Lymantriidae), tannic acid oxidation and the generation of peroxides are suppressed. The superior defense of O. leucostigma against oxidative stress imposed by the oxidation of ingested polyphenols can be explained by the presence of higher concentrations of ascorbate and glutathione in the midgut lumen. In O. leucostigma at least 50% of the ingested ascorbate present in the anterior midgut is still present in the posterior midgut, whereas in M. disstria, only 10% of the ascorbate is present in the posterior half of the midgut. We propose that the maintenance of higher levels of ascorbate in the midgut lumen of O. leucostigma than in M. disstria is explained by the secretion of glutathione into the midgut lumen by O. leucostigma, thereby forming a complete ascorbate-recycling system. The concentration of glutathione in the midgut lumen of O. leucostigma is 3.5-fold higher than in M. disstria and more than double the concentration in the diet. Our results emphasize the importance of a defensive strategy in herbivorous insects based on the maintenance of conditions in the gut lumen that reduce or eliminate the potential prooxidant behavior of ingested phenols.     

Effects of acidic pH on predation rates and survivorship of damselfly nymphs

Environmental variables such as pH critically affect the physiology and ecology of aquatic organisms. Our lack of information regarding indirect effects of acid precipitation requires investigation of acid stress and response to acid stress by species of freshwater communities. Damselfly nymphs Enallagma civile (Odonata: Coenagrionidae) are abundant aquatic predators and food items for other secondary consumers.We investigated the effects of low pH on mortality, survivorship, predation rate, respiration rate, and caloric content of nymphs of E. civile. We hypothesized that acid stress would increase mortality and decrease predation rate among surviving individuals. We hypothesized that respiration rate and caloric content as measures of physiological condition would decrease in acid-stress conditions.We cultured individual early-instar nymphs in the laboratory with a controlled diet of mosquito larvae. The nymphs were reared to final instar in pond water and artificially reconstituted soft water (RSW) adjusted to pH 3.5, 4.5, 5.5, and ambient conditions of the collection site.Predation rate was significantly lower in RSW than in pond water in treatments of equivalent H⁺ concentrations. This indicated stress from differences in dissolved solids in RSW compared with pond water. Acid conditions significantly affected predation rates in pond water, but only at pH 3.5 and only for the largest instar. Mortality increased and survivorship decreased at low pH. Early-instar mortality and survivorship were particularly susceptible to acid stress. Treatments of pH 4.5, 5.5 and ambient did not significantly alter respiration rate or caloric content. However, respiration rate and caloric content increased at pH 3.5.Damselfly nymphs tolerated pHs as low as 4.5 without significant changes (including predation rate). However, the effects of pH 3.5 were significant and greatly lowered the predation and fitness of the nymphs.author for correspondence     

棉铃虫4龄幼虫的摄食行为和中肠蛋白酶活性的变化

通过测定棉铃虫4龄幼虫,每隔12 h的摄食量、体重净增量以及丝氨酸蛋白酶活性的变化,揭示了它的取食行为及中肠蛋白酶活性的变化规律。结果表明:棉铃虫4龄幼虫取食及消化呈有规律的变化,蜕皮后4龄初,摄食量、体重净增量、类胰蛋白酶、类胰凝乳蛋白酶和总蛋白酶活性均逐渐增加;进入中期(36～48 h),各项指标达到最高值;其后,随着摄食量的下降,各项指标逐渐下降。同时, 连续观察幼虫取食高峰期的取食行为6 h,结果表明: 其间一次连续取食的平均时间为(3.6±0.4) min/次, 总平均取食时间为(62.4±8.9) min,约占整个时间的1/6。该文还对棉铃虫4龄幼虫不同时段取食行为与蛋白酶活性变化之间的相关性及调控机制作了初步讨论。     

A reinvestigation of the effects of cortisol on growth in insects

Incorporation of cortisol, or various derivatives and analogues of cortisol, into the diet had no effect on the rate of growth of Tenebrio molitor. Injections of cortisol, or its derivatives, into fourth instar larvae of Schistocerca americana gregaria, had no effect on their rate of growth. The moult to fifth instar was normal. Administration in the diet, or injections of cortisol had no effect on the rate of growth of fourth and fifth instar larvae of Manduca sexta, or on the insect's uptake and utilization of food. The development of larvae to pupae and adults was unaffected. It is concluded, contrary to the findings of two previously published reports, that the vertebrate hormone cortisol does not affect the growth and development of insects.     

