The influence of black bean aphid, Aphis fabae Scop., and its honeydew on leaf growth and dry matter production of sugar beet

A. fabae populations, started at the 3–4 leaf-stage of sugar beet in the glasshouse and peaking at 3000 individuals per plant, reduced leaf area by 64% at the 14 leaf-stage. The size of the heavily-infested leaves number 5 to 10 was reduced by 80% or more. The rate of leaf growth regained normal values after the aphid populations collapsed, but the infested plants did not make up for the decrease in leaf area production that had been incurred during the infestation. Total dry matter production over a period of 15 weeks was reduced by 47%. Honeydew had no effect on leaf size or dry matter production. Sugar beet plants in the field became infested with A. fabae at the 2–3, 4–5 and 6–8 leaf stages. Maximum populations of 800, 2100 and 2200 aphids per plant were recorded, respectively. The pertinent reductions in leaf area were 91%, 67% and 34% at the 10–12 leaf-stage and 79%, 65% and 14% at harvest while the total dry matter produced was reduced by 91%, 79% and 16%. Neighbouring plants of the early-infested sugar beet plants gained significantly higher weights than control plants. Honeydew had no effect on leaf area or dry matter production. The consequences of these results for our understanding of Aphis fabae injury in sugar beet and aphid control in the field are discussed.     

Comparison of Physiology and Anatomy of Seedlings and Regenerants of Sugar Beet

Whole sugar beet (Beta vulgaris L. cv. Ras poly) plants were grown in the greenhouse from the same seed stock used for an in vitro shoot tip culture. In vitro produced sugar beet plants exhibited a high content of chlorophylls a and b, carotene, and total and soluble sugars. On the other hand, total protein content of in vivo plants was higher than that of in vitro plants. No differences were found by SDS-PAGE analysis in the nature and contents of soluble proteins of in vitro propagated plants and greenhouse-grown plants. Surfaces of epidermal cells were larger and palisade and spongy paranchyma tissues were thicker in leaves of regenerants than in leaves of seedlings. Vascular tissues in leaf petioles in regenerants were flat and more differentiated than in seedlings. Closed and undeveloped stomata were found on the abaxial leaf surface of regenerants, whereas in seedlings the stomata were open. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of size and duration of aphid infestation on host plant quality, and its effect on sugar beet yellowing virus epidemiology

This paper studies the influence of previous infestation on the host quality of sugar beet (Beta vulgaris L.) for aphids and the influence of previous infestation on sugar beet yellowing virus epidemiology. Sugar beet previously infested with Myzus persicae (Sulzer) or Aphis fabae Scopoli (Homoptera: Aphididae) had an improved host quality for subsequently infesting aphids of the same species. There was a significant negative relationship between the number of M. persicae infesting a plant and the proportion of those that died with a dark deposit in their stomachs, and a significant positive relationship between the number that settled on a plant and the number that infested it previously. Nymphs feeding on previously infested plants grew more rapidly than those on control plants. The beneficial effect of previous infestation persisted for at least 2 weeks and prolongation of the infestation beyond 2 weeks was of no further benefit to the aphids. Field grown sugar beet, previously colonised by M. persicae, was more susceptible to natural infestation by M. persicae up to 5 days after exposure. Previously infested plants were also more susceptible to infection with beet mild yellowing virus (BMYV) but not beet yellows virus (BYV), suggesting that the aphids on the previously infested sugar beet settled more readily and were more inclined to feed (and thus transmit BMYV) than aphids on the previously uninfested plants. The consequences for the control of sugar beet yellowing virus vectors are discussed.     

The effect of plant age and infection with virus yellows on the survival of Myzus persicae on sugar beet

Survival of Myzus persicae confined in clip-cages on mature leaves of sugar beet declined as the plants aged. Death of aphids was often preceded by the appearance of a black deposit in the aphids' stomachs, which may have been the cause of death. Both the rate of death and the proportion of aphids dying with black deposits was significantly less when plants were infected with beet yellows virus or beet mild yellowing virus, by comparison with healthy plants. The implication of these phenomena on the onset of mature plant resistance is discussed.     

