General Production Information
Idaho ranks third in U.S. hop production accounting for 8% to 10% of the U.S., and 2% of the world hop production. Idaho hop production from 1993 to 1996 averaged 1,388 pounds per acre on 3,977 acres. Total production for that period averaged 5,521,210 million pounds with an on-farm value of $8,889,148 million dollars, annually. In 1998, production dropped well below that average to 4,529,000 pounds on 3,909 acres with a total farm-gate value of 6,838,394. Establishment costs for a hop yard range from $2,950 to $3,885 per acre. Production costs for an established yard range from $2,103 to $2,890 per acre. Gross returns have averaged $2,236 per acre since 1991.

Over 90% of the crop is generally contracted at the time of harvest. Some contracts prohibit the use of certain pesticides on the contracted crop, in order to meet specific brewing industry requirements or to comply with the import tolerance requirements of importing countries. In a given year 40% to 60% of the crop is exported to overseas markets.

Production Regions
Hop production in Idaho is concentrated in two geographically distinct areas: the cool, moist region of the northern Idaho panhandle in Boundary County (53 %) and the warmer, arid region of southwestern Idaho in Canyon County (47 %). Climate, soil type, hop varieties, and therefore, hop production practices, vary greatly between these two areas.

General Information
Hop is a specialty crop produced for the female flowers, or cones, which either raw or processed, are an essential ingredient in the production of beer. Lupulin glands on the hop cones contain soft resins (a and b acids) essential oils that impart bitterness, flavor, aroma, foam (head) characteristics, and preservative qualities to beer. The total amount and percentage composition of these compounds varies with variety and growing conditions. Because the brewing industry depends on hops to provide distinctive and proprietary characteristics to beer, a stable supply of high quality hops is a high priority.

Hop, Humulus lupulus L. (Urticales: Cannabaceae), is a perennial plant related to nettles and cannabis that produces climbing annual stems from a perennial rootstock and crown. The stem grows in a clockwise direction around its support and may reach a total length of 25 feet or more in a single growing season. The stem dies back to the crown after the hop flowers mature. Hop is dioecious, producing male and female flowers on separate plants. The commercial hop is a female plant with flowers (burrs) produced on side arms that develop along the stem. Burrs develop into hop cones. Fertile male plants are not tolerated in commercial hop yards as their pollen causes seeds to be produced reducing the brewing quality of the cones. Hops are vegetatively propagated, with new yards established by planting root cuttings (rhizomes) or, occasionally, softwood cuttings.

Cultural Practices
Hop plants are grown on a wire and cable trellis suspended about 18 feet above the ground on a regular arrangement of poles. Cement anchors, attached to trellis cables and buried five feet deep, surround the yard and hold the trellis upright under the weight of the developing crop. Plant spacing is either wide, with 14 to 15 feet between rows and 3.75 feet between plants with in rows, or narrow, with plants set 7.5 feet apart on a regular grid. With either planting arrangement there are approximately 55 poles and 900 plants per acre. Once established, the hop rootstock will produce indefinitely although industry practice is to rotate plantings every 10-15 years. Timing of rootstock replacement is influenced by declining yield caused by insect and disease and pests and by brewer demand for specific varieties. The major cultural practices used annually to produce hops include pruning, stringing, training, irrigating, and harvesting. Each of these is discussed below.

Pruning is an annual spring practice used to hold back the vigorous new annual growth until the proper training date for that variety. Pruning begins in late March and is accomplished either mechanically or chemically. Mechanical pruning uses tractor-drawn equipment with spinning steel "fingers," to remove early vine growth, debris from the prior season, and leave a clean, surface from which the new shoots will arise. Chemical pruning involves the use of desiccants to kill back early vine growth. Fields pruned chemically are often 'scratched' with a harrow to remove surface debris.

Stringing the trellises begins in early April as seven-man crews using tractor-drawn elevated platforms tie the coir (coconut husk fiber) twine to overhead trellis wires and secure the lower end of the twine into the hop crowns with small metal clips.

Training is the practice of wrapping the hop shoot in a clockwise direction around the stringing twine to facilitate rapid vine growth. Training begins in early May and is by the end of May. Due to the relationship between plant height and day length that determines flowering time, the training date is one of the most critical factors in determining yield. Training date is variety specific. Hop vines (or bines) in narrow spaced fields must be tied together, or arched, 6 ft. to 8 ft. above the ground in late spring to facilitate crop management activities.

Irrigation of hop fields begins in the latter part of May or early June, depending on weather and growing area. The hop field will require approximately 30 inches of water during a normal growing season. Various methods of irrigation are utilized, including rill, sprinkler and drip. In Idaho most irrigation is by rill method, where water is siphoned out of head ditches into smaller ditches (rills) alongside the rows of hop plants.

