Can you recall the smell of cut grass? Many plants, including cranberries, emit different smells, or volatiles, when damaged by cutting, but

Dr. Elvira de Lange in the greenhouse

also when damaged by herbivorous insects. These volatiles could then be used by predators to find prey, or by parasitoids to find hosts. However, fertilizer applications may affect emissions of these herbivore-induced volatiles, and therefore the interactions between plants, herbivores, and their natural enemies.

At the P.E. Marucci Blueberry & Cranberry Research Center, Drs. Elvira de Lange and Cesar Rodriguez-Saona studied the effects of fertilizer on volatile emissions in potted cranberry plants. They tested six cranberry varieties: Howes, Early Black, Potter, Stevens, Franklin, and Crimson Queen, exposed to four fertilizer regimes: 0, 0.5, 2, and 4 g NPK controlled-release fertilizer.

A previous study showed that increased fertilizer rates decreased resistance to herbivores. Three insect herbivores gained more weight on plants exposed to the higher fertilizer regimes, and also showed increased survival on these plants. Possibly, this was due to increased levels of nutrients and reduced levels of defensive compounds.

Fertilizer regimes

To test whether fertilizer also affects plant volatile emissions, we placed bags around potted cranberry plants, and using small pumps, sucked the air out of the bags. Connected to the pumps were trapping filters containing an adsorbent material, which was used to trap the volatiles. The results showed that higher fertilizer rates increased the total emission of volatiles, that is, the emission of all individual volatile compounds combined. This effect was found for all tested cranberry varieties, and was due to an increase in plant fresh weight. Indeed, higher fertilizer rates increased plant biomass substantially. Exposure to higher fertilizer rates also led to the emission of compounds that were not emitted when plants were exposed to lower fertilizer rates.

The effects of these increased volatile emissions, and changed volatile blends, on natural enemy attraction still need to be investigated. However, it is known that volatiles not only attract natural enemies, but they can also attract, as well as repel, insect pests. It is important to study possible changes in insect communities in response to fertilizer applications, to determine when and where to apply chemical and biological products for optimal pest control.

Volatile collection apparatus

In cranberry, previous studies by Dr. Cesar Rodriguez-Saona showed that plant volatiles affect the attraction of an herbivore, Sparganothis fruitworm; several predators, such as lacewings, ladybeetles, and hoverflies; and some parasitoid wasps. Potentially, volatiles could be used to manipulate pests and natural enemies in agro-ecosystems, directing natural enemies towards crops, while directing pests away from them. Therefore, adjusting volatile emissions, through select fertilizer applications, use of synthetic volatiles, selection of particular varieties, or otherwise, could contribute to integrated pest management programs in cranberry.

This study is published in Plant Signaling & Behavior: https://doi.org/10.1080/15592324.2019.1616517

We thank Rob Holdcraft, Vera Kyryczenko-Roth, and the summer students in 2015 for assistance with the experiments. Funding was provided by Hatch Project No. NJ08192 and the New Jersey Blueberry and Cranberry Research Council, Inc., Cranberry Institute, Cape Cod Cranberry Growers Association, Canadian Cranberry Growers Coalition, and Ocean Spray Cranberries, Inc.

Earlier this year, Exirel® (FMC Corporation) was registered for use in cranberries. The Exirel label includes the target pests: cherry fruitworm, cranberry fruitworm, blackheaded fireworm, and Sparganothis fruitworm. The active ingredient (a.i.) is cyantraniliprole. This insecticide belongs to a class of insecticides known as anthranilic diamides (similar to Altacor). This class of insecticides is particularly effective against lepidopteran pests but Exirel has broader insecticidal activity that includes flies, aphids, weevils, etc. After ingestion, anthranilic diamides work by impairing muscle contraction, resulting in feeding cessation, lethargy, and partial paralysis, eventually leading to the death of the insect.

Exirel is considered a reduced-risk insecticide and compatible with biological control. It should thus be considered as a new alternative when rotating insecticides with different modes of action for preventing the development of resistant populations. Exirel has a 14-day pre-harvest interval (PHI) and a 12-hour re-entry interval (REI). A maximum of 0.4 lb a.i. can be applied per acre, which equates to three applications at the highest label rate (20.5 fl oz per acre).

Previous work in our laboratory has demonstrated that Exirel can provide similar, or sometimes even better, larval control as compared with other registered materials such as Intrepid, Delegate, and Altacor.

