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Beneficial Insects in Alfalfa Fields

Introduction

Insect fauna of alfalfa fields contains mainly two major groups, i.e. beneficial insects and destructive insects or insect pests. The Later one contains insects that can cause damage to different parts of alfalfa plants in various stages of growth. Alfalfa has many insect pests, such as Lygus bugs, alfalfa plant bugs, pea aphid, alfalfa weevils and grasshoppers. The beneficial insects can be divided into true predators and parasitoids. True predators are those insects that can consume the entire prey items. Parasitoids on the other hand attack and consume prey also, but consume one prey item to complete immature development; the adult stage is free living. Successful parasitoid handles its prey to keep it alive until the immature development of such parasitoid is completed and that often synchronize with the death of the prey.

The following is some information about beneficial insects that commonly found in alfalfa fields with description of their main characteristics, life history, their potential in controlling insect pests of alfalfa, and some notices on their behaviors, react to environmental conditions and some of the means they use to protect themselves.



True predators are those insects that can consume the entire prey items. Parasitoids on the other hand attack and consume prey also, but consume one prey item to complete immature development; the adult stage is free living. Successful parasitoid handles its prey to keep it alive until the immature development of such parasitoid is completed and that often synchronize with the death of the prey.

The following is some information about beneficial insects that commonly found in alfalfa fields with description of their main characteristics, life history, their potential in controlling insect pests of alfalfa, and some notices on their behaviors, react to environmental conditions and some of the means they use to protect themselves.

 

Lady Beetles

Also called ladybugs or ladybirds, they are the most commonly known of all beneficial insects. Adults and larvae are predaceous feed by their strong mouthparts on soft-bodied insects, mainly aphids. They also feed on the eggs of moths and beetles, Lygus and alfalfa plant bug nymphs, mites, thrips, and other small insects.

 

 

 

 

 

 

 

 

 
Description

Adults, 4-7 mm long, are domed shaped, oval or convex and often shiny. Legs and antennae are short and the head is hidden from above. Fore wings are usually reddish-orange to pale yellow with or without black spots or irregular marks. Some are black, often with red spots or markings. The area immediately behind the head, the pronotum, may also have a distinctive pattern. Eggs are tiny, about 1 mm, cream, yellow, or orange in colour, and oval to spindle-shaped. Larvae are elongate, alligator-like, have three pairs of prominent legs, and dark in color. They are covered with tiny tubercles or spine. Newly hatched larvae measuring about 1 mm, while fully growing larvae exceed 1 cm in length. Pupae may be dark or yellow-orange.

Life cycle

The length of life cycle varies depending upon climatic factors and food supplies. It usually requires 3-4 weeks to complete the life cycle, or it can expand up to six weeks in cooler seasons. Lady beetles overwinter as adults, often in aggregations, under rocks, park and leaf litter, and along hedgerows. In spring, the adults disperse and search for suitable prey and oviposition sites. This dispersal could be far away from overwintering or releasing sites. This phenomenon can affect the reliability of control by released adult beetles. An adult female eats about 300 median-size aphids before laying eggs, and 3-10 aphids are eaten for each egg she lays. Female may lay from 20 to more than 1,000 eggs in her lifetime. Eggs usually deposit near aphid colonies in clusters of 10 to 50 each (a female can consume 75 aphids per day, while male may consume 40 aphids per day in average). Eggs hatch in 3-5 days. Larvae involving through 4 larval instars during 20-30 days. One larva can eat about 400 medium size aphids during its life span (eats aphids about its own weight every day). The last larval instar is relatively inactive and attaching itself by the abdomen to a leaf or other surface to pupate. The pupal stage last from 3 to 12 days and then the adult emerges. Adults may live for a few months to over a year. Usually lady beetles have one to two generations per year.

Notices

During the autumn, lady beetles intend to overwinter. A few to several hundreds of adults are gathering in an aggregation and colonize an overwinter site. This site may be a base of a tree, a fencerow, fallen tree, rock of fallen leaves.

Adults emerging from overwinter and those releasing in spring apt to disperse widely and leave the area of emergency or releasing, especially when temperature reaches 65F and above. Thus we cannot rely on these beetles to control the potentially growing aphid population. Also releasing beetles collected from distance areas may be of less value as a control tool. Relying on the local population of lady beetles and attempting to attract them to the target fields would be a better method.

Protection

Lady beetles have many interesting ways to protect themselves. Their external coloration warns birds that they will taste bad. They also may “play dead” to prevent the danger of other predators. Lady beetles probably produce bad smelling odor, perhaps a fluid from joints in the legs, which make them detestable for predators. The fearsome looking of the larvae is another mean of protection of these beetles. 