In rill irrigated hop yards, cultivation for weed management, and to mechanically maintain hill size and rill integrity, occurs four to six times during the season. Drip irrigated hop yards are generally cultivated fewer times per season.

Harvest begins in late- August, and progresses through late-September with each variety reaching peak maturity at a different time. Harvest begins in the field as the hop vines are cut at the ground and at the overhead support wires, and are placed into a trailer or truck bed. The vines are transported to stationary picking machines that are capable of picking 8 acres in a single 10-12 hour shift or 15 acres if picking runs round the clock. Most U.S. hop growing operations have one picking machine for each 250-300 acres of hops.

The vines are hung upside down on hooks and carried into the picking machine, where hops and leaves are stripped from the vine and sent through a series of cleaning devices to remove leaves and other debris.

The stripped vines and other debris are chopped and spread back onto the fields and service roads. To date, this activity has not resulted in the build up of disease or insect pest in hop yards. The introduction of hop powdery mildew into the Pacific Northwest, this practice may force growers to change this practice. Powdery mildew cleistothecia (spores from sexual reproduction between two different mating types) can exist on dead plant debris. If powdery mildew cleistothecia become common other methods for disposing of plant debris may need to be found.

Cleaned cones are transported by conveyor belt to the hop kilns. Kiln floors are each approximately 32' x 32', and hold some 15,000 pounds of hop cones. Cones are spread to a depth of about 32 inches. Once the kiln is filled, cones are dried as hot air (140°F) from oil or gas burners is forced through the bed of green hops. The drying process requires about 9 hours, reducing the hops to 30% of the green weight, with 8-10% moisture content. Hops are removed from the kiln floor and cooled for 24 hours. After cooling, the hops are compressed into 200-pound bales, wrapped in burlap, subjected to quality inspection, and transported to cold storage warehouses.

IPM Programs
The hop industry has received an EPA PESP grant and is in the process of collecting information for the purposes of building an industry-wide IPM program.

Resistance Management Strategies
The hop industry is actively pursuing three alternate chemistries for each major pest of hops for the purposes of resistance management.

Insect Pests
Regardless of production practices, arthropods represent a major factor limiting hop production in Idaho. Although numerous arthropods are known to attack hop, the most serious pests in the Idaho hop-growing region are the twospotted spider mite, Tetranychus urticae, the hop aphid, Phorodon humuli (Schrank) (Homoptera: Aphididae), the black vine weevil, Otiorhynchus sulcatus (Fab.) (Coleoptera: Curculionidae), and the California prionus, Prionus californicus. A review of the life cycle, damage to hops, and current management practices for each of these arthropods is provided below. Hop aphid and two-spotted spider mite are the two most important pests. The majority of efforts to control non-disease hop pests are targeted at these two species.

The twospotted spider mite, Tetranychus urticae
Damage to hop. Spider mites damage hops by feeding directly on hop leaves. Leaf feeding by spider mites results in silvering and browning of hop leaves, and reduced plant vigor. Severe infestations can cause complete defoliation and are accompanied by heavy production of webbing that can hinder chemical and biological control efforts. Most economic damage, however, is associated with spider mites feeding on hop cones. Spider mites not only contaminate the cones by their presence, but their feeding results in dry, brittle, discolored (red) cones. Spider mite-damaged cones tend to shatter so that both quality and quantity of yield is reduced. When infestations are severe, total crop loss can occur. Damaging populations of twospotted spider mite (TSSM) are favored by hot, dry climates. In the hop growing regions of Washington and southern Idaho, where these conditions prevail, spider mite populations frequently reach outbreak levels. Without control of these pests, there would be 100% crop loss.

Management
Miticide sprays are scheduled based on scouts monitoring TSSM populations. Various sampling schemes have been developed to serve different purposes. Economic thresholds shift with the season: two colonizing females/20 leaves in late June, 10 females or 50 motile forms/leaf in late July, and 50 females/leaf in mid-August.