Olfactory response by the aphidophagous gall midge, Aphidoletes aphidimyza to honeydew from green peach aphid, Myzus persicae

Female adults of the aphidopagous gall midge, Aphidoletes aphidimyza (Rondani) (Diptera: Cecidomyiidae), showed an olfactory response to honeydew excreted by the green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae) under laboratory conditions. The response was only elicited by treatments with honeydew, whereas aphids, aphids with pepper plants or leaves, and pepper plants or leaves were not attractive to the midges. Dose‐dependent responses were observed from whole honeydew, honeydew volatiles extracted in pentane, and honeydew volatiles captured on Porapak Q®. When honeydew was eluted with three sequential pentane washes, a positive response was only observed from the midges for the first wash. Female midges laid more eggs on pepper plants infested with higher densities of M. persicae. The olfactory response of midges to honeydew is discussed with respect to prey location.     

Transmission of potato leafroll virus in relation to the honeydew excretion of Myzus persicae

Honeydew excretion of single Myzus persicae nymphs on potato leafroll virus (PLVR)-infected Physalis floridana was studied during the acquisition access period (AAP) in relation to the efficiency of virus transmission.
With increasing length of the AAP, the percentage of nymphs that transmitted the virus increased. These nymphs produced significantly more honeydew droplets during the AAP on PLRV-infected P. floridana plants than nymphs which failed to transmit the virus. However, the number of honeydew droplets excreted during the AAP by transmitting nymphs did not affect the length of the latency period. Nymphs which infected the first test plant after a short latency period produced a similar amount of honeydew during the AAP to those with a longer latency period.
Honeydew excretion recorded on plants of varied age, showed that nymphs feeding on bottom leaves of infected plants produced more honeydew droplets than on comparable leaves of healthy plants. On infected plants, nymphs produced more honeydew droplets on bottom leaves with pronounced symptoms than on top leaves that hardly showed any symptom of PLRV infection.
The concentration of viral antigen measured by ELISA was lower in top leaves than in bottom leaves of infected plants. Nevertheless, nymphs feeding on top leaves transmitted the virus more efficiently than those which used bottom leaves as virus source. When bottom leaves were used as a virus source, the percentage of viruliferous nymphs decreased with plant age. These results indicate that the availability of virus for acquisition by aphids declines with increasing plant age and symptom severity.     

Why do ants shift their foraging from extrafloral nectar to aphid honeydew?

When aphids parasitize plants with extrafloral nectaries (EFNs) and aphid colony size is small, ants frequently use EFNs but hardly tend aphids. However, as the aphid colony size increases, ants stop using EFNs and strengthen their associations with aphids. Although the shift in ant behavior is important for determining the dynamics of the ant–plant–aphid interaction, it is not known why this shift occurs. Here, we test two hypotheses to explain the mechanism responsible for this behavioral shift: (1) Extrafloral nectar secretion changes in response to aphid herbivory, or (2) plants do not change extrafloral nectar secretion, but the total reward to ants from aphids will exceed that from EFNs above a certain aphid colony size. To judge which mechanism is plausible, we investigated secretion patterns of extrafloral nectar produced by plants with and without aphids, compared the amount of sugar supplied by EFNs and aphids, and examined whether extrafloral nectar or honeydew was more attractive to ants. Our results show that there was no inducible extrafloral secretion in response to aphid herbivory, but the sugar concentration in extrafloral nectar was higher than in honeydew, and more ant workers were attracted to an artificial extrafloral nectar solution than to an artificial aphid honeydew solution. These results indicate that extrafloral nectar is a more attractive reward than aphid honeydew per unit volume. However, even an aphid colony containing only two individuals can supply a greater reward to ants than EFNs. This suggests that the ant behavioral shift may be explained by the second hypothesis.     

The impact of Meu1-mediated resistance in tomato on longevity, fecundity and behavior of the potato aphid, Macrosiphum euphorbiae

