VARIETAL RESISTANCE OF LETTUCE TO ATTACK BY THE LETTUCE ROOT APHID, PEMPHIGUS BURSARIUS (L.)

Striking varietal differences in susceptibility to attack by the lettuce root aphid, Pemphigus bursarius L., were first found in lettuces grown at Wellesbourne in 1955. Subsequent work has confirmed that White Favourite and Imperials E-4 and 19551-M are highly resistant; Imperial 45634-M, Continuity and Iceberg are markedly resistant. Midas, Salad Bowl and Imperial 4164 each appear to be a mixture of susceptible and resistant plants.
Immigrant winged forms of P. bursarius showed no preference for colonizing any particular variety of lettuce, and it seems that resistance to attack results from antibiosis.
It is suggested that varietal differences in the composition of root sap may account for the differences in susceptibility to lettuce root aphid, but analyses of water and alcohol extracts of root sap from resistant and susceptible lettuce varieties have not shown consistent differences.     

OBSERVATIONS ON BLINDNESS IN OATS

The relative importance in North Wales of various factors said to cause blindness in oats was investigated. It is concluded that a direct attack on the developing panicles by frit fly larvae and by adults and larvae of thrips was of little importance in causing the blindness encountered. Most of the blindness appeared to result from adverse physiological conditions probably operative during early growth. Varying varietal susceptibility to blindness was noted, and the effect on the subsequent panicles of an early insect attack on the vegetative parts of the plant is discussed.     

The susceptibility of verieties of carrot to attack by the aphid, Cavariella aegopodii (Scop.)

Representative varieties from the carrot groups, Chantenay, Nantes, Berlikum and Autumn King, together with three Australian varieties, were tested for susceptibility to attack by Cavariella aegopodii (Scop.). Tests were done at different average temperatures in cages in the insectary by inoculating plants with apterous forms of the aphid and frequently checking the extent of colonization over the next 20 or so days. Field sowings of the varieties were also made in each of 3 years, and over the immigration period and the subsequent population development and decline of the aphid, regular counts were made of the alates settling on a standard number of plants of each variety, and of the progeny produced. Some differences in susceptibility to attack were noted but, because the main development of C. aegopodii on field carrots is limited to two generations, these were too small to be of practical value in terms of plant resistance, and on all varieties large numbers of aphids could be found. Although the Australian variety Osborne Park was thought to have some resistance to C. aegopodii in New Zealand, it proved to be of average susceptibility. Autumn King appeared to be the most aphidsusceptible variety but could be tolerant to motley dwarf virus. Nantes was not as tolerant to aphid attack as the other varieties and as it is also intolerant to motley dwarf virus, might suffer much damage in years favouring aphid expansion. Berlikum seemed to be the least susceptible to aphid attack. Temperature differences affected expressions of varietal susceptibility less than they affected aphid fecundity and a few degrees increase in temperature more than halved the number of young born in a generation.     

Changes in phytohormones and fatty acids in wheat and rice seedlings in response to Hessian fly (Diptera: Cecidomyiidae) infestation

Phytohormones and fatty acids (FAs) play important roles in plant resistance to insects and pathogens. In this study, we investigated the similarities and differences in the accumulations of phytohormones and FAs in the resistant wheat (Triticum aestivum L.) 'Molly' and the nonhost rice (Oryza sativa L.) 'Niponbare' in responses to Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae), larval attacks. Using chemical ionization-gas-chromatography/mass spectrometry, we analyzed the concentrations of 13 phytohomones and FAs at the attack site of wheat and rice plants at 1, 6, 24, or 48 h after the initial attack. Hessian fly attack resulted in increases of salicylic acid (SA), 12-oxo-phytodienoic acid (OPDA), palmitic acid (FA16:0), but a decrease of abscisic acid in both wheat and rice plants. In addition, the accumulation of jasmonic acid (JA) increased, whereas the accumulation of cinnamic acid (CA) decreased in wheat plants, but no changes were observed in the accumulation of JA, and the accumulation of CA increased in rice plants after Hessian fly attack. However, the accumulations of benzoic acid, strearic acid (FA18:0), and oleic acid (FA18:1) increased in rice plants, but no changes were observed in wheat plants after Hessian fly attack. Hessian fly-induced changes were more rapid in wheat plants in comparison with those in rice plants. Our study suggests that SA and OPDA may be involved in resistance of wheat and rice plants to Hessian fly and that the R gene-mediated resistance responses are more rapid than nonhost resistance responses.     

