Effects of plant spacing and interplanting with oilseed rape on colonisation of dwarf hops by the damson-hop aphid, Phorodon humuli

The pattern of colonisation of dwarf hops (Humulus lupulus) by damson-hop aphid (Phorodon humuli (Schrank)) migrating from Prunus spp. was investigated at six plant spacings and where some of the hops were replaced by oilseed rape (Brassica napus L.), a non-host of the aphid. The number of migrant aphids that accumulated on hop stems (bines) increased with increasing bine size and density. The numbers of aphids that colonised hops interplanted with oilseed rape reflected the density of the hop plants only and not the overall plant density. As the physical size of the wind shadow within which flying aphids can manoeuvre and land was unimportant unless provided by a host plant, the finding supports the theory that flying aphids respond to olfactory stimuli associated with their hosts. Variation in bine height (as a measure of plant size) explained 29–93% of the variance in aphid counts during the 3 years' study and bine density 1–14%. Standardising the data as the numbers of aphids per metre of bine and taking a square-root transformation of these standardised counts improved the precision of the analyses and, by stabilising variances, facilitated comparisons between years as growth became more vigorous as the plants matured. Each year, the rate of increase in numbers of aphids settling on plots of hops declined curvilinearly with increasing bine density. Maximum colonisation by P. humuli occurred at a bine density of five per metre row, a density similar to that used commercially by growers of dwarf hops.     

Sex pheromone and visual trap interactions in mate location strategies and aggregation by host-alternating aphids in the field

Abstract. Field observations were made on the responses of males and gynoparae of three host-alternating aphid species, the blackberry-cereal aphid, Sitobion fragariae (Walker), the bird cherry-oat aphid, Rhopalosiphum padi (L.) and the damson-hop aphid, Phorodon humuli (Schrank) to species-specific sex pheromones released from transparent and coloured water traps.Pheromone traps caught significantly more males than did control traps without pheromone, whereas transparent, light green, yellow and orange traps caught most insects.Measurements of the distance over which sex pheromones function indicated that male P.humuli detect the pheromone 2–6 m from the source and can fly upwind to a source in wind speeds of 0.7 m s^-1.In all three species significantly more gynoparae were caught in pheromone traps than in control traps, suggesting that pheromone released by adult sexual females may assist late-flying gynoparae to locate a suitable host plant on which to deposit their progeny.The response is relatively stronger for males than gynoparae, but the pheromones appear to act as both sex and aggregation pheromones.     

The effect of aphid prey quality on searching behaviour of Adalia bipunctata and its susceptibility to insecticides

After consumption of one individual of a suitable prey, such as Phorodon humuli (Schrank) (Aphididae), Adalia bipunctata L. (Coleoptera, Coccinellidae) adults changed from extensive to intensive searching behaviour. However, after the consumption of one individual of three other aphid species: Aphis fabae Scop., Aphis craccivora Koch or Aphis spiraephaga Müller, which are unsuitable prey, this coccinellid remained immobile for 12–76 min and did not switch to intensive search after moving off. Adult A. bipunctata fed an unsuitable prey, A. fabae, for 2 weeks were 2–4 times more susceptible to pyrethroid insecticides (permethrin, deltamethrin, fenpropathrin, esfenvalerate, alfa-cypermethrin and bioresmethrin) and insect growth regulators (diflubenzuron, triflumuron, fenoxycarb, flufenoxuron, teflubenzuron) than those fed a suitable prey, P. humuli.The searching behaviour of coccinellids could thus be another criterion for determining the suitability of aphids as prey. The fact that the nutritional quality of aphids can affect the susceptibility of predatory coccinellids to insecticides should be borne in mind in integrated pest management programs.     

Differential photoperiodic responses in genetically identical winged and wingless pea aphids, Acyrthosiphon pisum, and the effect of day length on wing development

Abstract. Winged and wingless individuals of a pink clone of the pea aphid, Acyrthosiphon pisum (Harris), showed differences in the response curves for photoperiodic induction of both males and sexual females (oviparae). The critical night length (CNL) for ovipara induction in winged aphids was 0.75 h shorter than in wingless aphids, whereas the CNL for male induction in winged aphids was 1.0h longer than in wingless aphids. This means that in winged aphids the CNL for male induction in winged aphids was 0.5 h longer than that for ovipara induction, while in wingless aphids the CNL for male induction was 1.0–1.5 h shorter than that for ovipara induction, and also the shapes of the curves differed.
Winged aphids were produced by wingless mothers which were crowded as young adults. However, when young adults were crowded in long nights, winged aphids were not produced, and the CNL for wing inhibition was between 9.5 and 10h. This effect of photoperiod on wing induction was maternal.     

Dispersion analyses and resource utilization of aphid parasitoids in a non-depletable environment

Dispersions and resource utilization of primary and secondary parasitoids developing in non-depletable primary host populations were determined for an aphid-parasitoid community occurring on strawberries. Analyses of dispersions based onGreen 's coefficient andLloyd 's Patchiness Index indicated parasitized aphids were highly aggregated initially, became less aggregated as density increased, and remained aggregated following collapse of the aphid populations. The “index of aggregation” values calculated usingTaylor 's Power Law concurred with results from the other indices, and the similarity of the regression coefficients from both seasons suggests that the index of aggregation may be characteristic for communities as well as species. Analysis withIwao 's regression of mean crowding on the mean generated similar results when population data were stratified temporally, and also indicated that the individual was the basic unit of the population. In a non-depletable environment, oviposition of individuals exhibiting an aggregated dispersion pattern within clumps of hosts provides primary parasitoids with a suitable trade-off between energy utilization or genetic potential, and losses associated with hyperparasitism.     

