Ecology and management of agricultural pests
Our laboratory has been involved for many years on the biology and control of the whitefly Bemisia tabaci, a cosmopolitan pest well-known for its monumental outbreaks that have occurred in different parts of the world. The irrigated desert valleys of the southwestern United States are sites of year round production of various vegetable and field crops and have been especially prone to outbreaks of B. tabaci. The conditions that occur in these desert valleys, in contrast to other agricultural regions where B. tabaci does not reach outbreak proportions, have been summarized in a conceptual synthesis called the outbreak pyramid.
The essence of the outbreak pyramid is that in circumstances where the bottom two layers are broad with agro-ecological potential, the biotic characteristics inherent in outbreak-prone organisms permit exploitation of the resources in excess of both natural and managed controls. Conceptually, the volume of the management (IPM) layer at the top of the pyramid is much less than the three layers that form the base. In practice, management efforts that are targeted at individual fields, even if they provide superior results, are often eventually overwhelmed by regional population increases of B. tabaci on crop and non-crop vegetation. It is not a failure of management per se to control regional populations of B. tabaci, but rather a function of the combined potential contained within the climate, agriculture and biotic levels of the pyramid that ultimately exceed the capacity of local crop protection.
The challenge for management of B. tabaci in an outbreak-permissive environment such as the Imperial Valley, CA is to improve control tactics and expand the top layer of the pyramid. This is the only component of the outbreak pyramid that is pliable – the climate and biotic characteristics of the pest are essentially fixed, whereas agriculture develops in regions over many years and is not readily changed. Expansion of the top layer of the pyramid is best achieved by developing a knowledge-driven system that relies upon scientifically-valid information to enable confident pest management decisions to be made.
· Control efficacies
· Action thresholds
· Sampling programs
· Integration of chemical, natural and cultural controls
· Conservation of chemistry and natural enemies
Arthropod vectors of plant pathogens
In addition to being a potentially devastating direct pest of crops, B. tabaci is also a vector of numerous plant viruses that often severely reduce crop yields if infection rates increase early and rapidly. While the practical aspect of plant virus epidemics and the role played by B. tabaci is challenging from an applied perspective, there are also interesting questions concerning the interrelationships of vector, virus and host that are worthy of exploration.
One area of study currently being pursued concerns the effect that virus infection in a plant has on vector performance. Previous work with aphid-borne viruses led to the vector dependency hypothesis which posits that a plant pathogen with greater dependency on a principal vector species will tend to reciprocate the benefit derived from its vector relationship. In contrast, less or non vector-dependent plant pathogens gain less or not at all from vector associations and therefore do little or nothing to benefit the vector-pathogen relationship. Vector-dependent plant pathogens require movement to new hosts that is provided by vector insects, in most cases the only mechanism of dispersal for the pathogen. Reciprocation to the vector can occur through the medium of their mutual host plant. Infection-induced changes in host plant chemistry may actually benefit a vector nutritionally. This concept was first stated over 50 years ago:
“An agent which, through the disease it causes, actually multiplies its own vectors, has an obvious evolutionary advantage over one that does not.”
J. S. Kennedy, 1951, Nature 168: 890-894
After demonstrating support for this hypothesis with an aphid-potato virus experimental system (Entomol. Exp. Appl. 69:51-60; Ann. Ent. Soc. Am. 91:661-667), a new system is being tested that consists of B. tabaci as the vector, cotton leaf crumple virus (CLCrV) as the plant pathogen, and cotton as the host plant. Preliminary data suggests that Gossypium hirsutum is less suitable as a host when infected with CLCrV than uninfected. Further study will examine the suitability of G. thurberi, native to the southwestern USA in common with CLCrV, as a host plant for B. tabaci when infected or uninfected with CLCrV. The vector dependency hypothesis predicts that performance on CLCrV-infected plants will be enhanced since B. tabaci is the only known vector of CLCrV.
Insecticide resistance monitoring and management
Resistance management is a fundamental component of good IPM and is dependent upon continuous monitoring of pest populations to determine resistance levels and, if necessary, make appropriate changes in pesticide use. Current management schemes for B. tabaci are still highly dependent on insecticide treatments in regions of heavy B. tabaci pressure such as the southwestern US, northern Mexico, southern Spain, etc. These regions face continuous challenges in maintaining production quality and quantity in the face of sometimes unrelenting whitefly pressure.
Worldwide, a shift to newer chemistry has been paramount in instituting more effective management for controlling outbreak populations on multiple continents. In particular, systemic application of the neonicotinoid insecticide imidacloprid with its long residual activity has been instrumental in preventing early-season buildups in vegetable crops. Outstanding field efficacy of imidacloprid has been remarkably sustained for over 12 years of intensive, multiple crop use in the southwestern US. However, resistance has occurred in certain locations under heavy B. tabaci pressure such as Almeria on the southern coast of Spain. Resistance monitoring data collected in Arizona and California have also pointed to changeable levels of susceptibility to imidacloprid in B. tabaci populations collected on various crops and time of the year. Vigilance in the field in the form of monitoring and management of insecticide resistance is essential to sustaining the détente that has been achieved between pest management and B. tabaci in the USA.
Results of systemic uptake bioassays of Bemisia tabaci adults collected at various times of the year in central Arizona. Mortality for each replication at each dose is presented for all eight samples. Mean mortalities at each dose is represented by the traversing blue line at points of intersection with black range lines; horizontal green lines represent mean mortality across all doses for each date. The LC50s computed for each date appear above each mortality trace.