The effect of the tomato resistance gene, Meu1, on feeding, longevity, fecundity and developmental rate of the pink biotype of the potato aphid, Macrosiphum euphorbiae (Thomas) (Hemiptera, Aphididae), was determined using nearly isogenic tomato (Lycopersicon esculentum Mill, Solanaceae) lines. Aphid mortality was significantly higher on resistant plants, with 60% of the aphids dying by the 4th day of exposure. By the 10th day, all the aphids on the resistant plants were dead whereas 100% of the aphids on susceptible plants were alive. Meu1-mediated resistance resulted in significantly decreased fecundity with a ten-fold decrease in the net fertility rate (4.5 and 45.7 progeny per aphid on resistant and susceptible tomato, respectively). A qualitative analysis showed that honeydew was produced by aphids on resistant and susceptible plants, suggesting that aphids initiate feeding on both plant types. However, significantly lower quantities of honeydew were present when aphids were caged on resistant plants. There were also significant differences in aphid location on resistant and susceptible leaves. Experiments evaluating behavior in less than 24 h showed that aphids left resistant leaves after relatively short exposure (3–6 h). Aphids transferred from resistant to susceptible tomato at intervals between 3 h and 24 h resumed feeding as evidenced by presence of honeydew. Although the mechanism by which Meu1-mediated resistance operates is not yet known, our data suggest that resistance factors act rapidly after initiation of feeding and that lower fecundity and longevity are related to reduction in aphid feeding.     

Plant–rhizobia interactions alter aphid honeydew composition

Both above- and below-ground interspecific interactions contribute to ecosystem functioning in terrestrial systems, and the integration of below- and above-ground interactions is crucial for deepening our knowledge of nutrient cycling and community dynamics in terrestrial ecosystems. The present study explored the effects of plant–microbe interactions on aphid honeydew quality and quantity and important factors mediating ant–aphid mutualisms and below-ground nutrient dynamics. Soybean aphids (Aphis glycines) were inoculated onto two closely related strains of soybean plants: a nodulating strain that associates with rhizobia and a non-nodulating strain that does not harbor any nitrogen-fixing bacteria. As expected, prior to aphid inoculation, nodulating plants were significantly taller and had more leaves than non-nodulating plants. Aphids feeding on nodulating strains were found to reach slightly larger colony sizes and produce honeydew with significantly different sugar profiles than those feeding on non-nodulating plants. The honeydew collected from aphid colonies feeding on nodulating plants contained 160 % more total sugars than honeydew collected from colonies feeding on non-nodulating plants, but there was no difference in total amino acid-N content in honeydew from colonies feeding on the different plant strains. We discuss the implications of honeydew composition for nutrient cycling and community dynamics and suggest areas of future research to elucidate the consequences of altered aphid honeydew composition on ecosystem properties.     

THE DISTRIBUTION OF VIRUSES IN DIFFERENT LEAF TISSUES AND ITS INFLUENCE ON VIRUS TRANSMISSION BY APHIDS

Exposing both surfaces of leaves systemically infected with cabbage black ring spot virus (CBRSV) or henbane mosaic virus to ultra-violet radiation decreases the infectivity of expressed sap to about one-fifth. As irradiation probably inactivates virus mainly in the epidermis, which occupies about one-quarter the volume of the leaves, these viruses seem to occur at much higher concentrations in sap from the epidermis than in sap from other cells. By contrast, tobacco mosaic virus seems not to occur predominantly in the epidermis.
CBRSV and henbane mosaic virus are normally transmitted most frequently by previously fasted aphids that feed for only short periods on infected leaves, but aphids treated like this transmit rarely from leaves that have been exposed to ultraviolet radiation. Irradiation has relatively little effect on the proportion of aphids that transmit after long infection feedings. Fasting seems to increase transmission by increasing the probability that aphids will imbibe sap from the epidermis of leaves they newly colonize. With longer periods on infected leaves, the ability of fasted aphids to transmit probably decreases because they then feed from deeper cells and their stylets contain sap with less virus. Only virus contained in the stylets seems to be transmitted, not virus taken into the stomach. About half the transmissions of henbane mosaic virus by aphids that have colonized tobacco leaves for hours may be caused by insects that temporarily cease feeding on the phloem and newly penetrate the epidermis.
Irradiating infected leaves affected the transmission of sugar-beet mosaic virus in the same way as that of henbane mosaic virus, but had little effect on the transmission of beet yellows virus, whose vectors become more likely to transmit the longer they feed on infected plants.     

