Insect Vector Quantification and Dynamics

The main insect vectors that transmit viruses, bacteria, and mollicutes belong to the order Homoptera, including aphids, whiteflies, psyllids, planthoppers, and leafhoppers.

Mites, nematodes, fungi, and higher animals can sometimes act as vectors for transmitting pathogens.

Spores of plant pathogenic fungi can be transmitted to flowers by thrips and various pollinators, or to the bark and wood of trees by bark boring beetles.

Pathogen characteristics that influence disease development include pathogen population size (number of propagules), vector relationships (specificity and persistence), level of virulence, replication, and movement inside plants, and ecology (soilborne or foliar, survival).

For many plant virus diseases, the epidemiologically important propagule is often the vector.

Phloem-limited bacteria or mollicutes may share the same epidemiologically important propagule as plant viruses.

Traps such as water pan traps, yellow sticky traps, vertical nets, and suction traps are used for quantifying insect vectors. The catch of insects in traps depends on factors such as trap size, color, height, and location.

Methods for determining the proportion of infective insects involve placing insects on susceptible seedlings and using ELISA or PCR to test for pathogens.

Real-time quantitative PCR and amplicon sequence specific methods are used to identify and quantify viruses in plants or vectors.

Vector relationships are characterized by specificity and persistence, which have a significant impact on epidemic development over time.

The five steps in the transmission of vector-borne pathogens include acquisition, movement, multiplication, inoculation, and movement in the host plant. These steps contribute to epidemic development.

Factors affecting transmission and epidemic development include the proportion of infected source plants, vector density, vector aggregation, vector movement, presence of alternate hosts, and the multiplication rate of the pathogen in the host tissues.

Factors affecting the spatial spread of vector-borne diseases include vector movement and aggregation, wind direction, and the presence of alternate hosts. For example, aphids have two forms of adults, apterae and alatae, which affect local movement and dispersal over short and long distances. Disease gradients are estimated from observed frequencies at different distances from the source plant, and transmission and inoculation close to the source may not always lead to new infections.

Vector-borne diseases are often distributed in patches in the field, and the distribution can be affected by vector movement and aggregation. Disease gradients are estimated from observed frequencies at different distances from the source plant. These patterns impact disease management strategies by influencing control methods and the effectiveness of spraying or prevention strategies.

Control methods for vector-borne diseases depend on the specific pathogen, vector, host, and other means of dispersal. Spraying of the vector may not always be effective, and prevention of vector landing on the crop, host plant resistance, and a combination of different control tactics are often needed to reduce epidemic development.

The 'multiple infection transformation' is a concept used to estimate the number of inoculations for vector-borne diseases. It involves using the transformation ln[1/(1-x)], where x is the proportion of diseased plants or diseased area. This transformation helps to estimate higher numbers close to the source than the observed infections.

Wind direction affects the spatial spread of vector-borne diseases, with aphids being carried further downwind than upwind from an in-field source. The effect of wind on disease spread is exemplified by the spatial spread of black currant reversion virus and the distribution of wheat streak mosaic virus, both transmitted by vectors.

Survival and overwintering of vectors and infected plants impact the availability of initial inoculum for the initiation of epidemics by providing sources for transmission. These factors affect the timing and intensity of disease outbreaks, as well as the potential for secondary spread within a season.

Vector-borne diseases are generally polycyclic, with several transmission cycles within one season. Exceptions to this pattern include viruses transmitted by nematodes with long generation times such as Longidorus and Xiphinema, which have one or two generations per year. The exceptions are due to the specific life cycles and transmission dynamics of these vectors.

The proportion of aphids molted, adult weights, and nymphs per female on virus-infected plants compared to virus-free plants can provide insights into the impact of viruses on vector populations by indicating potential effects on aphid development, reproduction, and overall fitness in relation to virus infection.