- Life History
- Insecticidal Control and Insecticide Resistance
- Cultural Control
- Physical Control
- Biological Control
- Plant Resistance
- Intergated Pest Management
- Control in Home Gardens
- References Cited
Colorado potato beetle, Leptinotarsa decemlineata (Say), is a leaf beetle in the family Chrysomelidae. The adult is approximately 3/8 inch (10 mm) long, yellowish in color with dark orange head and ten black longitudinal stripes on its back (Fig. 1A). The eggs are yellowish-orange, usually found in clusters of 20-50 eggs on the undersides of leaves (Fig. 1B). The larva is between 1/8 to 1/2 inch (3 to 13 mm) long and slug-like in appearance. It is red in color with two rows of black spots along each side and black legs and head (Fig. 1C). The yellowish or pinkish pupa (not shown) lives in the soil and is not easily seen in the field.
Fig. 1. Colorado potato beetle life stages. A-adult; B-eggs; C-larva.
Colorado potato beetle is the most important insect defoliator of potatoes. It also causes significant damage to tomato and eggplant. One beetle consumes approximately 40 cm2 of potato leaves at a larval stage, and up to additional 9.65 cm2 of foliage per day as an adult (Ferro et al., 1985). In addition to impressive feeding rates, Colorado potato beetle is also characterized by high fecundity, with one female laying 300-800 eggs (Harcourt, 1971). Furthermore, the beetle has a remarkable ability to develop resistance to virtually every chemical that has ever been used against it.
Fig. 2. Colorado potato beetle damage to potato plants. A-untreated potato plot adjusted to a potato field protected from the Colorado potato beetle damage by insecticide applications; B-close-up photograph of defoliated plants.
Since Colorado potato beetle shifted from its original wild hosts in southwestern North America, it has spread throughout the rest of the continent and has invaded Europe and Asia. Currently its distribution covers about 8 million km2 in North America (Hsiao, 1985) and about 6 million km2 in Europe and Asia (Jolivet, 1991). It has appeared recently in western China and Iran. Potentially the Colorado potato beetle can occupy much larger areas in China and Asia Minor, spread to Korea, Japan, Russian Siberia, certain areas of the Indian subcontinent, parts of North Africa, and the temperate Southern Hemisphere (Vlasova, 1978; Worner, 1988; Jolivet, 1991).
Fig. 3. Schematic diagram of the Colorado potato beetle life history. Only one generation is shown, but two-three generations are possible in the areas with warmer climates. Please click on the picture for a bigger image.
The beetles overwinter in the soil as adults, with the majority aggregating in woody areas adjacent to fields where they have spent the previous summer (Weber and Ferro, 1993). The emergence of post-diapause beetles is more or less synchronized with potatoes. If fields are not rotated, they are colonized by overwintered adults that walk to the field from their overwintering sites or emerge from the soil within the field (Voss and Ferro, 1990). If fields are rotated, the beetles are able to fly up to several kilometers to find a new host habitat (Ferro et al., 1991; 1999). Once they have colonized the field, the overwintered beetles first feed and then oviposit within 5-6 days depending on temperature (Ferro et al., 1985; Ferro et al., 1991).
Eggs are usually laid on the underside of potato leaves. Upon hatching, larvae may move over short distances within potato canopy and start feeding within 24 hours of eclosion. Development from the time of oviposition to adult eclosion for pupae takes between 14-56 days (de Wilde, 1948; Walgenback and Wyman, 1984; Logan et al., 1985; Ferro et al., 1985). The optimal temperatures range between 25-32ºC and appear to differ among populations of different geographic origins. The larvae are capable of behavioral thermoregulation via moving within plant canopies (May, 1981; Lactin and Holliday, 1994), thus optimizing their body temperature compared to the ambient temperature. Pupation takes place in the soil near the plants where the larval development has been completed.