Foliar uptake of Cd by pea (Pisum sativum) and sugar beet (Beta vulgaris)

Pea ( Pisum sativum L. cv. Fenomen) and sugar beet ( Beta vulgaris L. cv. Monohill) were cultivated in nutrient media without or with 10 μM CdCl₂. Leaves of the same size and stage of development, detached or still attached to the intact plants, were submerged into redistilled water containing 1 to 250 μM CdCl₂. The uptake experiments were run for 1 to 8 h at pH 3.6 and 5.1. Cuticular transpiration rate, density of leaf and density of stomata were also measured. Percentage of open stomata was studied at different pH.
Foliar uptake of Cd into the leaf is evident since Cd is transported from the exposed part of the pea leaves, through the petioles and into the stipules, and since the Cd concentration of the leaves increases with time and external Cd concentration. The foliar uptake depends on the permeability of the cuticular membrane, which is increased by a high intrinsic Cd level, which in turn enhances the foliar uptake of Cd in sugar beet. Higher cuticular permeability in pea than in sugar beet is shown by a 2.5 times higher cuticular transpiration rate and a 4 times lower density of leaf for pea, which causes a 7 times higher foliar uptake in pea than in sugar beet. Low pH decreases the net uptake of Cd, probably by an exchange reaction in the cutin and pectin of the cuticular membrane. Stomata are not directly involved in the Cd uptake, and the differences in the sum total of stomatal aperture area per unit leaf area is not related to differences in foliar uptake of Cd. Percentage of open stomata, calculated as average of both sides of the leaves, was not affected by changes in pH: but especially at high pH. proportionally more stomata were open on the adaxial than on the abaxial side.     

Onset of a decline in the quality of sugar beet as a host for the aphid shape Myzus persicae

The results presented in this paper suggest that sugar beet became less suitable as a host for aphids after late June and early July, when the plants were at the 10–12 leaf stage. This was indicated by a faster rate of aphid mortality and greater incidence of dark stomach deposits after this time. Increased aphid mortality coincided with a change in physiology of sugar beet leaves from being net sinks to sources of assimilate. Aphids which fed on older leaves, irrespective of plant age, suffered greater mortality than those feeding on young heart leaves. The incidence of dark deposits in aphid stomachs associated with this mortality was greater on outer than on inner leaves of old, mature and young plants. It is suggested that the incidence of the dark deposit is an indicator of declining plant quality. The consequences of this for the spread of beet yellowing viruses are discussed.     

The use of kairomones for foraging decisions by an aphid parasitoid in small host aggregations

1. The wasp Diaeretiella rapae uses honeydew emitted by its host, the cabbage aphid Brevicoryne brassica, as a kairomone (chemicals emitted by an organism as part of its activity and used by its natural enemies to their advantage). The role of the kairomone in foraging decisions by the parasitic wasp was explored by manipulating the amount of honeydew and the number of aphids in a colony independently. The count-down patch-exploitation mechanism (Iwasa et al., 1981) was employed to predict the results of these manipulations and contrast them with the predictions of Waage's (1979) model. 2. Kairomonal activity of honeydew decreased as the honeydew aged and lost its activity completely within 72 h. 3. The wasp was exposed to different amounts of honeydew on (a) aphid-free leaves and (b) leaves bearing colonies of 150 aphids. The parasitoid search time on both leaf types increased with increasing honeydew contamination. On aphid-bearing leaves, the number of attacked aphids in the colonies also increased with increasing honeydew contamination. The presence of aphids reduced the parasitoid search time compared to search time on aphid-bearing leaves with the same level of honeydew contamination, as predicted by the count-down model. 4. Parasitoids exposed to the same amount of honeydew, but a different number of aphids in a colony on the leaf, first increased the number of aphids attacked in relation to the number of aphids in the colony. But as the colony reached a certain size, the number of aphids attacked levelled off, despite the increase in the number of aphids in the colony. Search time was variable but did not exhibit any trend as the number of aphids in the colonies increased. 5. These results suggest that honeydew level is used by D. rapae as a cue for assessment of the number of aphids in the colony. In such cases, a count-down exploitation mechanism gives the best results to a forager. Many parasitoids may use kairomones for patch assessment and will therefore employ a count-down rule during patch exploitation.     

Leaf phosphate status and photosynthesis in vivo: Changes in light scattering and chlorophyll fluorescence during photosynthetic induction in sugar beet leaves

Light scattering and chlorophyll fluorescence were measured in vivo for leaves of sugar beet plants cultured with low levels of phosphate (P). Light scattering during photosynthetic induction was markedly increased in low-P compared to control leaves. This effect was reversible, disappearing within 24 h after the P supply was increased. The fluorescence induction curves also exhibited significant and reversible differences between low-P and control leaves. The changes in light scattering and chlorophyll fluorescence correlated well with changes in the rate of photosynthesis in vivo. We suggest that the increase in light scattering during induction in low-P plants may be due to a decreased ability of the Calvin cycle to utilize assimilatory power generated photochemically.     

