Biological Control of Insect and Weed Pests in North Dakota Agriculture (continued - part 3 of 4)

E-1225, January 2002

Contents

Introduction
Biological Control
        - Parasites
        - Predators
        - Diseases
Maintaining Natural Order
        - Canola Insect Pests
        - Potato Insect Pests
        - Sunflower Insect Pests
        - Sugarbeet Insect Pests
        - Corn Insect Pests
       - Small Grain Insect Pests
Weeds
       - Leafy Spurge
       - Purple Loosestrife
Conclusions
Biological Control References

Diseases also occur among insects. Insect diseases are caused by fungi, viruses, bacteria, protozoans, and other microorganisms. Insect-parasitic nematodes are also included in this group of natural enemies. Insect-parasitic nematodes are small worms that attack and kill insects that live in moist habitats. A few species are currently being sold commercially for insect control. Insect pathogens, including nematodes, are an important component in suppressing pest species. Some insect pathogens and nematodes are commercially available and can be manipulated to achieve biological control of specific pests. Both diseases and nematodes, like parasites, tend to be specific to certain species or groups of pests; they do not harm nontarget organisms, such as beneficial insects, animals, humans, or plants. They can quickly spread through an insect population causing rapid mortality in a short period of time, and can be important in the natural control of pest populations. This phenomenon, called an epizootic, occurs when the insect pest population level is high or environmental conditions are especially suitable for the pathogen or disease-causing organism, enabling the disease organism to spread from insect to insect very quickly. In high-value crops, the pest population usually cannot be allowed to reach a level where an epizootic can occur. However, epizootics can be an important natural control of pests of forests, rangeland, and certain typesof field crops.

Insect viral pathogens vary in how they attack and kill their host. Most insect viruses need to be ingested to successfully infect their host, though some can be transferred from the parent insect to the offspring through the egg. Symptoms usually occur within a few days after the virus is ingested. The infected insect (Figure 15) will appear sluggish, feeding will stop, and the cuticle will have a pale discoloration and will often hang from its legs. The infected insect will die one to two days after the symptoms appear. The decomposing cadaver will burst, liberating the viral particles into the environment. Important groups of viruses that attack insects are the nuclear polyhedrosis viruses (NPV), cytoplasmic polyhedrosis viruses (CPV) and granulosis viruses (GV). Viruses usually attack the caterpillar stage, such as the Helicoverpa NPV that invades the corn earworm larva or the codling moth GV that infects the codling moth larva.

Figure 15. Caterpillar killed by viral infection. (Click for a 22KB color photo of a caterpillar killed by viral infection.)

The bacteria most important in insect pest management are in the genus Bacillus. Species in this genus form spores that are toxic to the insect when ingested. Symptoms of infected insects include a loss of appetite, sluggishness, discharge from the mouth and anus, discoloration and liquefaction and putrefaction of the body tissues (Figure 16). Bacillus thuringiensis (commonly called Bt) is the most widely-used bacterium for insect pest control. Different strains of Bt are specific against caterpillars, mosquito larvae and some beetles and their larvae. Bacillus popillae and B. lentimorbus cause "milky disease" of white grubs. "Milky disease" refers to the white discoloration of the insect blood. The spores of B. popillae and B. lentimorbus survive in the soil and are ingested by the grubs as they feed on roots of grasses. Bacillus thuringiensis, B. popillae, and B. lentimorbus have been formulated into microbial insecticides by several companies for application to crops in an augmentative manner. Most recently, genes that produce Bt toxins have been genetically engineered into crop plants (corn, cotton, tobacco and potato) for season-long protection against larval pests and some adult insects. Larval and adult pests ingest the toxin after they have fed on the foliage and subsequently die. A beneficial soil bacterium, Saccharopolyspora spinosa, produces a natural metabolite, Spinosad, when cultured under aerobic fermentation conditions. Spinosad has been formulated as a microbial insecticide. Insects become poisoned with Spinosad when they ingest treated foliage or come into contact with the microbial sprays of the metabolite. Sickened insects stop feeding, become limp and are unable to move, and may appear to have weak tremors. Spinosad is effective against a wide spectrum of insect pests, including armyworms, European corn borer, diamondback moth, leafminers, and Colorado potato beetle.