Diapause is facultative, and the beetles can have between one and three overlapping generations per year. It takes a few days for the newly emerged adults to develop their reproductive system and flight muscles (Alyokhin and Ferro, 1999). After development has been completed, the beetles mate and start laying eggs. The reproduction continues until diapause is induced by the short-day photoperiod, then the beetles migrate to overwintering sites (mainly by flying), and enter the soil to diapause. Those beetles that emerge under short-day photoperiod do not develop their reproductive system and flight muscles that season. They feed actively for several weeks and then either walk to the overwintering sites or burrow into the soil directly in the field (Voss, 1989).
Colorado potato beetle's diverse and flexible life history is well-suited to unstable agricultural environments, and makes it a complex and challenging pest to control. Flight migrations closely connected with diapause, feeding and reproduction allow the Colorado potato beetle to employ "bet-hedging" reproductive strategies, distributing its offspring in both space (within and between fields) and time (within and between years). Such strategies minimize the risk of catastrophic losses of offspring, otherwise quite possible in unstable agricultural ecosystems (Solbreck, 1978; Voss and Ferro, 1990).
Chemical Control and Insecticide Resistance
Since 1864, hundreds of compounds were tested against the Colorado potato beetle (Gauthier et al. 1981), and insecticides still remain the foundation of the Colorado potato beetle control on commercial potato farms. Currently, more than 30 active ingredients are registered for use against this pest in the United States. Insecticide efficiency and availability, however, vary from area to area. The same is true about pesticide regulations. Therefore, people considering using chemicals to control beetles should contact a local extension agent or other qualified professional.
An important thing to keep in mind is that Colorado potato beetle has a legendary ability to develop resistance to a wide range of pesticides used for its control. High predisposition to resistance development seems to be an inherent characteristic of this species. It is probably caused, in large part, by the coevolution of the beetle and its host plants in the family Solanaceae, which have high concentrations of toxins, namely glycoalkaloids (Ferro, 1993). The first instance of Colorado potato beetle resistance to synthetic organic pesticides was noted for DDT in 1952 (Quinton, 1955). Resistance to dieldrin was reported in 1958, followed by resistance to other chlorinated hydrocarbons (Hofmaster et al., 1967). In subsequent years the beetle has developed resistance to numerous organophosphates and carbamates (Forgash, 1985). Presently it is resistant to a wide range of insecticides, including the arsenicals, organochlorines, carbamates, organophosphates, and pyrethroids. Resistance crisis was temporarily abated with the introduction of highly effective neonicotinoid insecticides. However, the first cases of beetle resistance to neonicotinoids have been already observed in several field populations (Alyokhin et al., 2006; 2007; Mota-Sanchez et al., 2006).
The major problem area is the Northeastern United States (Forgash, 1985); however, resistance has also been detected in Michigan (Ioannidis et al., 1991), Canada (Stewart et al., 1997), and Europe (Forgash, 1985; Boiteau, 1988). In some cases, a new insecticide failed after one year (e.g., endrin) or even during the first year of use (e.g., oxamyl) (Forgash, 1985). Resistance mechanisms are highly diverse even within a relatively narrow geographical area (Ioannidis et al., 1991). Furthermore, the beetles show cross-resistance to organophosphates and carbamates, and multiple resistance to organophosphates, carbamates, and pyrethroids (Ioannidis et al., 1991). In addition to the resistance to synthetic insecticides, the beetle has a capability to develop resistance to the Bacillus thuringiensis subsp. tenebrionis delta-endotoxin (Whalon et al., 1993; Rahardja and Whalon, 1995).
Colorado potato beetle populations can be reduced through the use of relatively common cultural practices such as crop rotation, manipulation of planting time and crop varieties, use of mulches, cover and trap crops (Hough-Goldstein et al., 1993). Crop rotation for the Colorado potato beetle control had been first recommended as early as 1872 (Bethune, 1872), and since then proved to be a good control strategy not only for the beetle, but also for a number of potato pathogenes and weeds (Casagrande, 1987). At the rotated field, peak density of the beetle egg masses could be less than 10% of that of the non-rotated field (Lashomb and Ng, 1984). Wright (1984) reported that when potatoes were planted following a non-host grain crop (rye or wheat), early season Colorado potato beetle adult densities were reduced by 95.8%.