The following insect pests bear special mention for early-season scouting in cranberry bogs:

Blackheaded fireworm – Blackheaded fireworm eggs overwinter on the beds and usually hatch by around mid-May. It is important to catch the first generation, if possible, because the second generation occurs during bloom and is typically much more destructive. Blackheaded fireworm larvae can be detected by sweep net sampling and it is a good idea to look along the edges of beds where vines first begin to grow. Remember: blackheaded fireworm is much easier to control if detected during the early part of the season.

Blackheaded fireworm larva

Spotted fireworm – overwinters as a 2nd instar larva. They complete two generations a year. Larvae feed between uprights they have webbed together. First-generation larvae injure the foliage causing it to turn brown as if burned. In New Jersey, first generation adult moths emerge the first week of June, followed by a second-generation of adult emergence in early August. Eggs are laid in masses on weedy hosts. Larvae from second-generation adults emerge in mid-August, and may feed on fruit. Populations of spotted fireworm are regulated by their natural enemies, in particular Trichogramma wasps that parasitize the eggs.

Sparganothis fruitworm – This insect is a serious pest in most cranberry-growing states. Sparganothis fruitworm completes two generations a year and overwinters as an early-instar larva. Larvae from the 1st generation feed on foliage. In New Jersey, first generation adult moths emerge from mid-June through the first weeks in July; pheromone traps are commonly used to monitor adult flight and population size. Second-generation eggs are laid on cranberry leaves, and larvae will feed on fruit.

Cranberry blossomworm – Adults lay their eggs in October in cranberry beds. The eggs overwinter and hatch over a period of several weeks. Early instars can be found during the first week of May. Larvae go through 6 instars to complete development. Because the first instars feed during the day (and also at night), scouting can be done during the daytime using sweep nets to estimate larval abundance. Larvae turn nocturnal during the later instars. At this time, night sweeping (9 pm – 1 am) is recommended for sampling. Larvae complete their development by June-July. Older instars are very voracious and capable of destroying 100 blossoms within a 3-week period. There is a pre-pupal that lasts until the end of August and a pupal stage that lasts until October. Adults emerge from end of August to end of October.

Spotted fireworm larva

Lepidopteran Pests Monitoring and Control – Use sweep netting for monitoring early lepidopteran pests (pre-bloom). A sweep set consists of 25 sweeps and 1 sweep set is recommended per acre (this may vary depending the size of bogs). The action threshold for false armyworm, blossomworm, other cutworms, and gypsy moth (we use a combined threshold from adding all these caterpillars per sweep) is an average of 4.5 caterpillars in sets of 25 sweeps. For brown and green spanworms is an average of 18 per sweep set. The action threshold for blackheaded fireworm and Sparganothis fruitworm is an average of 1.5 per sweep set. We recommend the use of the reduced-risk materials Intrepid, Altacor, Exirel, or Delegate if populations exceed action thresholds. These are reduced-risk, softer insecticides that are very effective against lepidopteran pests. More information on these (and other) lepidopteran pests will be provided as the season progresses.

Sparganothis fruitworm larva

Leafhoppers –Blunt-nosed leafhoppers transmit cranberry false blossom disease. This leafhopper has one generation a year. Nymphs may be found from the end of May, while adults are found in highest numbers during July. Eggs are laid in August-September. The eggs overwinter and hatch in May or June. The nymphs go through 5 instars to complete development.

Leafhopper Monitoring and Control: Leafhopper nymphs can be sampled using sweep nets (as described above for lepidopteran pests). Nymphs before bloom are small; thus, you may need to freeze the samples (to kill them), and then count the number of nymphs under a microscope or using a magnifying lens. There is no threshold based on sweep net counts, so decisions should be made by comparing current numbers with prior infestation history and/or incidence of false blossom disease on those beds.
In cases of high numbers of blunt-nosed leafhopper nymphs, we recommend application of a broad-spectrum insecticide, such as Diazinon (no aerial applications allowed) or Lorsban (only pre-bloom applications allowed for Ocean Spray growers). Broad-spectrum insecticides will disrupt biological control particularly the natural enemies (predators and parasitoids) of Sparganothis fruitworm, so their use should be restricted only to areas of high leafhopper populations.

Blunt-nosed leafhopper nymph

Cranberry blossomworm larva