Figure 16. Caterpillar killed by a bacterial infection. (Click for a 14KB color photo of a caterpillar killed by a bacterial infection.)

Insect pathogenic fungi produce spores that germinate when they come in contact with the insect cuticle and when temperature and moisture conditions are favorable. Germinating spores penetrate the insect cuticle and invade the body cavity. Hyphae rapidly grow, filling the body cavity with a fungal mass, killing the insect. The fungus also may produce a toxin. Hyphae penetrate outward through the softer parts of the insect and under favorable moisture conditions produce spores that ripen and are released into the environment to complete the life cycle (Figure 17). Insects that are attacked by fungi often retain their shape but usually become hardened or "mummy-like" and appear "fuzzy" from the fungal growth. There are many genera of fungi that attack insects. The most important ones are Metarhizium, Beauveria, Entomophthora and Zoopthora. Metarhizium anisopliae and B. bassiana attack a wide range of insects, such as grasshoppers, true bugs, aphids, caterpillars, and beetles. Entomophthora muscae attacks many types of adult flies including the seed corn maggot and the hover fly, a beneficial. Zoophthora radicans attacks the potato leafhopper and many aphids, and Z. phytonomi infects the alfalfa weevil.

Figure 17. Hyphae of pathogenic fungi produced on killed potato beetle. (USDA) (Click for a 10KB color photo of pathogenic fungi produced on killed potato beetle.)

Maintaining Natural Order

Natural enemies do not destroy or eradicate all insect pests. However, they often prevent pest populations from becoming too high. Most biological control agents at work in agricultural and urban environments are naturally occurring ones that provide excellent regulation of many pests with little or no assistance from humans. In some cases, they do this year after year because the natural enemy is an established part of the environment. The existence of naturally occurring biological control agents is one reason many plant-feeding insects do not ordinarily become economic pests. The importance of such agents often becomes quite apparent when insecticides applied to control a key pest cause an outbreak of secondary pests because of the chemical destruction of important natural enemies. The second species, released from the pressure imposed by its enemies, now may increase to damaging numbers and require further insecticidal treatment.

If insecticides eliminate natural enemies, populations of pests may increase and emigrate to surrounding habitats, at a considerable distance from the site where the application took place. Therefore, the impact of insecticides may extend over long periods of time and large areas until natural enemies are restored. Obviously if insecticides are used often, the normal balance of biological control agents may never be reattained. There is great potential for increasing the benefits derived from naturally occurring biological controls through eliminating or reducing the use of insecticides toxic to natural enemies. The successful combination of insecticide use and biological control in an IPM program depends on more knowledge of the system -- the ecology and the behavior of pests and natural enemies -- plus the incorporation of available tools and techniques.

Canola Insect Pests

Crucifer flea beetle (Phyllotreta cruciferae), diamondback moth (Plutella xylostella) and bertha armyworm (Mamestra configurata) are the three major pests of concern for canola grown in the upper Midwest.

Crucifer flea beetle is the first pest to occur in canola fields in North Dakota and northwestern Minnesota. Overwintered adults move into canola fields in May and June and begin feeding on newly emerged seedlings, causing serious damage. Although natural enemies do not play a key role in reducing damage, cultural control methods can help reduce plant losses. A firm seed bed that is well tilled and adequately fertilized will help plants outgrow beetle damage.

Bertha armyworm is native to North America. This pest, along with the true armyworm and variegated cutworm, belongs to a group of insects called the "climbing cutworms." Bertha armyworm can be a significant pest in canola; however, adverse environmental conditions and natural enemies help to keep this pest insect at low population levels during most years. There are a number of diseases and parasites that attack the bertha armyworm including: a nuclear polyhedrosis virus, an ichneumon parasitic wasp, Banchus flavescens, and a tachinid parasitic fly, Athrycia cinerea. Populations of these natural enemies build up slowly and have their greatest impact a year or two after the peak of an outbreak of bertha armyworm. Therefore, severe infestations only last two or three years.