Rapid low temperature-induced stomatal closure occurs in cold-tolerant Commelina communis leaves but not in cold-sensitive tobacco leaves, via a mechanism that involves apoplastic calcium but not abscisic acid

Commelina communis stomata closed within 1 h of transferring intact plants from 27 degrees C to 7 degrees C, whereas tobacco (Nicotiana rustica) stomata did not until the leaves wilted. Abscisic acid (ABA) did not mediate cold-induced C. communis stomatal closure: At low temperatures, bulk leaf ABA did not increase; ABA did not preferentially accumulate in the epidermis; its flux into detached leaves was lower; its release from isolated epidermis was not greater; and stomata in epidermal strips were less sensitive to exogenous ABA. Stomata of both species in epidermal strips on large volumes of cold KCl failed to close unless calcium was supplied. Therefore, the following cannot be triggers for cold-induced stomatal closure in C. communis: direct effects of temperature on guard or epidermal cells, long-distance signals, and effects of temperature on photosynthesis. Low temperature increased stomatal sensitivity to external CaCl(2) by 50% in C. communis but only by 20% in tobacco. C. communis stomata were 300- to 1,000-fold more sensitive to calcium at low temperature than tobacco stomata, but tobacco epidermis only released 13.6-fold more calcium into bathing solutions than C. communis. Stomata in C. communis epidermis incubated on ever-decreasing volumes of cold calcium-free KCl closed on the lowest volume (0.2 cm(3)) because the epidermal apoplast contained enough calcium to mediate closure if this was not over diluted. We propose that the basis of cold-induced stomatal closure exhibited by intact C. communis leaves is increased apoplastic calcium uptake by guard cells. Such responses do not occur in chill-sensitive tobacco leaves.     

THE EFFECT OF SUCROSE SPRAYING ON SYMPTOMS CAUSED BY BEET YELLOWS VIRUS IN SUGAR BEET

When leaves of sugar-beet plants infected with beet yellows virus were sprayed daily with 10% sucrose solution, yellowing symptoms were intensified. When glasshouse plants were shaded so that the light intensity was reduced to less than half of full daylight, yellowing symptoms were suppressed more completely on un-sprayed than on sprayed plants. Spraying with 2–5 % sucrose solution had similar, but slightly smaller effects.
Spraying with sucrose solution increased the carbohydrate content of the leaves, and the effects on symptom intensity and carbohydrate content were closely correlated. The regression coefficients of symptom score on total sugar content were nearly the same for shaded and unshaded plants. As the severity of symptoms was increased by supplying carbohydrate without change in the light conditions, it is concluded that light intensity affects symptom expression by varying the carbohydrate content of the leaves through its influence on photosynthesis.
Sucrose spraying increased the yield of roots of healthy and infected plants, and most of the increase was sucrose. This shows that sprayed sugar was translocated to the roots from the leaves of both healthy and infected plants.
Measurements of changes in carbohydrate content between evening and morning samplings confirmed that movement of carbohydrate out of infected leaves is not stopped by infection.     

Diurnal pattern of aphid feeding and its effect on cotton leaf physiology

Cotton aphid (Aphis gossypii G.) populations seemed to fluctuate over the past years in cotton (Gossypium hirsutum L.) perhaps as a result of excessive use of insecticides for controlling more problematic pests. Contradictory plant responses have been observed depending upon the aphid/plant system, and it is unclear if cotton aphids, abiotic stress or both are responsible for cotton yield reduction in aphid-infested fields. Our objectives were to investigate the diurnal changes in the physiology of cotton leaves following aphid herbivory, and the diurnal pattern of aphid feeding. The experiment was conducted in a growth chamber using the cotton cultivar ‘Stoneville 474’. Leaves of the same age and size were infested with wingless adults plus nymphs. Cotton aphids were allowed to increase in numbers without restriction for 9 days, after which the amounts of carbohydrates in aphid-honeydew, and the number of honeydew droplets excreted per aphid were measured. Photosynthetic rates, dark respiration rates and foliar non-structural carbohydrates were measured. The amount of individual carbohydrates found in the honeydew was significantly different with time. The total amount of carbohydrates excreted per aphid within a 24-h period averaged 2.5 μg. The number of honeydew droplets excreted per aphid varied significantly from time to time period. Cotton aphids did not significantly alter photosynthesis or respiration rates or non-structural carbohydrates on leaves. Aphid populations of approximately 300 per leaf on the 9th day of infestation did not appear to significantly alter the physiology of cotton leaves.     