Uptake and fate of specific antibody in feeding larvae of the sheep blowfly,Lucilia cuprina

Abstract. The quantity of specific antibody ingested by larvae of Lucilia cuprina and its fate after ingestion were studied in larvae grown on sheep and on an artificial diet. Larvae grown to late first or early second instar on sheep vaccinated with horse myoglobin contained 66% less specific antibody detected by enzyme linked immunosorbent assay than larvae grown to a similar stage on an artificial diet containing 75% serum from the same sheep. A similar result was obtained when larvae were grown to mid-third instar. Larvae grown on sheep to first or second instar contained approximately the same quantity of specific antibody per unit weight of larvae as those grown to third instar. Larvae grown on diet to third instar contained 22% less specific antibody per unit weight than those grown to first or second instar. In larvae grown on diet to late third instar, ingested diet retained 91 ± 12% of its original specific antibody activity in the crop, 50 ± 11% in the anterior midgut, 8 ± 2% in the posterior midgut and 13 ± 6% in the hindgut. The mean concentration of total immunoglobulin detectable in the haemolymph of individual third instar larvae grown on diet was 1.7 ± 2.8 ug/ml. Assays of specific antibody in the haemolymph of similarly reared larvae indicated that all or most of this immunoglobulin remained functional. The implications of the quantities and distribution of ingested functional antibody found in feeding larvae of L.cuprina are discussed in relation to the possibility of vaccinating sheep against these larvae and the selection of likely internal targets as sources of potential protective antigens.     

单宁酸对淡色库蚊抗氰戊菊酯品系和敏感品系幼虫生长发育的影响

单宁酸是一种植物次生代谢物, 为探索其用于蚊幼虫防治的可能性,在室内测定了其对淡色库蚊Culex pipiens pallens抗氰戊菊酯品系和敏感品系1～4龄幼虫的毒性,并观察了其对存活幼虫生长发育的影响。结果表明,淡色库蚊敏感品系幼虫对单宁酸的敏感性比抗氰戊菊酯品系的要高,1～4龄幼虫分别高6.4、4.9、4.7和2.0倍。4个龄期幼虫中,无论是敏感品系还是抗氰戊菊酯品系,均是1龄幼虫对单宁酸的敏感性最高,3龄幼虫最低。在1 000 mg/L单宁酸持续作用下,敏感品系和抗氰戊菊酯品系各龄幼虫的存活率,均随处理时间延长而降低。与对照相比,饲养在100 mg/L～500 mg/L单宁酸溶液中的存活幼虫发育历期延长,敏感品系和抗氰戊菊酯品系发育历期分别延长了34.5～38.3 h和59.2～93.4 h。其中,125 mg/L浓度处理的敏感品系1～4龄幼虫,其发育历期与对照的差异达到了显著水平(P<0.05);抗性品系则在250 mg/L作用下也达到了差异显著水平(P<0.05)。但100～250 mg/L单宁酸处理淡色库蚊抗氰戊菊酯品系和敏感品系1龄幼虫,对其存活幼虫的化蛹率、羽化率和成虫性比均无显著影响。表明单宁酸对淡色库蚊幼虫的影响主要是延迟其生长发育,且影响程度与蚊虫对氰戊菊酯的敏感性有关。     

Characterization and cDNA cloning of midgut carboxypeptidases from Trichoplusia ni

Carboxypeptidase A and carboxypeptidase B activities from the midgut of Trichoplusia ni larvae were characterized. In the T. ni larval midgut, the primary digestive carboxypeptidase activity was attributed to carboxypeptidase A, which was eight times more active than carboxypeptidase B. Both the midgut carboxypeptidase A and carboxypeptidase B exhibited maximal activities at pH 8.0-8.5 and were similarly susceptible to inhibition by potato carboxypeptidase inhibitor and phenanthroline. The midgut carboxypeptidase activities were analyzed in T. ni larvae fed on various diet sources and the results indicated that midgut carboxypeptidase activities per milligram of gut were similar regardless of the amount of dietary proteins or amino acids. However, midgut carboxypeptidase A activity was significantly higher in larvae exposed to soybean trypsin inhibitor and was significantly lower in larvae fed on broccoli foliage. From the T. ni larval midgut, five putative carboxypeptidase cDNAs were cloned, demonstrating that midgut carboxypeptidase activities are composed of multiple carboxypeptidase types. Sequence analysis indicated that the midgut carboxypeptidases were produced as secreted proenzymes which could be activated after removal of an N-terminal activation fragment by a trypsin. Two cloned cDNAs are predicted to code for carboxypeptidase A and one cDNA is predicted to code for a putative carboxypeptidase B. The other two cDNAs are highly similar to carboxypeptidase A and carboxypeptidase B in sequences, but their activity was not predictable.     