Diamondback moth was introduced from Europe about 150 years ago and has since established throughout North America wherever its hosts plants are grown. Larvae are the damaging stage, and they feed on plants in the mustard family (canola, mustard), cole crops (broccoli, cabbage), and several greenhouse plants. Diamondback moth does not overwinter well in the northern states, and the severity of an infestation in a given year depends on overwintering populations in southern states and southerly winds to transport adult moths northward. Natural controls, including weather and beneficial organisms, canhave a significant impact on diamondback moth population levels. Fungal diseases in the class Entomophthorales cause natural disease outbreaks in larval populations (Figure 18). Disease outbreaks usually occur later in the season after populations have reached higher levels. Infection at higher rates can limit development of additional generations late in the season. Three parasitic wasp species are known to attack the diamondback moth in western Canada. Diadegma insulare and Microplitis plutellae attack the larvae, and Diadromus subtilicornis attacks the prepupa and pupa stages. In addition to the parasitic wasps, other beneficial organisms that attack diamondback larvae include lacewings, predaceous plant bugs and ground beetles, minute pirate bugs, spiders and birds.

Figure 18. Fungal infection of diamondback moth. (USDA) (Click for a 19KB color photo of fungal infection of diamondback moth.)

Lygus bugs have been observed during the past few production seasons in North Dakota, and yield losses may have been attributed to lygus bug populations in northwestern Minnesota during the 1999 production season. Lygus bugs are a major pest of canola in Canada. The parasitic wasp Anaphes iole can be used in an augmentative approach against lygus bugs. This wasp attacks lygus bug eggs on the flowers and buds. Releases are made when lygus bugs are found laying eggs during the flowering stage. Anaphes iole does well in warm and dry to humid environments. Some commercial insectaries may provide A. iole at a reduced rate for growers interested in experimenting on untested cropping systems.

Green peach aphid (Myzus persicae) and turnip aphid (Lipaphis erysimi) also have recently been observed in canola fields during the flower and pod development periods but have not reached levels that would be damaging to the canola crop. This may be attributed to the presence of natural enemies in the canola fields when aphids occur. Lady beetles are usually the dominant beneficials in canola fields when aphids are present and probably are maintaining aphids at low levels, although fungal pathogens play an important role as aphid densities increase.

Potato Insect Pests

The Colorado potato beetle (Leptinotarsa decemlineata), green peach aphid and potato leafhopper (Empoasca fabae) are the major pests of potato crops in the upper Midwest. Predators (lady beetles, nabids, lacewing larvae and syrphid fly larvae), parasites (aphid parasites) and fungal diseases can effectively maintain aphid populations below damaging levels.

Insecticides and fungicides are commonly used in potato production. Because the green peach aphid is resistant to most insecticides used in potato production, their numbers increase substantially in the absence of their natural enemies. In addition, regular fungicide applications appear to reduce the effectiveness of beneficial fungal diseases that play an important role in keeping green peach aphid populations in check in potato fields.

The Colorado potato beetle is the major insect pest in potato crops and often determines the method of pest management. Microbial insecticides provide an option for beetle management and are less harmful to most natural enemies. Microbial insecticides give adequate control of the beetle and allow natural enemies to effectively reduce green peach aphid populations below damaging levels. Proper application timing is important to ensure adequate control of the beetle. Microbial insecticides have a short residual life and must be applied frequently. In addition, they are not effective against larger larvae (third and fourth instars) and adults and must be targeted toward the smaller larvae. The microbial insecticides should be applied between 15 and 30 percent egg hatch for best control. Frequent application should be continued because beetle eggs are laid over several weeks.

Sunflower Insect Pests

Sunflower is native to North America and, although hundreds of insects have been recorded on sunflower, only a small number have achieved pest status. Indigenous natural enemies have been a significant factor in preventing many plant-eating insects from becoming economic pests. Even the insect species that have become pests are subject to attack by numerous natural enemies. Insects of main concern to cultivated sunflower in the northern Great Plains include the red sunflower seed weevil (Smicronyx fulvus), the banded sunflower moth (Cochylis hospes), the sunflower beetle (Zygogramma exclamationis), the sunflower stem weevil (Cylindrocopturus adspersus), and the sunflower midge (Contarinia schulzi).

Larvae and pupae of the red sunflower seed weevil overwinter in the soil and are reported to be attacked by three different predators in North Dakota, including larvae of two species of predaceous flies in the family Therevidae, Thereva candidata and Fucifera rufiventris. In addition, ants of the species Formica cinera also attack and consume overwintering seed weevil larvae in the soil. Larvae of the red sunflower seed weevil have been reported as hosts for a number of parasite species, including the pteromalids Trimeromicrus sp. and Pteromalus sp. and three species of braconids, Bracon mellitor, Nealiolus curculionis, and Triaspis aequoris. Female T. aequoris deposit eggs singly in weevil eggs and early-instar larvae and overwinter within the larva, killing it the following spring.

