The Biotic Challenge

Pest pressure can vary tremendously from one agroecosystem to another, even where the same crops and pests are present. For example, outbreak conditions of B. tabaci persisted for many years in the lower desert valleys of California, such as the Imperial Valley, but barely rose to the level of occasional pest in the San Joaquin Valley situated 500 km to the northwest. Both regions grow cotton, vegetables and alfalfa, but the San Joaquin Valley accumulates only half the degree-days and many fewer generations of B. tabaci compared to the Imperial Valley. In considering pest pressure in different types of major crops, the analysis of potential crop losses to animal pests19 revealed the least pressure in wheat at 8.7% and the greatest pressure in cotton at 36.8%. Actual losses for the same two crops were 7.9 and 12.3%, respectively, indicating the value of pest management efforts in cotton to be sure.19 It is probably not surprising that the warm climate grown cotton faces a greater onslaught of arthropod pests than the higher latitudes and cool season grown wheat. The appropriateness of and demand for different pest defense tactics probably vary considerably between these two crops. Cotton at one time was the biggest user of pesticides, but that designation has now shifted to maize for its tremendous usage of herbicides. However, more insecticides continue to be used in cotton than in other major crops.

The rationale for using insecticides to fight infestations may not always or may rarely be justified, depending on one's frame of reference. The rules of IPM are supposed to provide an objective basis for using an insecticide application or not, depending on whether an economic threshold (ET) has been attained.16 Too often the problem is that neither the ET nor the economic injury level (EIL) have been determined for a crop, nor has a sampling plan for evaluating pest densities been worked out for particular insect pests in a target crop, relying instead on a nominative ET that is determined arbitrarily. IPM then risks becoming more rhetoric62 than reality and an overdependence on pesticides continues as in the past. This is not to suggest, however, that there isn't a critical need for the right pesticide at the right moment in a pest population's development. Agriculture is replete with pest species that are superbly adapted at colonizing new fields as progression through the seasons and from one annual crop to the next takes place. Colonization is the establishment of a population of a species in a geographical or ecological space not occupied by that species.63 So for those itinerant species whose life histories often involve a colonizing episode, the absence of population pressures and natural enemies in a new colony with virtually unlimited resources provides exponential growth opportunities. Such colonizers are able to take advantage of their dispersal capabilities to escape natural enemies that have accumulated at one site, by colonizing a new site that is free of conspecifics and natural enemies, at least temporarily. It is this brief period in enemy free space that enables the colonizing species to stay ahead of its natural enemies, at least until intraspecific competition factors begin to slow population growth rates of the pest species.

A synoptic model of insect population dynamics was developed by Southwood and Comins64 that expertly accounts for differences in life history patterns and how these influence population growth rates of pest species. This model makes use of the concept of r- or K-selected species, or more realistically the r-K-continuum that distributes species according to their life history traits.65 The terms r and K are derived from the Verhulst logistic growth equation, with r representing the growth rate of the population and K representing the carrying capacity of the local environment. Many agricultural pests possess life history traits consistent with the concept of the r-strategy, including short lifespan, high fecundity and great dispersal ability. It is these r-strategists that are able to disperse to new crops and establish new colonies that initially see rates of growth that approach rm.66 One of the key features of the synoptic population model is the "natural enemy ravine'', which is deepest at an intermediate position between two stability points: an upper one determined by competition and a lower one determined by natural enemies.66 Natural enemies are effective only when pest densities are below a certain point in the ravine according to the model. For insect species that normally remain at low to modest numbers, an upset in the balance of control, perhaps in the form of an insecticide treatment, can cause a release of the population to the "epidemic ridge'' depicted in the synoptic population model. For the extreme r-strategists that frequently occupy the epidemic ridge, natural enemies may often be of little consequence due to the invasive and colonizing nature of the r-pests.66 The habitats they occupy, both natural and agricultural, are frequently characterized by their durational instability. Climate seasonality may drive instability in certain natural habitats in which the availability of herbaceous annuals are tied to a rainy season that soon gives way to warming and drying conditions, leaving an abundant plant biomass to wither and recede. For many r-selected insects that exploit annual plants, the consequence of dry-down is a crash in the population that somehow manages to persist thereafter on a dwindling number of hosts until replenishment returns once again with a new rainy season. In an irrigated, continuous cropping system, however, the bust cycle is much less common while the boom cycle predominates. The insect species that evolved to rapidly exploit ephemeral habitats have now been transposed to a modified environment that remains highly unstable, but in a completely different way from the natural habitat, and one that works to the advantage of the r-selected pest. Much the same as the natural habitat, the agro-habitat features ephemeral stands of herbaceous plants that grow rapidly and then decline, but the interval between growth and decline cycles is much shorter, and may actually be non-existent in certain high production agro-ecosystems. Thus, the r-selected pest does not fade away with a decline in food resources as occurs in the natural habitat, but instead disperses from one crop at the end of its growth cycle into an adjacent crop at the beginning of its cycle. The evolutionarily adroit r-selected pest has literally sensed declining conditions in the previous crop including intraspecific crowding, reduced dietary nutrients and water, and/or an accumulation of secondary defensive compounds within the mature crop plants.67 69 By dispersing to a new field with a fresh young crop, it gains a tremendous fitness advantage by colonizing vigorously growing plants that are uncrowded with conspecifics and on which prey densities have yet to establish, thus improving chances that lower numbers of natural enemies will be present. For a while at least, the new crop represents an unlimited resource upon which population growth rates of the r-selected pest will be maximized.