Chemical Controls:
Abamectin (Agri-Mek 0.15EC, 16 fl oz./A). 14 day PHI. This product is the industry's main miticide. 100% of the acres are treated.
Propargite (Omite-CR, Omite 30WS). 14 day PHI. Rates of 1.5-2.25 lbs. AI/A for Omite-CR, or 1.5 lbs. AI/A for Omite 30WS. Omite, first registered on hops in 1971, has been slowly fading in performance. Resistance has been implicated but no definite studies documenting resistance have been done. Approximately 5%-10% of the acreage is treated on a regular basis.
Dicofol (Kelthane MF, 1.0 - 1.25 lbs. AI/A). 7 day PHI. Used extensively in the late 1950s until the late 1960s when mites developed high levels of resistance. It is now of limited importance. In recent years, researchers suggested reserving use of this compound only for phytosensitive varieties with one application/season spaced three years apart.
In 1997 and 1998 a Section 18 request for cyhexatin (Pennstyl 5F) was submitted but not approved. In 1999 another Section 18 request was submitted and approved. Elf Atochem is pursuing a full Section 3 label for this compound on hops. 40% of the total acreage was treated in 1999.
In 1998 a crisis exemption was issued for hexythiazox (Savey 50WP), but none of the Idaho acres was treated because spider mite populations flared outside of the 60-day PHI. A 24c was obtained in 1999, restricting use to pre-bloom applications. Hexythiazox is an ovicide, with end use dates of July 1. The industry estimates 1999 usage will result in approximately 10%-20% of the acres treated.
The hop aphid, Phorodon humuli

Damage to hop
Economic damage to hop can result from direct feeding of aphids that lowers plant vigor and productivity through the consumption of water, carbohydrates and other nutrients being transported in the phloem tissues on which aphids feed. If severe hop aphid infestations are not controlled, defoliation of hop plants and loss of yield by premature drop of hop cones can occur. Contamination of hop cones with aphids and honeydew are the major cause of economic damage. Growth of sooty mold on aphid-produced honeydew discolors the hop cones, severely lowering crop value. Large infestations can render an entire crop unmarketable. In contrast to T. urticae, damaging populations of P. humuli are favored by cool, moist climates. If not properly managed, P. humuli is capable of causing serious damage to hops grown in any of the hop-producing regions of the U.S. P. humuli is also implicated in the transmission of several viral diseases of hop.

Management
Insecticide sprays are scheduled based on aphid abundance (leaf counts of less than 10 aphids/leaf). Sampling schemes have been devised, but industry-wide firm economic thresholds have been difficult to establish. Growers using drip irrigation may set thresholds slightly higher than growers using rill irrigation, as spray timing in drip irrigated yards is not complicated by coordination of spray and irrigation schedules. The two goals, preventing economic loss at bloom and preventing aphids entering cones late in the season, have different thresholds.

Most hop acreage in Idaho is treated one or two times per season although in bad aphid years some yards may be sprayed five times. The average probably is two times for the entire acreage. Without control, 100% of the hop acreage would be lost to this pest.

Chemical controls:
Diazinon (various trade names, 1 lb. AI/A). 14 day PHI. Still the most important aphicide in Idaho hop fields. The product provides early season aphid control and is important as a rotation partner to prevent resistance development to imidacloprid. It has limited residual activity. Acreage treated about 80%-100%.
Imidacloprid (Provado, 8 fl oz of 1.6 F product/A). 14 day PHI. This product is highly effective against but more costly than diazinon. It works well as a foliar spray but has also been applied under experimental use permits through drip irrigation systems and as a shanked-in soil application with excellent results. Such application methods may favor conservation of beneficial organisms. Percent acreage treated is about 40%-60%.
Bifenthrin (Brigade, 0.06-0.1 lbs. AI/A). 14 day PHI. A very effective, widely used compound used to treat about 75% of the acreage. While it also kills some adult mites, it causes spikes in mite populations later in the season. It has great value to the industry as a rescue treatment for lepidopteran pests.
Malathion (various trade names, 1.25 lb. AI/A). 10 day PHI. Used for approximately 40 years but recent use has been very limited, as efficacy is poor. No acreage is treated.
Phorate (Rampart 10-G, 2-3 lbs. product/A). 42 day PHI. Used early season it can be banded over the row after crowns begin regrowth, or incorporated 2-4 inches deep. Acreage treated is about 10%. The registrant did not renew the registration of this 24c in 1998. A tolerance remains and existing stocks can be used.
A registration for pymetrozine on hops is imminent. The industry hopes to be using this compound by the year 2000 growing season. It is a very effective aphid control.
The black vine weevil, Otiorhynchus sulcatus

Damage to hop
Adult O. sulcatus feed on hop foliage. Adult feeding is not reported to result in economic loss. Economic losses can result from larvae feeding on the roots of hop plants. Root damage by larval results in reduced nutrient uptake, water stress, and reduced plant growth. Premature leaf drop and plant death have been associated with feeding damage caused by black vine weevil larvae. Heavy infestations may require that individual hop plants or even whole hop fields be removed from production.

