Leaves aphids

Lettuce is the most pest- and disease-prone member of the Asteraceae. Common pests include slugs, cutworm, leaf aphids, and root aphids. Linder cover, downy mildew and gray mold (botrytis) can be a problem, especially in cool, damp conditions. Cultivars with resistance to aphids, downy mildew, and various physiological disorders are available. For more advice and information, see the A-Z of Plant Problems ipp.320-341). 

Bernasconi, M. L., Turlings, T. C. J., Ambrosetti, L., Bassetti, P. and Dorn, S. (1998). Herbivore-induced emissions of maize volatiles repel the corn leaf aphid, Rhopalosiphum maidis. Entomologia Experimentalis etApplicata 87 133-142. 

Homoptera Corn leaf aphid Rhopalosiphum maidis... 

If 50% of winter cereal plants have eggs present then consider a foUar insecticide treatment. Threshold for grain aphids at flowering is when two thirds of the ears are infected. Do not treat if more than 5% of aphids are mummified. Threshold for leaf aphids is when 50% stems are infected. 

Elder leaves contain the alkaloid sambucine, a precursor of hydrocyanic acid, which is somewhat toxic. Thus, the leaves are used only topically in ointments for sprains or bruises. Bruised leaves can be rubbed on the body or worn under a hat to prevent being pestered by insects. Cooled strained leaf tea can be applied to plants to discourage aphids. 

Pests are often classified according to where and how they feed on the plant (see facing page). Although this is convenient, these categories are not mutually exclusive. Many leaf-feeders also feed on stems some sapsucking aphids also feed on roots and flowers. 

A little tolerance is also to be advised. Plants are part of the natural world, and are inevitably going to be less than perfect a leaf spot here or a nibbled leaf there is not going to be life-threatening. The A-Z of Plant Problems (seepp.320-342) contains more detailed advice on specific problems affecting woody plants the entries on aphids, black spot, powdery and downy mildew, and rose rust are particularly applicable to roses. 

Symptoms Black or brown sootlike deposits on upper leaf surfaces and other plant parts. Plants may also be infested with sap-feeding pests such as aphids, whiteflies, scale, or mealybugs. 

Bystrum et al. first described morphologic changes in leaf surface waxes of table beet exposed to photochemical oxidants these changes were different from those associated with aphid feeding. Comparison of oxidant injury with that produced by insects has since received attention from Hibben, who found that ozone injury to the leaves of four tree species produced smaller flecks, randomly spaced and darker than fleck injury along veins induced by a mesophyll-feeding leafhopper. 

Several inferences can be drawn from these data that may suggest the impacts to be expected at the consumer and decomposer levels. Accelerated leaf drop may influence the development of pests—namely, aphids, scale insects, and red citrus mites. Pest populations might be increased if injured leaves had higher concentrations of amino acids or free sugars before abscission (see Chapter 11) or diminished if leaves fell too rapidly. Leaf and fruit drop would provide a larger substrate for populations of decomposer organisms at the soil surface. 

All these data support the idea that QA may function as chemical defense compounds. We also tested whether this chemical defense is relevant for the survival of a lupin plant. Lupins offer a unique chance to explore this question experimentally plant breeders have selected "sweet" varieties, which have a very low alkaloid content. These varieties can be compared to semi-bitter or bitter ones. We have grown Lupinus albus strains that differ in their alkaloid content in our experimental garden and greenhouse and have monitored their susceptibility to attack by plant pests. As can be seen from Figure 2, "sweet" lupins are preferentially eaten by rabbits (Cuniculus europaeus) or are infested by aphids (Aphidae) or leaf miners (Agromyzidae). Literature data also support the assumption that alkaloid-rich lupins are much more resistant to plant pests than "sweet" varieties (30-32). We conclude therefore, that QA are indeed important for the fitness of a lupin plant and that they constitute a major part of its chemical defense system, in which... 

Arachis hypogaea Leaf bud petioles Procyanidin Affects fecundity of groundnut aphid (Aphis craccivora) 366... 

The significance of such spatial arrays lies in the behavioral responses of insects foraging in these trees. If certain leaf types are unavailable while others are preferred, then such spatial arrays force insects to move about in search of good feeding sites (29). For insects which spend much time (or all of their lives) feeding in one place (sessile species, such as aphids), this search is performed once after a suitcible site is located, these insects are restricted to one portion of their... 

A number of plants of the Solanum and Nlcotlana genera are particularly adept at producing sticky leaf exudates. In some wild potato species, an exudate is discharged from glandular hairs when aphids mechanically rupture the cell walls (33). The clear, water soluble exudate is stable in the absence of O2,... 

Many Insects have become specialists on crucifers and a few related plant families. These Include flea beetles, leaf beetles, cabbage root fly, aphids, cabbage butterflies and the dlamondback moth. At the same time, several polyphagous Insects such as the cabbage looper, armyworms and aphids are major pests of crucifers. Comparative studies on these specialists and generalists have provided valuable Information on host recognition and possible resistance mechanisms. 

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