Late and early planting is aimed to suppress the second generation larval populations. Because summer-generation adults emerge later in the season on the late-planted crop, the short-day photoperiod stimulates reproductive diapause, largely eliminating the second-generation larval impact on the crop. Early planting also eliminates the second generation larvae, in this case because the crop is already being removed at the time of their emergence (Weber and Ferro, 1995).
Trap crops may be used to attract beetles away from the main crop. It has been shown to intercept both overwintered beetles colonizing a field in the spring (Weber and Ferro, 1995), as well as the beetles moving away from senescing potatoes late in the season (Hoy et al., 1996).
Another promising technique of Colorado potato beetle cultural control is mulch applications. Larval populations of the beetle were significantly reduced in straw- mulched plots of potato (Stoner, 1993) and eggplant (Stoner, 1997). A peak of the small (1st - 2nd instar) larval populations was observed 1 - 2 weeks later on the mulched potato fields than on the unmulched ones (Stoner, 1993). Furthermore, the mulch may increase the time required by the beetles to find potatoes (Ng and Lashomb, 1983), decrease the likelihood of flying beetles locating the potato plants (East, 1993), increase the proportion of beetles leaving the area by flight (Weber et al., 1994), and increase predation on eggs and larvae (Brust, 1994). Overall, a six - ten cm layer of wheat straw produced 2.5-5 fold decrease in potato defoliation (Zehnder and Hough-Goldstein, 1990; Brust, 1994).
In addition to cultural control, a number of physical control methods can be used to suppress Colorado potato beetle populations. One possible method involves digging plastic-lined trenches along a field border in order to intercept post-diapause Colorado potato beetles colonizing the crop in the spring. In a one-month period, during which the majority of the beetles emerge from the soil, 1 m of such a trench can capture as many as 1,000 beetles (Ferro, unpublished data). Up to 95% of captured beetles are normally retained in the ditch (Misener et al., 1993).
Another method is to manipulate beetle diapause habitat in an attempt to enhance its overwintering mortality. In the experiment of Milner et al. (1992), wheat straw mulch was applied to the overwintering sites in the fall, and then removed together with the layer of snow covering it in January. This procedure rapidly depressed soil temperatures, and led to a significantly lower beetle survival (approximately 7% at disturbed habitats vs. approximately 26% at the undisturbed habitats).
Still other methods of physical control include propane flamers (Pelletier et al., 1995; Khelifi et al., 2007) and tractor-mounted vacuum collectors (Boiteau et al., 1992; Lacasse et al., 1998). Combining these two techniques increases their overall efficiency (Khelifi et al. 2007), making control level comparable to that of some insecticide treatments (Laguë et al., 1999).
High fecundity usually allows Colorado potato beetle populations to withstand natural enemy pressure. Still, in the absence of insecticides natural enemies can sometimes reach densities capable of reducing Colorado potato beetle numbers below economically damaging levels (Ferro, 1985).
Beauveria bassiana (Hyphomycetes) is a pathogenic fungus that infects a wide range of insect species, including the Colorado potato beetle. It is probably the most widely used natural enemy of the Colorado potato beetle, with readily available commercial formulations that can be applied using a regular pesticide sprayer. Applications of B. bassiana have been shown to reduce beetle populations by up to 75% (Cantwell et al.,1986). However, control is usually less effective compared to chemical insecticides (Campbell et al., 1985; Hajek et al., 1987).
Fig. 4. Dead Colorado potato beetle infected with Beauveria bassiana.