It is in agro-habitats where a series of nutritionally high-value crops are grown sequentially through an entire annual cycle, and then repeated each year, that some of the greatest potential for insect outbreaks can occur. This was the nature of agriculture in the Sudan Gezira during the outbreaks of B. tabaci in the 1960-70s as intensification measures were implemented. Similarly, temporally overlapping vegetable and field crops that are grown year-round characterize agriculture in the irrigated desert agro-ecosystems of the southwestern USA. This region was invaded by the B-biotype of B. tabaci in the late 1980s that continued to build numbers that culminated in unprecedented destruction in 1991.70,71 The clouds of B. tabaci adults that arose from cantaloupe fields in late summer (see Figure 9.3A) were an extreme example of the pest pressure that was present throughout the Imperial Valley of California in 1991 and for years thereafter. Of all the irrigated valleys in this region of the USA, the Imperial Valley has perhaps the largest contiguous area of harvestable land at ca. 232 000 ha. It is planted to a large diversity of field crops, including cotton and alfalfa, leafy vegetables such as lettuce and broccoli, and various cucurbits. The sequential planting of these crops beginning with curcurbits in the spring, cotton and alfalfa during the summer, and finally the leafy vegetables and second planting of cucurbits in the autumn into winter encourages increasing numbers of whiteflies (see Figure 9.3B), all the way into autumn until much cooler winter temperatures diminish populations.

The agricultural milieu of the Imperial Valley, California was perfectly tailored for an r-selected pest such as B. tabaci in terms of the sequence of suitable crops grown and the hot and arid climate that accelerates growth and reproduction and increases the number of generations per year. It wasn't until the B biotype was positively identified as a new biotype of B. tabaci72 that growers in the Imperial Valley and other parts of North America recognized that they were up against a new pest with superior biotic potential compared with the indigenous A biotype. Numerous management changes have since been

Figure 9.3 Outbreaks of Bemisia tabaci that occurred in the southwestern USA during the early 1990s including this decimated cantaloupe field (A) in the Imperial Valley, California, in September 1991 (Reproduced with kind permission from Springer). Despite improvements in management, whitefly pressure remained heavy each year, being most intense during the early autumn vegetable season like this broccoli field (B) in September 2005.

Figure 9.3 Outbreaks of Bemisia tabaci that occurred in the southwestern USA during the early 1990s including this decimated cantaloupe field (A) in the Imperial Valley, California, in September 1991 (Reproduced with kind permission from Springer). Despite improvements in management, whitefly pressure remained heavy each year, being most intense during the early autumn vegetable season like this broccoli field (B) in September 2005.

implemented and proven highly effective, yet tremendous pest pressure is still exerted each year in the southwestern USA as evidenced by dispersing adults infesting autumn grown crops (see Figure 9.3B). The most influential changes have involved the introduction of highly effective insecticides representing numerous new modes of action,73 but also cultural changes in terms of the relative areas of crops planted. In particular, cotton and cucurbit acreages have been severely reduced74 (see Figure 9.4) as Imperial Valley growers recognized that certain structural changes in the agriculture had to be made to contend with B. tabaci biotype B. Most growers recognized that the expense and effort

The Challenge of Green in a Pesticide-Dominant IPM World 20,000

The Challenge of Green in a Pesticide-Dominant IPM World 20,000

1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Year

Figure 9.4 A steep decline in area of three favored crops of Bemisia tabaci in the Imperial Valley, California (USA) following the destructive outbreak of 1991 (indicated by arrow). Crop areas have remained at reduced levels ever since, due to perennially high population densities of B. tabaci.74

1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Year

Figure 9.4 A steep decline in area of three favored crops of Bemisia tabaci in the Imperial Valley, California (USA) following the destructive outbreak of 1991 (indicated by arrow). Crop areas have remained at reduced levels ever since, due to perennially high population densities of B. tabaci.74

to fight back a perennial outbreak pest on susceptible crops, like cantaloupes, were too high and the risk too great to continue at the same level prior to the 1991 outbreak.

It was because of pests such as B. tabaci and agricultural environments like the Sudan Gezira or the Imperial Valley that Southwood66 suggested that insecticides would always have an essential role in managing explosive pest species. B. tabaci probably experiences at least 13 generations per year in Yuma, Arizona75 and the same in the Imperial Valley 80 km to the west. The rapid and regular turnover of crops in these regions is ideal for the highly polyphagous B. tabaci that is also a very adept disperser among fields. B. tabaci effectively rides wind currents between fields, and in a large and contiguous region like the Imperial Valley its chances of finding a suitable host crop are quite good. Its pattern of moving into a new field, often just as plants are breaking through the ground, always puts B. tabaci ahead of its natural enemies in terms of population growth and creating a lag period that natural enemies are not able to overcome. It was only through the commercialization of imi-dacloprid and subsequent neonicotinoid insecticides that retarding B. tabaci populations in the Imperial Valley became possible beginning in 1995. Soil-applied treatments of imidacloprid at the time of planting were very effective in suppressing the first couple of generations that normally occurred in the spring cantaloupe crop. This had a tremendous effect on the population dynamics of B. tabaci right through the summer and autumn cropping seasons, especially in concert with other modes of action that altogether contributed to the transformation of B. tabaci into a reasonably well managed pest.

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