Hop Aphid  The hop aphid, Phorodon humuli (Schrank),  overwinters as little black eggs on ornamental and fruit plums, Prunus spp.  The eggs hatch in February and March followed by several generations of females (males are produced only in the fall) usually causing a rapid increase in aphid numbers.  The aphids begin to fly to hops in early to mid-May.  The length of the flight season is very variable from year to year but flight usually ends by mid-July.  Hop is the only summer host and only wingless females are produced on hops during the summer.  The aphids feed on the leaves (usually the underside) and in the cones.  In the late August or early September, winged females begin to fly back to the plums where they give birth to a generation of wingless, egg-laying females.  The winged males appear about a month later, fly to the plums, and mate with the egg-laying females.  Then the overwintering eggs are laid (the aphids give live births at all other times of the year), completing the life cycle.  The immature aphids are translucent white to bright green (they tend to turn green as they age) and the wingless adults are green.  The winged aphids are mostly black.  The aphid can be separated from other aphids by the presence of frontal tubercles - two horn-like projections on the front of the head between the antennae. 
    The aphid reduces yield by sucking juices from the plant.  The aphids also produce honeydew, upon which sooty mold grows.  Mold contaminated cones are considered low quality by brewers and bring a low price and may even be unmarketable.  Hop aphids can also transmit viruses.  
    Currently, hop growers rely on chemicals to keep aphid numbers below damaging levels.  No economic injury level has been established but almost every year, aphid numbers are high enough to warrant at least one insecticide application.  See Beneficial Insects below.  

Twospotted Spider Mite  Spider mites are small spider-like, plant-feeding animals that are more closely related to spiders than insects.  They spin webs, are wingless, and have four pairs of legs (except that the first stage out of the egg, the larva, has three).  The twospotted spider mite, Tetranychus urticae, is the mite pest of hops.  Mated females overwinter in plant debris and emerge when green plants appear in the spring.  Females develop from fertilized eggs, while males emerge from unfertilized eggs.  The spherical, pearly-white eggs are laid on the leaf, often in the mite webbing.  The mites go through many generations per year and have a tremendous capacity to increase their numbers, especially in hot weather.  They suck sap from the leaves, removing nutrients from the plant.  Heavy infestations can defoliate hop plants.  No economic injury level has been established.  Although growers depend upon chemical acaricides to control mites, they are reducing chemicals to take advantage of biological control.  Under natural conditions, spider mites are usually kept at low numbers by predatory mites and insects.  However, the acaricides used to kill spider mites also kill the predatory mites and insects, allowing the mite populations to quickly rebound to damaging levels.  In addition, mites are notorious for developing resistance to chemicals, rendering chemicals ineffective.  The key to integrated pest control of mites appears to be reducing acaricide applications to increase and stabilize predator numbers and reduce the chance of mite resistance.

Black Vine Weevil  See our description under grapes.  The main damage to hops is caused by the larvae feeding on the roots.  Plant vigor declines over several seasons.  Two other species of root weevils are also reported from hops: the strawberry root weevil, Otiorhynchus ovatus, and the rough strawberry root weevil, O. rugosostriatus.  

                                

Armyworms  Two species of these caterpillars can be pests of hops: the Bertha armyworm, Mamestra configurata and the western yellow-striped armyworm, Spodoptera praefica.  Both species have dark backs and yellow stripes on their sides.  The larvae emerge in late summer and feed on hop foliage.  They often cut the cones from the plant, reducing yield.  In most years, their numbers are low but occasionally they become pests.  When they do cause damage, chemical control is about the only option.  The key to control is monitoring hop yards for caterpillars and damage.

The Hop Looper, Hypena humuli, is a noctuid moth that feeds on hops and stinging nettles (Urtica spp.). Until recently, we haven't considered it an important pest. However, probably due to the decreased use of broad-spectrum insecticides, the looper is becoming more common. Because it wasn't an important pest, we knew very little about this insect. Dr. Tess Grasswitz began studying the hop looper in 2004. Her research has added greatly to our knowledge of the hop looper's life history, impact of predation, and control.

The hop looper has three complete generations per year rather than two, which was previously believed. The third generation is the largest and most damaging. The adult moths overwinter and first appear in hop yards in early April. The female has a relatively long egg laying period, which results in a mixture of larval ages and sizes present at the same time.

Exclusion cage experiments showed that insect predators had a significant impact on hop looper larval mortality. Insect parasitoids, although relatively common, were never very abundant.

In larvicide trials, bifenthrin (Capture) at 6.4 oz/acre was the most effective material tested. However, Dipel and XenTari, two formulations of Bacillus thuringiensis (Bt), applied at 1 lb/ acre, controlled populations very well. XenTari worked slightly better than Dipel but the difference was not significant. XenTari at 0.5 lbs per acre reduced larval numbers but probably not enough for large populations. Some growers were concerned that Bt would not work well against large larvae, but these tests were done on populations of mixed larval sizes (22% of larvae were large 4th or 5th instar). Therefore, the Bt products did control the large larvae. The Bt formulations are toxic specifically to lepidoterous (moth) larvae. Therefore, so they should be less disruptive than bifenthrin to an IPM program. As always, read the label before applying a pesticide.

                 
        Ladybird beetle larva            Phytoseiid feeding on a mite egg

Beneficial Insects and Mites  Several species of insect predators prey on hop aphids.  The main ones are ladybird beetles (Coccinellidae), green lacewings (Chrysopidae), brown lacewings (Hemerobiidae), minute pirate bugs (Anthocoridae), big-eyed bugs (Lygaeidae), and damsel bugs (Nabidae).  Predators can be quite numerous and have promise for control in the future.  Spraying only when necessary will preserve these predators and give them a chance to work.  Wasp parasitoids are rare in hop yards but are abundant on plums in the spring.  Increasing the number of these wasps or introducing new species may reduce the number of aphids flying to hops in the spring and early summer. 
    Spider mites are attacked by several species of predatory mites and insects.  Neoseiulus fallacis and Galendromus (formerly Typhlodromus) occidentalis are the most common predatory mites on hops.  These predators can be quite numerous and effective in controlling spider mites.  Unfortunately, they are often susceptible to the same acaricides used to control spider mites, so chemical applications can be very disruptive to predators.  A small, black ladybird beetle, Stethorus picipes, can also be a very abundant and effective predator.  Minute pirate bugs, Orius tristicolor, big-eyed bugs, Geocoris sp., and six-spotted thrips, Scolothrips sexmaculatus, are also common predators.  These predatory insects are probably susceptible to most aphid sprays, so aphid chemical applications can result in mite outbreaks.  Some additional information can be found in our grape page.