 

II. Minute Pirate Bug

Minute pirate bugs are common predators of phytophagous mites, mite and insect eggs, aphids, thrips, and small caterpillars. Both adults and nymphs feed by sucking the body fluids of their preys by their piercing sucking mouth parts. Both nymph and adult can consume more than 30 spider mites per day. 

Description

Adult of minute pirate bug is tiny, about 3 mm length, very flat, and oval-shaped. The head and thorax are shiny black. The forewing has a small dark area at the base followed by a wide pale brown area, then a smaller triangular dark area, and the reminder of the forewing is membranous. Egg is very small, about 0.5 mm length, and pale white in color. The red eye of the embryo appears through the eggshell before hatching. The nymph is small, resembles the adult but is wingless, teardrop-shaped, yellow to brown in color, and fast moving.

Life cycle

Under optimum conditions, the development of minute pirate bug from egg to adult takes about 3 weeks. The minute pirate bug overwinters as an adult in leaf litter or under tree bark or boards. The adult emerges in spring and start feeding on the available hosts and then mating is took place. Shortly after mating, two to three days, the female embed eggs in plant tissues. In adequate conditions, adults live about 35 days and female lays an average of 129 eggs. Depending on climatic conditions, 2-4 generations each year may be seen in the Northwest and Midwest. Adults of last generations move into inactivity when day length drops below 16 hours. An incubation period of about 6 days at 23°C is required for egg hatching. Nymphs hatch from eggs and pass through five instars before becoming adults. About 19 days are required to complete the nymphal stage.   

Minute pirate bugs are effective searchers on their preys and are voracious general predators. They are able to aggregate in areas where their preys are found in high density and depress the prey population.

Minute pirate bugs may occasionally feed upon pollen and plant nectar where their preys are not available. They can survive by feeding on plant juice and pollen, however their reproductive may be impaired.

Minute pirate bug may occasionally cause mild stinging sensation to human when the adult attempts to penetrate the skin with their mouthparts.   

 

III. Lacewing

Lacewings are common predators on aphids and other pests like spider mites, thrips, white flies, small caterpillars, Lygus and alfalfa plant bug nymphs, leaf miners, beetle larvae, and eggs of leafhoppers, and moths. Some species of lacewings are important predators in greenhouses. Larvae, known as aphid lions, are the active predators in many species of lacewings, while adults of these species feed only on nectar, pollen and aphid honeydew. In some species, like Chrysopa oculata, both adults and larvae are predators.

 

 

 

 

 

 

Description

Adults of lacewing have slender bodies and about 12-20 mm long. They have beautiful appearance with green in color body, long antennae and bright golden eyes. They have large, transparent, veined, pale green wings. The wings are held tent-like over the body when at rest. Eggs are less than 1 mm long, oval in shape and are laid individually or in clusters at the top of hair-like filaments. These filaments are pale green turning gray after several days. Larvae, or aphid lions, are very active, grow from >1 mm to 6-8 mm. They are alligator-like in shape, and gray or brownish in color. The larvae have well-developed legs and suctorial mouth parts with large grooved, sickle-shaped mandibles (jaws). Mature larvae spin spherical, white pupal cocoons by using silk secrete from the Malpighian tubules, which attached at their tips to the rectum.

 

 

Life cycle

Lacewings have complete metamorphosis. Adults appear in summer and lay their eggs on foliage attached to their remarkable hair-like stalks. Each adult female lays several hundreds of eggs, over 200, during her life span (about 2-3 weeks). Adults are nocturnal, active at night. The incubation period of eggs is about 3-6 days after which the larvae are hatching. The ant lions pass three larval instars before pupate in the silky pupal cocoons. The larval stage lasts for 2-3 weeks. Each larva will devour 200 or more pets or pest eggs a week during its developmental period. They hold their preys by their legs, inject them with venom, and insert their jaws inside the prey bodies and suck their body fluids. Lace wings in northern the Midwest overwinter as pupae while most species in other eastern, southern and western areas of North overwinter as adults. Lacewings have wide range of generations, 1-10, according to climatic conditions. In most of the agricultural areas of the Canadian Prairies they have 1-2 generation per year. 

Notices

The lacewing larvae are considered as general predator, however their favorable food is aphids. Sometimes they may have difficulty find prey in crops with hairy or sticky leaves. There is some differentiation in desiccation tolerance among lacewings species. Therefor, some tolerate species, like C. carnea, are recommended for release and control in dry areas, while desiccation susceptible species, like C. rufilabris, are recommended for humid areas and greenhouses.

Protection

Adults of lacewing also called “stink flies” because their ability to produce stink odor when caught as a means of defense against enemies.

