John L. Capinera
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Increased concern over invasion by high-profile and damaging insects requires the answers to questions about the origin of pests, period of invasion, taxa, feeding behavior, damage frequency, and the influence of crop characteristics on pest species richness. John L. Capinera he successful tive) and nonindigenous (invader, adventive, exotic) pests establishment of vegetable crops in the United States and Canada. Tin North America of invading pests is not a new phe- Analysis nomenon. Since the earliest arrival To determine the origin and biological charac- of European explorers and colonists, teristics of vegetable pests, I reviewed the original pests of plants have accompanied move- ment scientific literature of over 330 pests known to feed of people, food, and plant materials to the “New on vegetable crops (Capinera 2001 and references World.” The long sea voyage during the initial therein), and compiled data on pest origin, period stages of colonization likely inhibited extensive of introduction, host range, portion of plant dam- transport of many short-lived pests. Up until 1800, aged, and damage frequency. In most cases the lit- only about 36 species of insects invaded the United erature contains reference to the likely source of these States (Simberloff 1986). However, by the late pests (126 of 135 invaders). However, for some 1800s not only was the speed of transport greatly cosmopolitan pests or poorly studied organisms increased, but transport of living plant material the source is unknown or uncertain. Similarly, the (and associated pests) was commonplace. period of detection in the United States and Canada Sailer (1978, 1983) analyzed the invasion of varies from precise to unknown, with knowledge of the United States by arthropods. He reported that the period of establishment (detection) limited to 87 initially the invaders (immigrants) consisted mostly of the 135 invading pests. Lack of data stems mostly of Coleoptera, which arrived principally in ship from the early period of settlement, but invasions ballast. The dominance by beetles decreased as more since 1900 are fairly well documented. Lepidoptera and then Homoptera invaded, often The host range of the vegetable pests is quite in association with living plant material. The rate well documented in the literature, although there is of invasion increased greatly after the 1860s as a tendency for species that are better studied to international commerce expanded. Sailer also noted have longer lists of hosts. Also, during periods of that there was a slight reduction in the rate of inva- great abundance (outbreak), pests often use hosts sion commencing in the 1920s as the Plant Quar- that they will not normally accept. Thus, outbreak antine Act of 1912 was implemented. species probably have inflated host ranges. For the Invasion by high-profile and damaging species purposes of this analysis, I have defined a narrow in recent years has increased the awareness and host range to consist of consumption of vegetable concern by the scientific community, government, plants from a single botanical family. A moderate and the general public over invading species (U.S. host range consists of consumption of vegetable Congress 1993, Simberloff et al. 1997, Sakai et al. plants from two to five families, and a wide host 2001). The pattern of invasion is poorly docu- range is defined as consumption of greater than mented, despite heightened concern, and many ques- five families of vegetable crops. tions remain. Has the increase in international Damage frequency is based on the literature for commerce and tourism in recent years resulted in each pest, as described elsewhere (Capinera 2001), higher rates of invasion and establishment of pests? and considers damage to both home gardens and Are we at greater risk from certain types of pests, or commercial vegetable production. Damage fre- from certain sources? Are certain taxa of pests more quency is designated as rare when there are few likely to invade and establish successfully, and are all reports of serious injury in the literature. Damage crops equally at risk? Here I address these questions frequency is considered periodic when there is gen- and present a comparative analysis of indigenous (na- eral recognition that the organism is capable of 20 AMERICAN ENTOMOLOGIST • Spring 2002 causing crop injury, but the pest does not cause marily cantaloupe and honeydew), onion, pea, pep- loss annually, and vegetable growers do not nor- per, potato, spinach, sweet corn, sweet potato, to- mally take preventative measures to guard against mato, and watermelon. Data on crop acreage and injury. Pest damage is considered frequent when value are from Capinera (2001), and consist of the potential