ONE HOST SHIFT LEADS TO ANOTHER? EVIDENCE OF HOST‐RACE FORMATION IN A PREDACEOUS GALL‐BORING BEETLE

Abstract.— We show that a predator, the tumbling flower beetle Mordellistena convicta (Coleoptera: Mordellidae), has formed host races in response to a host-plant shift and subsequent host-race formation by its prey, the gall-inducing fly Eurosta solidaginis (Diptera: Tephritidae). This fly has formed two host races, one that induces stem galls on the ancestral host plant, Solidago altissima (Compositae), and another that induces stem galls on the closely related S. gigantea . We found that subpopulations of M. convicta that attack E. solidaginis galls on the different host plants have significantly different emergence times and, although slight, these allochronic differences are consistent across a range of temperatures. More importantly, we found that beetles assortatively mate according to their natal host plants, and female M. convicta preferentially attack and/or their offspring have higher survival in galls on natal host plants. Our data suggest that subpopulations of M. convicta that attack E. solidaginis galls on S. altissima and S. gigantea have formed host races. This is one of the first studies to demonstrate that a host shift and subsequent host-race formation by an herbivorous insect may have resulted in subsequent diversification by one of its natural enemies.     

Does R gene resistance allow wheat to prevent plant growth effects associated with Hessian fly (Diptera: Cecidomyiidae) attack?

Resistance genes (R genes) are an important part of the plant's immune system. Among insects, the Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae), larva is the target of the greatest number of characterized R genes (H1-H32). The biochemical/molecular mechanism of R gene resistance to Hessian fly is not well understood. In the absence of an effective R gene, larvae caused extensive growth deficits (> 30 cm) in wheat seedlings. In the presence of one of three effective R genes, H6, H9, or H13, larvae caused small growth deficits (approximately 3-4 cm) in two leaves (third and fourth) that were actively growing during the first days of larval attack. After larvae died on R gene plants, the fifth leaf and tiller leaves exhibited small increases in growth (2-4 cm). Growth responses of susceptible and resistant plants diverged at a time when Hessian fly larvae were establishing a nutritive gall tissue at feeding sites. The results of this study support the hypothesis that R gene resistance cannot prevent initial larval attack, but, by stopping the formation of the larval gall, it prevents the most serious consequences of larval attack.     

Hessian fly (Mayetiola destructor) attack causes a dramatic shift in carbon and nitrogen metabolism in wheat

Carbon and nitrogen (C/N) metabolism and allocation within the plant have important implications for plant-parasite interactions. Many plant parasites manipulate the host by inducing C/N changes that benefit their own survival and growth. Plant resistance can prevent this parasite manipulation. We used the wheat-Hessian fly (Mayetiola destructor) system to analyze C/N changes in plants during compatible and incompatible interactions. The Hessian fly is an insect but shares many features with plant pathogens, being sessile during feeding stages and having avirulence (Avr) genes that match plant resistance genes in gene-for-gene relationships. Many wheat genes involved in C/N metabolism were differentially regulated in plants during compatible and incompatible interactions. In plants during compatible interactions, the content of free carbon-containing compounds decreased 36%, whereas the content of free nitrogen-containing compounds increased 46%. This C/N shift was likely achieved through a coordinated regulation of genes in a number of central metabolic pathways, including glycolysis, the tricarboxylic acid cycle, and amino-acid synthesis. Our data on plants during compatible interactions support recent findings that Hessian fly larvae create nutritive cells at feeding (attack) sites and manipulate host plants to enhance their own survival and growth. In plants during incompatible interactions, most of the metabolic genes examined were not affected or down-regulated.     