Predation is reported to be important in reducing populations of the banded sunflower moth. Eggs and larvae of the banded sunflower moth are consumed by minute pirate bugs, nabids, green lacewings, and several species of lady beetles (Hippodamia spp.). A number of species of predaceous ground beetles are present in the fields and probably consume overwintering larvae. Analysis of gut contents of beetles showed that the carabid beetle, Pterostichus lucublandus (Figure 19), attacks and feeds on banded sunflower moth larvae. Studies in Canada revealed that at least two species of parasites attack banded sunflower moth larvae and are often able to maintain the pest below economic levels. Studies in North Dakota, Minnesota, and South Dakota on cultivated sunflower showed that the banded sunflower moth is parasitized by the braconids Chelonus phaloniae and Macrocentrus ancylivorus and the ichneumonid wasps Glypta prognatha (Figure 20), Mastrus sp., and Trathala sp. The most abundant parasites of the banded sunflower moth in cultivated sunflower fields are C. phaloniae and G. prognatha, and the relative impact of each parasite species on the banded sunflower moth appears to vary from year to year, with a total of 40-70 percent of overwintering larvae parasitized.

Figure 19. Carabid beetle feeding on sunflower beetle larva. (Click for a 37KB color photo of a carabid beetle.)





Figure 20. Glypta prognatha, a parasite of the banded sunflower moth larva. (Click for a 24KB color photo of a parasite of the banded sunflower moth larva.)

Based on a multiple-year study of the sunflower beetle in Manitoba, Canada, a number of predators were determined to be feeding on the different life stages of the sunflower beetle. Adult beetles were attacked by redwinged blackbirds and the stinkbug, Podisus maculiventris. The latter species also attacks larvae in North Dakota sunflower fields. Sunflower beetle eggs are consumed by the melyrid beetle, Collops vittatus, and the lady beetles, Hippodamia tredecimpunctata and H. convergens, and green lacewings consume both eggs and larvae. Sunflower beetle larvae are also fed on by the stink bugs, Perillus biocolatus and P. circumcinctus. The carabid beetle, Lebia atriventris, which was reported feeding on sunflower beetle larvae in Manitoba, is a common inhabitant of sunflower fields in North Dakota and Minnesota. Nabids are common predators in sunflower fields and also are suspected of feeding on sunflower beetle larvae. Parasites have been reported to attack three stages of the sunflower beetle. Studies in North Dakota, Minnesota, and Manitoba have shown eggs are parasitized by the pteromalid Erixestus winneman and larvae are attacked by the tachinid fly Myiopharus macellus. The life history of M. macellus is well synchronized with the life history of its host, and the rate of parasitization is high in both Canada and the U.S. (up to 70 and 100 percent, respectively). Adult sunflower beetles are parasitized by the tachinid, Myiopharus sp. Approximately 0.2 to 17 percent of adults in Manitoba are attacked, but less than 2 percent of adults are parasitized in North Dakota and Minnesota.

No predators have been reported to attack the sunflower stem weevil. However, this is probably due to a lack of research rather than an absence of predators. The sunflower stem weevil is parasitized by a number of wasp species. Anaphes pallipes (Hymenoptera: Mymaridae) was reared from eggs of the weevil. Larvae are attacked by the following Hymenoptera in the northern Great Plains: Nealiolus curculionis (Braconidae); Quadrastichus ainsliei (Eulophidae); and Mesopolobus sp. (Pteromalidae). Nealiolus curculionis represented over 96 percent of the parasites attacking sunflower stem weevil larvae from 1980 to 1991 in North Dakota and is a consistent mortality factor even when the weevil density is low.

No predators have been reported to attack the sunflower midge. However, a number of generalist insect predators are on the sunflower heads when midge eggs and larvae are present. The parasites of the sunflower midge are largely unknown. Only one undescribed species has been discovered, Inostemma sp., and the impact of this larval parasite has not been studied.

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E-1225, January 2002