Contrasting diel activity and feeding patterns of four instars of Rhyacophila dorsalis (Trichoptera)

1. Ontogenetic shifts in prey choice and predator behaviour can affect food‐web structure. Therefore, it is important to establish if the diet and feeding activity differ between life‐stages of the same species. This hypothesis was tested for second, third, fourth and fifth larval instars of Rhyacophila dorsalis by comparing their diel activity and feeding patterns. Second to fifth instars collected from two streams were used either for gut analyses or for observations of their activity and feeding patterns in three stream tanks. Food was provided in excess; being organisms living in bryophytes on top of a large stone in each tank, augmented by different‐sized larvae of Ephemeroptera, Simuliidae and Chironomidae. As few first instars for gut analyses were found in the field, the diet of first instars reared in the laboratory was also studied. 2. Larvae for gut analyses were taken 1 h before dusk or dawn (n = 50 larvae per instar for each day or night sample). First and second instars fed on the smaller food items with no significant day‐night differences in diet. Gut contents indicated a progressive trend from feeding chiefly at night in third instars to almost exclusively at night in fifth instars. Fourth and fifth instars fed on the larger food items, whilst the diet of the third instar larvae overlapped with that of both the earlier and later instars. 3. Diel activity patterns of single larvae differed between instars but not within each instar (n = 20 larvae per instar). Second instars were active throughout the 24 h, with peaks at dusk, around midnight, dawn and around midday. A similar pattern was shown by third instars but the peak of activity at midday was less than the other three peaks. Prey were captured only during these peaks for both instars. Fourth and fifth instars were most active, and fed only, at night. They used an ambush strategy to capture more active prey at dusk and dawn (e.g. Baetis, Gammarus), and a searching strategy to capture more sedentary prey during the night (e.g. chironomids, simuliids). These experiments provided support for the hypothesis under test. If competition and/or interference occur between instars, then it could be reduced between earlier and later instars because of differences in their diet and diel pattern of feeding activity.     

Digestive proteinases of larvae of the corn earworm, Heliothis zea: characterization, distribution, and dietary relationships.

Proteinases and peptidases from the intestinal tract of fifth-instar larvae of Heliothis (= Helicoverpa) zea (Boddie) (Lepidoptera:Noctuidae) were characterized based on their substrate specificity, tissue of origin, and pH optimum. Activity corresponding to trypsin, chymotrypsin, carboxypeptidases A and B, and leucine aminopeptidase was detected in regurgitated fluids, midgut contents, and midgut wall. High levels of proteinase activity were detected in whole midgut homogenates, with much lower levels being observed in foregut and salivary gland homogenates. In addition, enzyme levels were determined from midgut lumen contents, midgut wall homogenates, and regurgitated fluids. Proteinase activities were highest in the regurgitated fluids and midgut lumen contents, with the exception of leucine aminopeptidase activity, which was found primarily in the midgut wall. Larvae fed their natural diet of soybean leaves had digestive proteinase levels that were similar to those of larvae fed artificial diet. No major differences in midgut proteinase activity were detected between larvae reared under axenic or xenic conditions, indicating that the larvae are capable of digesting proteins in the absence of gut microorganisms. The effect of pH on the activity of each proteinase was studied. The pH optima for the major proteinases were determined to be pH 8.0-8.5 for trypsin, when tosyl-L-arginine methyl ester was used as the substrate; and pH 7.5-8.0 for chymotrypsin, when benzoyl-L-tyrosine ethyl ester was used as the substrate.     

Cyclic AMP phosphodiesterase activity in the midgut of Manduca sexta

Midgut preparations from Manduca sexta larvae exhibit potent cyclic AMP phosphodiesterase activity. The enzyme exhibits a pH optimum at pH 8·8 and the assay reaction is linear for at least 30 min. Enzyme activity is greatest in larvae during the second day of the fourth and fifth instars and decreases at the end of the instar. Midgut cyclic AMP phosphodiesterase activity rises again in the pharate pupa and during pharate adult development. Whether these alterations in emzyme activity reflect humorally controlled morphogenetic changes in the midgut or are involved in the physiological functions of the midgut is as yet not known.