A number of predatory and parasitic arthropods attack the Colorado potato beetle (Hough-Goldstein et al., 1993). The lady beetle Coleomegilla maculata consumes eggs and small larvae (Groden et al., 1990; Hazzard et al., 1991), killing up to 37.8% of eggs for the first Colorado potato beetle generation and up to 58.1% of eggs for the second generation (Hazzard et al., 1991). Predaceous stink bugs Perillus bioculatus and Podisus maculiventris attack beetle larvae. Inundative releases of these predators suppressed beetle density by 62% (Biever and Chauvin, 1992), reduced defoliation by 86% (Hough-Goldstein and McPherson, 1996), and increased potato yields by 65% (Biever and Chauvin, 1992) over the untreated control. Adult ground beetles Lebia grandis feed on the Colorado potato beetle eggs and larvae, while larvae of the same species parasitize the Colorado potato beetle pupae (Weber et al., 2006). The parasitic wasp Edovum puttleri was found to parasitize 71-91% of Colorado potato beetle egg masses on eggplant, killing 67-79% of the eggs per mass (Lashomb et al., 1987). The level of parasitism is somewhat lower in potatoes, rarely exceeding 50% (Ruberson et al., 1991; Van Driesche et al., 1991). Performance of Edovum puttleri in the field can be further improved by the supplementary use of an artificial carbohydrate source (Idoine and Ferro, 1990).
Several species of generalist predators also occasionally feed on the Colorado potato beetle. Fourteen species of carabid beetles, three species of Coccinellidae, and a spider, Xysticus kochi, are known to feed on the Colorado potato beetle in the former Soviet Union (Sorokin, 1976). Eight species of Lebia and five other ground beetle species attack this pest in Mexico (Logan, 1990). Another ground beetle, Pterostichus chalcites, has been observed feeding on the Colorado potato beetle in Delaware (Heimpel and Hough- Goldstein, 1992). The daddy-long-legs Phalangium opilio preys on the beetle's eggs and small larvae (Drummond et al., 1990).
Although Colorado potato beetles are fully capable of completely wiping out potato crops, at low-to-moderate beetle densities potato plants are fairly tolerant to the inflicted defoliation. They can tolerate 30-40% defoliation during early growth stages, 10-60% defoliation during middle growth stages, and up to 100% defoliation late in the season without noticeable yield reduction (Hare, 1980; Cranshaw and Radcliffe, 1980; Ferro et al., 1983; Shields and Wyman, 1984; Zehnder and Evanylo, 1988). In the same time, currently there are no truly resistant cultivars. Conventional potato breeding is complicated by tetraploidy in S. tuberosum (Grafius and Douches 2008). Genetically modified potatoes expressing Bacillus thuringiensis delta-endotoxin that is toxic to the Colorado potato beetle were introduced in the U.S. in 1995, but then discontinued after only five years of use, in large part because of consumer concerns about genetically engineered foods.
Cultural practices may enhance plant ability to resist the Colorado potato beetle. Lower beetle densities have been recorded in plots receiving manure soil amendments in combination with reduced amounts of synthetic fertilizers compared to plots receiving full rates of synthetic fertilizers, but no manure (Alyokhin et al., 2005). No reduction in plant vigor in the absence of synthetic fertilizers was observed in that study. Subsequent field-cage and laboratory experiments (Alyokhin and Atlihan, 2005) confirmed that potato plants grown in manure-amended soil were indeed inferior Colorado potato beetle hosts compared to plants grown in synthetically fertilized soil.
Integrated Pest Management
The secret of Colorado potato beetle’s success as a pest is its diverse and flexible life history coupled with a remarkable adaptability. Therefore, to be successful in our control efforts we also need to be diverse and flexible in our approaches, as well as adaptable to ever-changing circumstances. Mindless reliance on a single tactic is doomed to fail, no matter how fundamentally sound this tactic is. The only sustainable way to manage this insect is integration of multiple control techniques based on a scientifically sound understanding of its biology.
Control in Home Gardens
In addition to creating problems in commercial production, the Colorado potato beetle is also a concern for home gardeners. When garden is limited to a few potato, tomato, or eggplant plants, hand-picking overwintered adults and egg masses early in the season is the simplest management approach. Most damage is done by larvae, so removing their parents and unhatched eggs should provide fairly good protection of the plants later in the season. It is no more time-consuming than other gardening practices, does not require expensive purchased inputs, and environmentally friendly. It can also be a relaxing and somewhat therapeutic experience – after all, from the biological point of view we have evolved to be hunters and gatherers, not computer programmers or hedge fund managers. The picking should be done for several weeks because overwintered beetles exit diapause and colonize host plants over approximately one-month time window.
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