Winter is usually dry and sunny in West Hawaii. Though we have had more rain than usual this year, winter often presents ideal conditions for grasshoppers to breed and feast in our gardens.
Grasshoppers have chewing mouth parts. Their damage usually appears as chewed sections on the edge or in the interior of a leaf. The damage may look similar to that caused by slugs, caterpillars or Chinese rose beetles, so be sure to identify the pest before you choose a treatment. Grasshoppers have a wide range of preferences, but when they find leaves they like, they can cause major damage to your garden.
Since grasshoppers travel long distances and move often, you may find that discouraging them is the best way to prevent their damage. Plants such as cilantro and calendula can repel grasshoppers, if you plant them ahead of grasshopper season. Most alliums, including chives, onions, leeks and society garlic also discourage them.
Several nontoxic substances can deter grasshoppers. Make your own garlic oil by placing minced garlic in either neem or mineral oil and letting it sit for a day before straining out the garlic. A mix of about 2 tablespoons of garlic oil, some chili pepper sauce, water and enough liquid soap to allow the oil to disperse can effectively deter grasshoppers. Adding fish emulsion can increase the repellent odor while fertilizing your plants. Neem oil is another effective repellent that can keep grasshoppers at bay as well as prevent them from laying eggs or feeding and developing properly.
If grasshoppers are already present, trapping is a good low-tech, low-toxic solution. Use a can or similar container and bury all but the very top in the ground. A mixture of molasses and water in the container will attract grasshoppers and they will drown.
Another preventive measure against grasshoppers is to encourage their predators. Ducks, chickens, turkeys and other large birds are natural enemies of the grasshopper. Rodents also enjoy making a meal of grasshoppers and their egg pods. Toads, spiders and praying mantises will feed on grasshoppers. Cats love chasing grasshoppers and will usually kill them as they play with them and often eat them as well. Encouraging these predators can help reduce grasshopper damage.
If you notice that grasshoppers make regular visits to your garden, you could foil their attempts to eat your plants by placing covers over their favorites. Floating row covers are made of a lightweight fabric that can be placed over taller shrubs or over hoops around lower plants and secured with garden staples. These covers can keep insects out while allowing light and water in.
If you find many grasshoppers in your garden and their damage is severe, several immediate solutions can help. Grasshoppers lay their eggs in the soil making it difficult to kill them before they hatch. Once they hatch, however, they go through a gradual metamorphosis where they shed their skin and increase in size. The younger and smaller they are the more vulnerable they are to insecticidal remedies, so act as soon as you see them in your garden. Their metamorphosis takes four to six weeks to go from egg to nymph. They molt several times before becoming adults.
Several low-tox mixtures can kill grasshoppers. Four ounces of black strap molasses in one quart of water sprayed directly onto grasshoppers will clog their pores and prevent breathing. Diatomaceous earth can be dusted directly onto plant leaves and will scratch the waxy layer of the grasshoppers underbelly, eventually causing death. A cocktail including 1 cup of diatomaceous earth, 1 gallon of water and 2 tablespoons of black strap molasses sprayed on leaves where grasshoppers feed can also kill them. The fatty acids in an insecticidal soap can be sprayed on vulnerable leaves and will adhere to the grasshopper’s body, eventually dissolving its exoskeleton and causing death.
If a stronger response seems necessary, organic compounds of rotenone and pyrethrins can be effective against grasshoppers. The granular form of the protozoan Nosema locustae is also deadly to grasshoppers though harmless to mammals and other insects except crickets. Grasshoppers that ingest this become infected with a disease that severely impairs the insect’s growth process.
Watch for grasshoppers in your garden this winter and try to prevent their damage however you see fit.
Tropical gardening helpline
Phyllis asks: I have a lot of young strawberry guava, waiwi, trees in my yard which I would like to remove. Should I cut them down to the ground? How do I treat the stumps to kill them? I know I have to remove the cuttings from the yard as they come back easily.
Answer: Strawberry guava trees can be a real nuisance. You can try the following strategies to control strawberry guava without herbicides. First, cut all the stems very low to the ground, then smother the stumps with black plastic temporarily or, for a longer term, use cardboard covered with thick mulch. Wood chips work well.
Prevent any light from reaching the stumps. Meanwhile, look for root shoots and cut them as soon as they appear. It is also a good plan to remove the cuttings as some of them will likely sprout.
If you want to kill the stumps and you are not organic, paint them with an herbicide designed for stump killing. It may take several applications to work, but it should eventually kill the stump and roots. For best absorption of a systemic herbicide designed to kill, apply it immediately after you cut the stump.
Email plant questions to konamg@ctahr.hawaii.edu for answers by certified master gardeners. Some questions will be chosen for inclusion in this column.
Diana Duff is a plant adviser, educator and consultant with an organic farm in Captain Cook.