Differences in the relative resistance of two carrot cultivars to carrot fly attack over five seasons

A comparison was made over five seasons of carrot fly (Psila rosae) damage on two carrot cultivars, Sytan and Danvers, which represented the extremes of resistance discovered in screening trials at Wellesbourne. Plants were harvested regularly during the seasons and at each harvest the number and weight of roots was recorded and carrot fly damage was assessed using various techniques; in 1979-80 the numbers of carrot fly larvae and pupae in and around root samples were counted. The experiments confirmed repeatedly the relative resistance of cv. Sytan compared with Danvers. The estimated reduction in carrot fly larvae on Sytan relative to Danvers in early November in four seasons ranged from 40% to 67%. When tested against first generation carrot fly attack the reduction in larvae on Sytan was 54%. There were 45% fewer mines per root on Sytan resulting in less damage at each harvest in all seasons. Larvae took longer to develop on Sytan than Danvers and 18% more plants of Sytan survived carrot fly attack. Differences in seedstock, season, sowing time, generation of carrot fly, plant size and density did not account for observed differences in damage between the two cultivars. The studies indicated satisfactory practical techniques for assessing carrot fly damage in cultivar screening trials.     

No fitness cost for wheat's H gene-mediated resistance to Hessian fly (Diptera: Cecidomyiidae)

Resistance (R) genes have a proven record for protecting plants against biotic stress. A problem is parasite adaptation via Avirulence (Avr) mutations, which allows the parasite to colonize the R gene plant. Scientists hope to make R genes more durable by stacking them in a single cultivar. However, stacking assumes that R gene-mediated resistance has no fitness cost for the plant. We tested this assumption for wheat's resistance to Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae). Our study included ten plant fitness measures and four wheat genotypes, one susceptible, and three expressing either the H6, H9, or H13 resistance gene. Because R gene-mediated resistance has two components, we measured two types of costs: the cost of the constitutively-expressed H gene, which functions in plant surveillance, and the cost of the downstream induced responses, which were triggered by Hessian fly larvae rather than a chemical elicitor. For the constitutively expressed Hgene, some measures indicated costs, but a greater number of measures indicated benefits of simply expressing the H gene. For the induced resistance, instead of costs, resistant plants showed benefits of being attacked. Resistant plants were more likely to survive attack than susceptible plants, and surviving resistant plants produced higher yield and quality. We discuss why resistance to the Hessian fly has little or no cost and propose that tolerance is important, with compensatory growth occurring after H gene-mediated resistance kills the larva. We end with a caution: Given that plants were given good growing conditions, fitness costs may be found under conditions of greater biotic or abiotic stress.     

Exposure to host resistance mechanisms drives evolution of bacterial virulence in plants

Bacterial pathogenicity to plants and animals has evolved through an arms race of attack and defense. Key players are bacterial effector proteins, which are delivered through the type III secretion system and suppress basal defenses . In plants, varietal resistance to disease is based on recognition of effectors by the products of resistance (R) genes . When recognized, the effector or in this scenario, avirulence (Avr) protein triggers the hypersensitive resistance reaction (HR), which generates antimicrobial conditions . Unfortunately, such gene-for-gene-based resistance commonly fails because of the emergence of virulent strains of the pathogen that no longer trigger the HR . We have followed the emergence of a new virulent pathotype of the halo-blight pathogen Pseudomonas syringae pv. phaseolicola within leaves of a resistant variety of bean. Exposure to the HR led to the selection of strains lacking the avirulence (effector) gene avrPphB (or hopAR1), which triggers defense in varieties with the matching R3 resistance gene. Loss of avrPphB was through deletion of a 106 kb genomic island (PPHGI-1) that shares features with integrative and conjugative elements (ICElands) and also pathogenicity islands (PAIs) in diverse bacteria . We provide a molecular explanation of how exposure to resistance mechanisms in plants drives the evolution of new virulent forms of pathogens.     