to cause injury exists annually in ei- summed American and Canadian values. ther home gardens or on farms, and characterizes pests for which vegetable producers annually plan Origins of Vegetable Pests for suppressive or preventive actions. Pests are des- Of the 332 vegetable pests considered in this ignated as capable of periodic or frequent damage analysis, 99% were successfully classified as either if this pattern is exhibited even in a limited geo- indigenous (native) or nonindigenous (invaders), graphic area, because most pests have limited geo- and 59% appeared to be indigenous to the United graphic distribution. States or Canada (Appendix 1). Similarly, Pimentel Designation of the site of damage is based on (1993) estimated that 63% of major American veg- the scientific literature, and consists of blossom etable pests are indigenous, though this figure is (flower); fruit (seed-containing structure); foliage; based on a subset of vegetable crops. It is evident stem or vine; tuber (below-ground storage organ); that our native fauna displays considerable plas- Pest damage and root, bulb, or germinating seed (below-ground ticity in acquiring new hosts. This trend has been root-related structures). noted previously, and we can expect the number of is considered The 332 pests considered in this analysis are, in pests to increase with time, and especially with the frequent when nearly all cases, individual species. However, the area planted to each particular crop (Strong 1974, the potential economic literature tends to group some pests into Strong et al. 1977, McCoy and Rey 1983, Capinera complexes either because they are poorly known et al. 1984a), as indigenous species adapt to im- to cause injury or difficult to differentiate. Therefore, a few pests ported host plants, or crops are exposed to addi- exists annually in (e.g., false wireworms, Coleoptera: Elateridae; white tional potential pests in new geographic areas. either home gardens grubs, Coleoptera: Scarabaeidae) are treated as in- However, species accrual on introduced crops oc- dividual pests although more than one species is curs most rapidly soon after plant introduction, or on farms involved. Also, some literature is confusing because and species richness on crops does not increase pests originally thought to be a single species have indefinitely, leveling off after fewer than 300 years since been differentiated into species complexes (e.g., if there is not an increase in crop acreage (Strong spinach and beet leafminers, Pegomya spp., Diptera: 1974). Also, it is important to note that though Anthomyiidae; dingy cutworms, Feltia spp., Lepi- indigenous pests form a large assemblage, they do doptera: Noctuidae). These issues are minor, how- not necessarily cause damage frequently (see dis- ever, and likely have little effect on the analysis. cussion of pest damage below). The aforementioned data on pest characteris- Europe is the principal origin of the nonindig- tics were analyzed by 2 × 2 contingency table analy- enous vegetable pests found in North America. sis using Fisher’s exact test (Zar 1984). In the case About 54% are thought to have originated there, of feeding behavior, I compared the number of spe- or to have arrived in North America via that re- cies with wide host range to the number of species gion (Fig. 1). This is not surprising because the with narrow host range for both indigenous and United States and Canada were colonized princi- nonindigenous species. For the noctuid and pyralid pally by Europeans, who introduced European analysis, I also analyzed the number of species with crops, and possibly allowed European pest “hitch- narrow and wide host ranges for each family. For hikers” to arrive. The introduction of crops, the damage frequency, I compared the number of spe- repeated waves of European immigration, the ex- cies causing damage rarely or frequently for both tensive trade between Europe and North America, indigenous and nonindigenous species. and the introduction of European ornamental Regression analyses were conducted on area plants to North America all contributed to the pre- planted to major commercial vegetable crops, to- tal crop value, and unit value in relation to the total number of pest species or number of major pest species per crop. Major pest species were those capable of “frequent” and “periodic” damage, as described previously. Correlation analyses were performed on crop acreage and crop value data, and on crop acreage and crop unit value data. All data were log-transformed prior to linear regres- sion or correlation (Graphpad Software 1993). Such transformation linearizes curvilinear rela- tionships, and normalizes residuals and makes them homocedastic. Although not always improv- ing the fit of the model (Conner and McCoy 1979), this is a