 

IV. Damsel Bug

The damsel bugs (or nabids, from the family name Nabidae) of Nabis sp. are true bugs. Both adults and nymphs are predators on many small insects, mainly aphids, beside nymphs of Lygus and alfalfa plant bugs, leafhopper, small caterpillar, small coleopterous larvae like alfalfa weevil, mites and grasshoppers. Adult or nymph hides in flowers and jabbing the prey by piercing-sucking mouth parts, injecting enzymes that digest the body contents of the prey, and sucking the predigested fluid.

Description

Adult damsel bugs have elongated slender body of about 8-12 mm long. They are tan reddish brown to grayish brown in color. The membrane of each forewing is edged with a series of vein cells. The body narrows towards the head, with a long beak (rostrum). Front legs are modified for grasping preys and antennae are fairly long. Damsel bugs move rapidly and swiftly, and adults can fly for long distances. Eggs are fattened on top and less than 1 mm long. Nymphs resemble adults, however they lake complete wings and their reproductive system are underdevelopment.

Life cycle

The development of damsel bugs is incomplete (gradual) where three stages, adult, egg, and larva are composting the life history. By the end of growing season, adults of damsel bug overwinter in protected places in the field or near field margins. Overwinter may be taken place in alfalfa fields. They appear again in May or June and females start deposit their eggs inside soft plant tissues. Nymphs are hatching from eggs and the early nymphal instars start feeding on pest eggs or small insects. Nymphs develop gradually through five nymphal instars on about 50 days to become adults. Damsel bugs have only one generation per year.

Notices

Damsel bugs are aggressive and rapidly suck the body contents from their preys. Warm dry weather in the spring encourages the reproduction of damsel bugs and so increases their efficiency on controlling their favorable prey, aphids. If aphids’ population is not enough to supply enough food for damsel bugs, they will then switch to their second favorable, Lygus bugs. Late in the season, if the ratio between damsel bugs and Lygus is 2:1, the damsel bugs will maintain Lygus population in a level that will not cause an economic damage. However, cool, moist spring may allow aphid populations to build up before the damsel bugs population becomes large enough to provide control.

 

V. Ambush Bug

The ambush bug is a predatory true bug, which both nymph and adults can attack other insects. As its name describes, ambush bug hides in flowers waiting for any insects to visit and then grabs it by its strong forelegs. The ambush bug will inject its prey with venom, parlaying it and partially digesting its body contents.  The predigested fluid of the prey will then be sucked through its modified piercing sucking mouthparts.

 

 

 

 

 

 

 

Description

Ambush bugs are stout-bodied bugs with cryptic coloration and are 6-15 mm in length. Their color and pattern match the flowers they wait on for their preys, so they are hard to spot.

Life cycle

Adults deposit eggs inside plant tissues. Eggs hatch to nymphs, which similar to adults except the absence of complete wings and that they are still inactive reproductively. After several molts, these nymphs develop to mature adults. 

 

Notices

Ambush bugs attack any suitable prey that may visit the flowers where they hide. They usually attack preys larger than themselves. These preys could be wasps, bees, butterflies, or flies. Ambush bugs could eat beneficial pollinators as well as other pests that visit flowers.

 

VI. Assassin Bug

Like many other predatory bugs, both adults and nymphs of assassin bugs feed by external digestion. They pierce the body of their prey by their beak, inject a very toxic liquid that affects the nervous system and liquefies the body contents of their prey. Most other insects with piercing sucking mouth parts have two tubes in their beak; one for injecting the venom and second for sucking in their food, but assassin bugs have only one large tube that does both functions. This allows assassin bugs to inject a large amount of the toxic digestive secretion so that prey many times their size can be quickly overcome. These bugs get their names because of the speed they grab and poison their prey. Some assassin bugs are active hunters while others patiently wait their prey to come close enough to grab. They attack many of small and medium –sized insects, including plant bugs, caterpillars and beetle larvae.

Description

The assassin bugs are usually black or brown in color. They are about 12-25 mm long. The head is long with a powerful covered beak or rostrum that they use to pierce and suck out the tissues of their prey. The powerful forelegs are used in grabbing victims. These legs have sticky pads, which made up of thousands of tiny hairs that stick to the victims and prevent them from escape. Nymphs resemble adults except the absence of complete wings and that they are still inactive reproductively.

Life cycle

Most assassin bugs lay their eggs in the autumn in cracks and crevices that contain lots of leaves. After hatching from the egg, the nymph passes through five instars to become an adult. Assassin bugs overwinter in various stages, usually the adult stage, depending on the species. Usually have one generation per year.

Notices

Some tropical species of assassin bugs attack mammals, birds, and reptiles and suck their blood or use their beak to squirt their venom if they are attacked. Their saliva can cause severe irritation of the eye and nose and may cause temporary blindness.