Management
Currently black vine weevil is controlled by using insecticides targeted at adult weevils. Applications are made about three weeks after adult emergence, but before egg laying begins. It is important to apply insecticides at night when adult weevils are most active. No host-plant resistance to O. sulcatus in hop has been identified. Little research has been conducted with respect to the identification of arthropod natural enemies of O. sulcatus in hop. However, control by several parasitic nematodes has been demonstrated in field trials.

Chemical controls:
Bifenthrin (Brigade, 0.06-0.1 lbs. AI/A). 14 day PHI. A very effective compound used to treat about 10% of the acreage. Should not be used when mites are present.
The California prionus, Prionus californicus

Damage to hop
Prionus californicus is widely distributed in the Pacific Northwest and has been recorded as a pest of hops in Idaho for over 60 years. Adult P. californicus are not known to damage hop plants. Larvae feeding on the roots of hop plants can, however, cause serious economic loss. Feeding by P. californicus larvae damages plant roots resulting in decreased nutrient uptake, water stress and reduced plant growth. Severe infestations can completely destroy plant crowns resulting in plant death. Less severe infestations can result in the wilting, yellowing, and death of one or more bines of infested plants.

Management. Management of P. californicus in hop consists of identifying and removing infested plants. It may be necessary to dig up and remove all plants in severely infested fields. Fields taken out of production must be fumigated and/or left fallow for two-three years before being replanted to hops. There are no chemicals presently registered for preventing new, or controlling established P. californicus infestations in hop.

Loopers, Cutworms, and armyworms
These insects can cause serious damage at certain stages of growth, but outbreaks tend to be less widespread than those seen with mites and aphids.

Spotted cutworm (Amathes c-nigrum) and redback cutworm (Euxoa ochrogaster) are pests of early season crown growth. They are usually not a problem unless a heavy infestation feeds on newly trained vines. Bifenthrin is the only effective registered compound but treatment is usually not economically feasible.

Hop looper (Hypena humuli) is a summer defoliator. Damage is usually confined to the lower portion of the vine. Chemical control has been poor except for bifenthrin, which is rarely economically feasible.

Bertha armyworm (Mamestra configurata) and Corn earworm (Helicoverpa zea) are pests at harvest. Eggs are laid on pigweed or lambs quarter in the hop yard. Every three to four years, populations reach levels causing severe damage (30%). Caterpillars defoliate the weeds and move up into hops, chewing on the stems and causing cones to fall on the ground. Emergency harvest results in worms curling up inside the cones and later becoming a contaminant in the dryers. An effective control is a low rate of bifenthrin if the problem is recognized at least 14 days prior to harvest.

Chemical Controls:
Bifenthrin (Brigade, 0.06-0.1 lbs. AI/A). 14 day PHI. It has great value to the industry as a rescue treatment for lepidopteran pests. Acreage treated is about 10%.
Dibrom (Naled, 1 lb. AI/A). 7 day PHI. Works very well on worms only. Up to five applications may be made at 14-day intervals. Acreage treated is about 10%.
Bacillus thuringiensis var. kurstaki (Dipel, Javelin, 0.5 to 1 lbs. product/A). 0 day PHI. The material must be ingested by the worms to be effective. Little to none of the Idaho acreage is treated.
Diseases
Downy Mildew

Downy mildew is a serious threat to profitable hop production. Direct losses result from reduced yields, infected and shattered cones, and crown die-out. Certain hop varieties are more susceptible to the disease, and Oregon growers must deal with annual outbreaks, due to their moist climate. The fungus overwinters in infected hop crowns and first appears in the spring as an infected shoot, commonly known as a primary "spike." The spike has pale green or light yellow, slightly downward-cupped leaves, and shortened internodes. Under favorable environmental conditions, the under surface of the leaves becomes blackened with spores, which spread the disease to other shoots, causing lesions to develop on the leaves. Secondary spikes are formed from these shoots.

Spike formation halts growth of the infected shoot reducing or eliminating flower and cone production. Infection of hop flowers results in flower death directly reducing cone yield. Cone infection results will be death of a portion of the cone. The dead portion becomes blackened and is unacceptable to brewers. Crown infection with downy mildew in certain varieties results in crown death during the winter. Fungicides are utilized on an "as needed" basis to control this disease. Without control susceptible varieties will experience substantial yield loss and crown die-out, necessitating replanting (100% loss).