Correlation of volatile profiles of twenty mango cultivars with their susceptibilities to mango gall fly infestation

Mango gall fly (Procontarinia matteiana) is an orchard pest that parasitises flush leaves of mango and serious outbreaks may result in reduced fruit yield. The trigger for infestation is unknown, but terpenes emitted by the leaves appear to play a role in attraction. Metabolic profiles of three mango cultivars of varying susceptibility to mango gall fly attack were obtained by headspace profiling using GC-FID and GC-MS analysis. Chemometric models constructed from the data revealed that three terpenes, α- and β-pinene and camphene could be useful as biomarkers for susceptibility. Headspace profiles of twenty other cultivars, naturally exposed to gall fly, were obtained in the same way. Susceptibility or resistance of these cultivars was predicted using the developed orthogonal partial least squares model. Predictive outcomes were thereafter verified by visual examination of the leaves to detect gall formation, an indication of gall fly infestation. The model was found to predict the susceptibility or resistance of 90% of the cultivars accurately. This finding indicates the contributory role of the three terpene biomarkers in mango gall fly interaction and may direct future studies to determine their inter-relationship.     

Scarlet gilia resistance to insect herbivory: the effects of early season browsing,plant apparency,and phytochemistry on patterns of seed fly attack

Pollen-limited plants are confronted with a difficult tradeoff because they must present showy floral displays to attract pollinators and yet must also minimize their apparency to herbivores. In these systems, traits that increase pollinator visitation may also increase herbivore oviposition and overall plant resistance may therefore be constrained to evolve largely as a correlated response to selection on plant apparency or vigor. We used a family-structured quantitative genetic experiment to evaluate the importance of ungulate browsing, flowering date and plant height (traits that are related to overall vigor), and variation in a putative phytochemical defense (cucurbitacin production) on patterns of seed fly attack in a scarlet gilia population. We found significant genetic variation in the amount of insect damage plants experience in the field, providing evidence that resistance may evolve. In addition, we found that browsing reduced seed fly attack and that oviposition is strongly related to plant size and flowering date; large, early flowering plants experience high attack. In addition, we found that high cucurbitacin production was correlated with low seed fly damage, although this effect was relatively weak.We found directional selection on final plant height and flowering date; tall, early flowering plants had the highest reproductive success. In addition, we found negative directional selection on cucurbitacin production, which may indicate a high cost of cucurbitacin or other functions of this phytochemical. Although seed fly herbivory arguably decreases plant fitness, we found an unexpected positive relationship between damage and fitness. A negative relationship between fitness and damage may be masked in this system through strong positive indirect correlations between patterns of damage and levels of pollinator visitation. Finally, we found significant genetic variation in flowering date, plant height, and cucurbitacin production. Resistance to seed flies may evolve in this population, but largely as a non-adaptive correlated response to selection on overall plant vigor. Phytochemicals may play a more important role in defense in years with high seed fly attack, or when pollen-limitation is less severe.Co-ordinating editor: J. Tuomi     

Virulence in Hessian fly (Diptera: Cecidomyiidae) field collections from the southeastern United States to 21 resistance genes in wheat