 

VII. Syrphid Fly

Also known as hover or flower fly because it is hover and dart over plant canopy during its visiting to flowers. Larvae are predators on many of small-bodied insect, especially aphids. They have chewing mouthparts by which they tease their prey. Adults are not predators, feed on nectar and pollen and known as good pollinator.

Description

The adults of syrphid fly are about 6-18 mm long, black or brown in color and marked with yellow or white bands on abdomen. They resembling bees and wasps, but they can be distinguished that they have only two wings, their short antennae, their pattern of flight, and they are not able to sting. Eggs are chalky white with faint longitudinal ridge. Larvae are fleshy, 6-19 mm long, elongate, legless, slug-like, and tapered to a point anteriorly. They are usually green or brown with whitish strips on their back. It can be mistaken for alfalfa weevil larvae, but they do not have a black head capsule or legs. Last larval instar pupates in a tan-brown teardrop shaped puparium.

Life cycle

Most species overwinter as pupae either on the host plant or in soil. Adults emerge from pupae when condition promise. Females lay their eggs individually on leaves near aphid infestation or near other suitable host. Eggs hatch within 3-4 days into the soft-bodied maggot-like larvae. They develop through several instars before drop into soil or attach to plat to pupate. During its lifetime of 1-3 weeks, a single larva can consume about 150-400 aphids. If aphid population is very high, the larva can eat one aphid per minute. Number of generations varies among different species. There are at least three generations per year up to 7 overlapping generations each year.

Notices

It is very important to distinguish clearly between the larva of syrphid fly and the alfalfa weevil larva since the earlier is a beneficial insect while the later is a destructive one, which may need control.

 

VIII. Peristenus Spp.

Peristenus spp. are braconid wasps parasite Lygus bugs. P. howardi Shaw is a domestic species in Western Canada and Pacific Northwest. Another species, P. digoneutis Loan, was discovered in northern Europe where it is an effective parasitoid of the European tarnish plant bug, Lygus rugulipennis Poppius, which is an ecological equivalent of the tarnished plant bug, L. lineolaris Palisot. This European parasitoid was released in northeastern US in Early 80’s and found that it was established in alfalfa fields in New Jersey, New York and reached the southern part of Quebec. The targets of these parasitoids are Lygus bug nymphs and may also parasite other plant bugs of family Miridae.

Description
Adult of Peristenus spp. is a small, about 3 mm, brown wasp. Eggs are very small deposit inside host bodies. Larvae are about 1 mm and live inside host. Mature larva leaves the host and drop in soil to construct cocoon and pupate.

Life cycle

Peristenus spp. overwinter as pupa in its cocoon under the soil surface. Adults emerge in June and start inject their eggs singly into each tarnish plant bug nymph. The adults live 2-4 weeks. Eggs hatch in 5-7 days. The hatched larvae feed on the body content of their host and complete their development in about 7-10 days. The mature larvae emerge from the dying host and spin cocoons to pupate in them just under the soil surface. There are two generation per year for these wasps in alfalfa.

Notices

In the areas where these parasitoids already established, a parasitism rate of up to75% was recorded. Some studies found that a parasitism rate of 30% decreases pest numbers by 75%. These parasitoids may be dispersed to other crops and this phenomenon results in the oscillation of the parasitism rate from year to another. Some studies found that tillage may affect the overwintering pupae in soils and decrease the rate of adults’ emergence. More studies about the host range of the European parasitoid and the possibility of confliction with the domestic one are needed before any decision on releasing P. digoneutis in alfalfa in Western Canada.

 

IX. Big-eyed Bug

Another predator true bug of order Hemiptera. They are very fast and well-hunting predators. Big-eyed bugs prefer feeding on Lygus bugs, aphids and thrips, beside insect eggs, small caterpillars, whiteflies and mites. They appear in the field when Lygus bugs begin to reach economically damaging levels. Both adults and nymphs are predators. Sometimes they can confuse with Lygus and false chinch bugs, but the unique eyes and head can help in differentiate them from other true bugs.

Description

The big-eyed bug can be gray, brown or yellowish in color. Oval and somewhat flattened and about 3-4.5 mm long. The have distinct large bulbous eyes in both adult and nymphal stage. Nymphs resemble adults but don’t have fully developed wings or reproductive system.

Life cycle

Adults lay their eggs on the lower surface of plants near colonies of aphids, Lygus or other suitable host. Eggs hatch in 5-10 days depending on average temperature. The hatched nymphs undergo 5 nymphal instars before becoming mature adults. Larval stage may last for 2-3 weeks. A nymph can eat up to 1600 spider mites before reaching adulthood, while an adult reported to consume as many as 80 mites per days. Depending on the species they may overwinter as an egg or an adult. They have 2-3 generations per year. 

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