Effects of sucrose sprays and darkness on aphid colonization of sugar beet and on aphid transmission of yellowing viruses

In the glasshouse, adult, apterous Myzus persicae (Sulz.) and Aphis fabae Scop, settled better and deposited more larvae on sucrose-sprayed sugar-beet plants than on water-sprayed plants. M. persicae settled badly and deposited few larvae on plants that were kept in the dark before or after infestation. The effects of darkness on aphids were reduced by spraying the host plants with 10% solutions of sucrose before infestation. Viruliferous M. persicae transmitted beet yellows virus (BYV) and beet mild yellowing virus (BMYV) less efficiently to dark-treated plants than to those grown in normal daylight. Spraying sugar beet with sucrose before inoculation with viruliferous M. persicae increased the proportion of successful BYV transmissions but only when the plants were dark-treated. The effects of sucrose and darkness on settling and larviposition of aphids and on virus transmission may be related to changes in the concentration of carbohydrates, particularly sugars, in the leaves.     

罗汉松雌雄株叶形态结构的比较研究

该研究以雌雄异株植物罗汉松(Podocarpus macrophyllus)成熟叶为研究材料,采用光学显微镜、扫描电镜和透射电镜观察比较罗汉松雌、雄植株叶在形态、显微结构和超显微结构的差异,以明确罗汉松雌、雄株在进化过程中叶对环境功能的适应性。结果显示:(1)罗汉松雌株的叶片大于雄株,且两者的叶长、叶宽和叶柄长差异极显著,而叶柄厚、叶面积、叶体积、叶质量、比叶重(SLW)、面积与体积之比(A/V)等性状无显著差异。(2)雌株叶片的气孔相对较大,密度较高,且雌株气孔宽度极显著大于雄株;雌株叶片的上表皮长细胞宽度和下表皮短细胞宽度均显著大于雄株,但雌株叶片的上表皮长细胞和短细胞的长度则显著小于雄株。(3)罗汉松雌株叶片的栅栏组织厚度、海绵组织厚度、传输组织长度和宽度、上下角质层厚度、维管束厚度、叶片紧密度(CTR)及疏松度(SR)均极显著大于雄株,而雌株的下表皮厚度极显著小于雄株,但雌雄株叶片的上表皮细胞厚度和栅海比差异不显著;雌株叶片的栅栏组织细胞、叶绿体和线粒体均较雄株的长而细,且雌株的线粒体宽度极显著小于雄株。(4)罗汉松雌株叶片上表皮蜡质饰纹、下表皮角质层纹饰、气孔外拱盖纹饰及内缘类型等4个微形态特征与雄株差异明显。(5)叶表皮蜡质层能谱分析表明,罗汉松雌株叶片含有9种元素,而雄株叶片仅有8种(缺少K元素);且雌株的Si元素含量高于雄株,而雄株的C、O、Na、Mg、Al、Ca和Au元素含量均高于雌株。研究表明,罗汉松雌、雄植株之间存在明显的第二性征,雌株叶片结构有助于提高光合等性能以满足生殖需求;罗汉松雌、雄株叶形态结构的差异是其长期进化形成的有利于物种繁衍的适应策略。     

The Sources of Sugars in Sieve-tube Sap

All the leaves of rooted willow cuttings were enclosed in aglass chamber for 2 hours, during which time they assimilated20 µ. of ¹⁴CO₂. A number of groups of aphids were situatedalong the stem, their honeydew was collected hourly on filter-paper,and the specific activity of the honeydew sugars determined.The activity in the honeydew of groups nearest the leaves rosesharply to a peak, then fell, steeply at first, then more gradually. Twenty-four hours after ¹⁴CO₂ assimilation, substantial activitywas still evident in the honeydew. In the case of other groupsfarther from the leaves, the maximum activity reached was notso high, while those aphids with a number of other groups ofaphids between them and the leaves excreted very little ¹⁴C.They did, however, excrete sugar at about the same rate as theother group, indicating that the stem reserves compensated forthe decrease (due to the other feeding aphids) in sugars directfrom the leaves. In one instance a mean rate of translocationof 7.5 cm./hr. was obtained.