Genetic resistance in wheat, Triticum aestivum L., is the most efficacious method for control of Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae). However, because of the appearance of new genotypes (biotypes) in response to deployment of resistance, field collections of Hessian fly need to be evaluated on a regular basis to provide breeders and producers information on the efficacy of resistance (R) genes with respect to the genotype composition of Hessian fly in regional areas. We report here on the efficacy of 21 R genes in wheat to field collections of Hessian fly from the southeastern United States. Results documented that of the 21 R genes evaluated only five would provide effective protection of wheat from Hessian fly in the southeastern United States. These genes were H12, H18, H24, H25, and H26. Although not all of the 33 identified R genes were evaluated in the current study, these results indicate that identified genetic resistance to protect wheat from Hessian attack in the southeastern United States is a limited resource. Historically, R genes for Hessian fly resistance in wheat have been deployed as single gene releases. Although this strategy has been successful in the past, we recommend that in the future deployment of combinations of highly effective previously undeployed genes, such as H24 and H26, be considered. Our study also highlights the need to identify new and effective sources of resistance in wheat to Hessian fly if genetic resistance is to continue as a viable option for protection of wheat in the southeastern United States.     

Isotope technique for investigation of cold resistance in rice and sorghum varieties

Summary A radiochemical test procedure has been developed for investigation of the cold-resistance of young rice and sorghum varieties. It was found that the extent of Rb-ion exchange diffusion caused by cold injury is inversely related to cold-resistance among the varieties. Therefore measurements of changes in ion influx effected by exposure to low temperature can be used to estimate varietal differences in cold-resistance. This test procedure is proposed for the screening of newly bred or newly introduced rice and sorghum varieties for their temperature sensitivity.     

Impact of an insecticide resistance strategy for house fly (Diptera: Muscidae) control in intensive animal units in the United Kingdom

A strategy for house fly (Musca domestica L.) control in intensive animal units in the United Kingdom was proposed by the Pesticide Safety Directorate (PSD) of the Ministry of Agriculture, Fisheries and Food in 1993. An advice leaflet was circulated to farmers, and label recommendations for insecticides used to control house flies were altered to prevent their long-term and frequent use. A study was carried out between 1996 and 1998 to gather data on insecticide use and resistance in house fly populations and compared with results from a study carried out in 1990-1992 to assess the impact of the 1993 label recommendations. As in the 1990-1992 study, resistance to methomyl, azamethiphos and pyrethrins + piperonyl butoxide was assessed. Larvicide tests with cyromazine, which had recently been released in the United Kingdom, were also included in this study. Most of the farmers claimed to have received and read the PSD insecticide advice leaflet, and half claimed to have altered insecticide treatments as a result. Comparing results for insecticides used before and after 1993, the proportion of farmers claiming to have used each of the insecticides had decreased. However, there had been no amelioration in resistance to synergised pyrethrins, and the number of house fly populations with reduced response to the insecticide baits had increased between 1990-1992 and 1996-1998. All the house fly populations tested were fully susceptible to cyromazine. There is an urgent need, therefore, to devise new strategies and particularly to minimize the risk of selecting for resistance to cyromazine.     

Stable fly, Stomoxys calcitrans, mouthpart removal influences stress and anticipatory responses in mice

Abstract Biting fly attack induces a variety of stress and anxiety related changes in the physiology and behaviour of the target animals. Significant reductions in pain, or more appropriately, nociceptive sensitivity (latency of a foot-lifting response to an aversive thermal stimulus), are evident in laboratory mice after a 1 h exposure to stable flies, Stomoxys calcitrans. The role of the various components of biting fly attack in the development of this stress-induced reduction in pain sensitivity (analgesia) is, however, unclear. This study demonstrates that fly-naive mice do not exhibit a stress-induced analgesia when exposed to stable flies whose biting mouthparts have been removed. In contrast, mice that have been previously exposed to intact stable flies exhibit significant analgesia when exposed to flies that are incapable of biting. However, the level of analgesia induced is lower than that elicited by exposure to intact stable flies. Exposure to non-biting house flies, Musca domestica , has no effect on nociceptive sensitivity. It appears that the actual bite of the stable fly is necessary for the induction of analgesia and probably other stress and anxiety associated responses in fly naive mice. However, mice rapidly learn to recognize biting flies and exhibit significant, possibly anticipatory analgesic responses to the mere presence of biting flies.     