Biology of the hop cyst nematode Heterodera humuli

In an intensively sampled English hop garden, cysts of Heterodera humuli were found in every sample taken. Most occurred in the top 15 cm of soil, decreasing in number by half with every 15 cm in depth. Numbers of second-stage juveniles fluctuated during the vegetative period in a pattern which indicated that more than one generation, possibly two to three, were produced. The generation time was about 1.5 months. In pot experiments under controlled conditions (20°C; 16 h light/day) the life cycle lasted 40 days. The optimal temperature was 20°C for the development of H. humuli in the roots of its host Humulus lupulus. Most H. humuli juveniles invaded hop roots at 15°C, but egg hatch was greatest at 20°C. In moist soil, second-stage juveniles survived for at least 54 days and thereafter invaded roots and reproduced.     

Time-specific life tables for the pea aphid,Acyrthosiphon pisum (Harris), on alfalfa

Time-specific life tables were constructed for three pea aphid, Acyrthosiphon pisum (Harris ) (Homoptera: Aphididae), populations using a modification ofHughes' analytical procedure. All populations were studied on second-growth alfalfa (mid-June to mid-July) in south central Wisconsin; data for two populations were collected during 1980, and data for the third population were collected during 1982. The intrinsic rate of increase (r_m) estimated on a physiological time (day-degree) scale under field conditions but in the absence of natural enemies, provided a reliable estimate of potential population growth rate and was used in preference toHughes' approach of estimating potential population growth rates directly from stage structure data. Emigration by adult alatae and fungal disease were the major sources of A. pisum mortality in each of the three populations studied. These factors were most important because of their impact on reducing birth rates within the local population. Parasitism was never greater than 9 percent. Mortality attributable to predation ranged from 0.0 to about 30.0%; however, even at the highest predator densities A. pisum populations increased exponentially.     

Pheromone Traps for the Dissemination of an Entomopathogen by the Damson-hop Aphid Phorodon humuli

Two types of sex pheromone-emitting trap, a simple louvred trap and a commercially-produced wasp trap (Waspy®) designed or modified as delivery systems for entomopathogens, were tested using an isolate of the pathogen Verticillium lecanii . Their efficiencies for capturing winged autumn females (gynoparae) and males of Phorodon humuli in a hop garden and a plum orchard were compared with each other and with catches by yellow-painted water traps. Gynoparae rarely entered either type of pathogen dissemination trap. Males only entered traps that released (1 R , S ,4a R , 7 S , 7a S )-nepetalactol, the sex pheromone of P. humuli . In the hop garden, approximately five times as many males entered the modified Waspy® as entered the louvred traps, whereas in the plum orchard similar numbers of males were caught in both traps. On average, males visited the modified Waspy® trap for 34.1 ±7.9 (SE) min. Males spent about 50% of their time in the arena where the entomopathogen would normally be present and a further 20% of the time on the inside of the arena lid alongside the pheromone source. Out of a sample of 16 live aphids removed from a trap dispensing V. lecanii , 9 died from infection by the pathogen and 5 of the 16 initiated colonies of the fungus after they walked on a sterile agar plate for five minutes. None of the 15 aphids collected from traps without V. lecanii became infected or initiated colonies of the fungus on agar plates.     

Xylem ingestion by aphids – a strategy for maintaining water balance

A new waveform, pattern G, detected during recording of electrical penetration graphs (EPG's) from aphids was strongly correlated with uptake from radioactively labelled artificial diets. During pattern G, maxillary stylet tips were located within xylem conductive elements and it was concluded that pattern G is representative of ingestion of xylem sap. Dehydrated aphids showed a higher incidence and greater duration of xylem uptake, suggesting that its occurrence is related to the water balance of the insect.

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Résumé Une nouvelle onde, de type G, décelée pendant l'enregistrement des électropénétrogrammes (EPG's) des pucerons est nettement associée à la prise d'aliments marqués radioactivement. Pendant l'onde G, les extrémités des stylets maxillaires sont situées dans les éléments conducteurs du xylème et l'on en conclu que l'onde G représente l'ingestion du contenu du xylème. Les pucerons déshydratés présentent une plus grande fréquence et une durée plus longue d'absorption de xylème, ce qui suggère qu'elle est liée à l'équilibre hydrique de l'insecte.

     

Interactions between TIME and PIN could play a role in the system that controls the duration of diapause development and synchronization with seasonal cycles in the silkworm Bombyx mori*

The significance of winter cold in the termination of diapause was investigated with regards to TIME and PIN in eggs of the silkworm Bombyx mori. TIME (time interval measuring enzyme) is an ATPase that can measure time intervals by exhibiting a transitory burst of activation of the enzyme in accordance with diapause development, which requires cold for resumption of embryonic development in the silkworm. The possible timer function of TIME comprises a built‐in mechanism in the protein structure. TIME is a metallo‐glycoprotein consisting of 156 amino acid residues with a unique sequence in the N‐terminal region to which a sugar chain is attached. PIN (peptidyl inhibitory needle) inhibits the ATPase activity of TIME. PIN is not a simple enzyme inhibitor, but holds the timer by forming a time‐regulatory complex with TIME. The carbohydrate moiety of TIME is essential for the assembly of a high‐affinity PIN‐binding site within the timer motif of the TIME structure. The binding interaction between TIME and PIN was much tighter (nearly 1000 times) at 25°C than that at 4°C, as measured by fluorescence polarization. Because the logEC₅₀ at 4°C was approximately 7 nmol/L, PIN must dissociate from TIME at the physiological concentration of TIME in eggs in the winter cold. Based on the results of our study, we propose that the dissociation of the TIME–PIN complex in the winter cold cues a series of conformational changes of TIME, ultimately reaching the active form of ATPase which in turn causes the completion of diapause development and initiates new developmental programs.