Chemical Controls:
Metalaxyl (Ridomil Gold, 0.25lbs. AI/A). 45 day PHI. Used as a tank mix with copper compounds (2 lbs. Kocide 101) for resistance management. This fungicide is the most effective against downy mildew. One treatment per year is applied on 25%-100% of the total acreage, depending on environmental conditions.
Fosetyl-al (Aliette WSP, 2 lbs. AI/A). 24 day PHI. This fungicide is also effective against downy mildew, and is alternated with copper sprays for resistance management. Depending on environmental conditions, 25%-100% of the total acreage may be treated.
Copper (various formulations). 14 day PHI. This fungicide is effective against downy mildew and can be used alone or tank mixed with metalaxyl. Depending on environmental conditions, 25%-100% of the total acreage may be treated.
Powdery Mildew

Powdery mildew is a serious disease in hops in other hop growing areas of the world. The disease was largely responsible for eliminating commercial hop production in New York during the early part of this century. Prior to 1997, powdery mildew had not been observed in the Pacific Northwest. The hop industry has attempted to protect their growing region from introduction of this disease through the use of state hop quarantine laws. As the disease was not present, no fungicides were registered to control it.

The Idaho hop industry discovered hop powdery mildew in 1998. Approximately 20 hop fields were infected with infections ranging from light to heavy in individual fields. One lesion in a single field was found prior to harvest in 1999. Without control 100% loss would be expected on susceptible varieties. It is expected that varieties currently exhibiting some resistance/tolerance for the disease will rapidly become susceptible due to disease mutation.

Chemical Controls:
In 1998, Section 18s were requested and granted for fenarimol and myclobutanil, and a crisis exemption was issued for use of tebuconazole. Preliminary results indicate good activity at field rates. 100% of the acreage was treated with one or more of these compounds.
In 1999, Section 18s were requested for trifloxystrobin, myclobutanil, and tebuconazole. None of the Idaho acres was treated with any of these compounds.

Biological Controls:
AQ10 (Ampelomyces quisqualisis a hyperparasite of powdery mildew. As a parasite it is only able to survive when its host is already present in the field. It is only effective during early season in high humidity and at low temperatures. Mildew rates must be 2% or lower for any control, as the parasite cannot keep up with mildew growth rates. At rates higher than 2%, and during different environmental conditions, the AQ10 fungus is not effective against mildew. Growers use this product in the early season, and then start preventative spray schedules. About 25% of the Idaho acreage was treated in 1998. None of the acres was treated in 1999.
Other diseases include Verticillium wilt, and crown and root rot caused by downy mildew or Phytophthora spp. Previously listed fungicides control these diseases. The primary viral disease in hops is necrotic ringspot virus, which is spread through propagation material. The U.S. hop industry promotes the use of virus-free rootstock by growers and regular replanting of hop yards to avoid viral problems. Nematodes are not an economic concern at this time.

Weeds
Few herbicides are available for hops, and weed control is largely done by cultivation, which can exacerbate water quality problems in return flows (the majority of Idaho hop acreage is rill irrigated). Desiccants are very important tools for the hop industry, as early season growth must be burned back for several weeks prior to optimum training dates for the specific hop variety. Desiccation is also important for the prevention of powdery mildew through spike removal. In late June to early July, the lower leaves need to be removed from hop vines in order to improve airflow through the hop yard, and to control diseases and pests. Herbicides for adequate weed control and desiccation efforts are generally lacking in hop production. Without control losses will vary by variety.

Chemical Controls:
Paraquat (Gramoxone Extra, 0.47 - 0.78 lbs. AI/A). 14 days PHI. Used for chemical pruning on hops. Marginally effective when used alone and temperatures are below 7°F. Approximately 100% of the acreage is treated.
Endothall (Des-I-Cate, 0.5-1.0 lbs. AI/A). 28 days PHI. A desiccant used for chemical pruning. It does not have any weed control activity. Growers may tank-mix endothall and paraquat to enhance the activity of both products. Temperatures of at least 75°F are needed for acceptable desiccation. Approximately 100% of the acreage is treated.
Norflurazon (Solicam DF, 2.0-4.0 lbs. AI/A). Controls grasses and some broadleaf weeds in established hops. Apply in the fall or within one week after stringing in the spring. Approximately 20% of the acres are treated.
Trifluralin (Treflan HTP, TR-10, EC and 5 formulations, 0.5-0.75 lbs. AI/A). A dormant application. Controls grasses, but has to be soil incorporated which restricts weed control to cultivated areas. Weed control within the crown is not possible using trifluralin, as foliar spray results in damage to the hop crown in the following year. Less than 5% of the acreage is treated.

Contacts
Author, Industry, and Technical Contact

Jim Barbour, Hop Pest Management
University of Idaho
Parma Research and Extension Center
Parma, ID 83660

Phone: (208) 722-6701
FAX: (208) 722-6708
Email: jbarbour@uidaho.edu

Prepared September 1999