Leaf growth signals the onset of effective plant resistance against Hessian fly larvae

For plant resistance that is induced rather than constitutive, the precise timing of a sequence of events must be considered (i.e., initial detection of the insect by the plant's surveillance systems, up-regulation of signaling and defense pathways, achievement of effective levels of defense, and finally down-regulation of signaling and defense). Here, we provide a timeline for the interaction between resistant wheat ( Triticum aestivum L.) (Poaceae) and the Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae). To create this timeline, we measured the daily growth of the third, fourth, and fifth leaves of susceptible and resistant plants. Because each leaf had a different spatial relationship to the site of larval attack (i.e., the sheath epidermal cells of the third leaf) and a different pattern of growth relative to the 3–5 days that larvae attacked resistant plants, we learned different things from each leaf. The third leaf shows how quickly responses of susceptible and resistant plants diverge (i.e., 36–60 h after initial larval attack). The fourth leaf shows that, for both susceptible and resistant plants, negative effects of larval attack extend beyond the third leaf. These negative effects are more severe for susceptible plants, but even in resistant plants continue for several days after larvae have died. The fifth leaf is interesting because it shows how rapidly the resistant plant recovers from larval attack. Thus, 204–348 h after initial attack, a time when the fourth leaf of resistant plants is showing reduced growth and the fifth leaf of susceptible plants is showing zero growth, the fifth leaf of resistant plants shows a small increase in growth. Grasses with resistance gene-mediated resistance may have a two-fold strategy, using resistance mechanisms to stop Hessian fly larvae from further attack and tolerance mechanisms to protect resources for future plant growth.     

A novel wheat gene encoding a putative chitin-binding lectin is associated with resistance against Hessian fly

The gene-for-gene interaction triggering resistance of wheat against first-instar Hessian fly larvae utilizes specialized defence response genes not previously identified in other interactions with pests or pathogens. We characterized the expression of Hfr-3 , a novel gene encoding a lectin-like protein with 68–70% identity to the wheat germ agglutinins. Within each of the four predicted chitin-binding hevein domains, the HFR-3 translated protein sequence contained five conserved saccharide-binding amino acids. Quantification of Hfr-3 mRNA levels confirmed a rapid response and gradual increase, up to 3000-fold above the uninfested control in the incompatible interaction 3 days after egg hatch. Hfr-3 mRNA abundance was influenced by the number of larvae per plant, suggesting that resistance is localized rather than systemic. In addition, Hfr-3 was responsive to another sucking insect, the bird cherry-oat aphid, but not to fall armyworm attack, wounding or exogenous application of methyl jasmonate, salicylic acid or abscisic acid. Western blot analysis demonstrated that HFR-3 protein increased in parallel to mRNA levels in crown tissues during incompatible interactions. HFR-3 protein was detected in both virulent and avirulent larvae, indicating ingestion. Anti-nutritional proteins, such as lectins, may be responsible for the apparent starvation of avirulent first-instar Hessian fly larvae during the initial few days of incompatible interactions with resistant wheat plants.     

Tolerance and antibiosis resistance to cabbage root fly in vegetable Brassica species

Four accessions of the wild species Brassica fruticulosa Cyrillo (Brassicaceae) were studied in order to identify its tolerance and antibiosis resistance to the cabbage root fly, Delia radicum L. (Diptera: Anthomyiidae), in comparison to a widely cultivated cauliflower cultivar and a rapid cycling Brassica oleracea L. line. Antibiosis was prominent, as the insects reared on resistant accessions showed reduced individual pupal weight, total pupal weight, adult dry weight, and the longest average fly eclosion time. Host plant resistance, however, did not affect the sex ratio of adult flies. A study of the root architecture of plants with and without root fly inoculation revealed differences in the structure within B. oleracea accessions. A long main root and a high number of lateral roots appeared to be important characteristics for a Brassica type, with a higher tolerance level to cabbage root fly attack.