Using Soap to Kill Insects

 Is soap good to kill insects in the garden?  

 Yes it is, in fact, gardeners have been using soaps as an insecticide since the early 1800’s.  But there are some qualifications.  An insect wears its skeleton on the outside – called an exoskeleton.  This is the hard shell we see on many insects like beetles, grasshoppers and cockroaches.  Soap will breakdown this exoskeleton, causing desiccation and thus killing the insect.  But soap has no residual affect and therefore must be sprayed directly on the pest.  Soaps will not kill insects that land on plants after they have been sprayed.

The other problem with soaps is that some plants are very sensitive to them and could exhibit some burning on the leaves - called phytotoxicity.   This is especially true if you are mixing your own concoction.  Use only a mild dishwashing soap with no additives. Purchased products are well refined and would have fewer problems with phytotoxicity.    

Here is a University of Hawai`i recipe for a good oil and soap mixture:  Make a concentrate of one tablespoon dishwashing liquid and one cup of vegetable oil.   When ready to use, shake well and mix 1 – 2 ½  teaspoons of the this mixture into one cup of water.  Spray plants thoroughly, every 5 – 7 days as needed.  Don’t spray in the heat of the day.

The Plague of Powdery Mildew

Powdery mildew (PM) is a menace to many gardeners. The disease will turn large zucchini leaves white and later brown. A whitish cast will appear on many common garden plants such as tomatoes, beans, cucumbers, mustard, peas, and collards. Infected leaves often turn yellow to brown and may shrivel making the plant unproductive and eventually die.

The white fungal growth can develop both on the upper and the lower surfaces of the leaf, and sometimes, on flowers and fruit. The cottony like threads of this fungus travel along the surface of the leaf, occasionally sending “roots” down into the leaf tissue in order to obtain nutrients.

Many different types of powdery mildew fungi are host specific. This means that one particular mildew fungus will only infect those plants in a particular genus or family; the PM on the beans will not attack papaya, and the PM on mangoes will not attack tomatoes. 

The fungus likes temperatures between 60 and 80 degrees F being sensitive to temperatures above 90. Cool days and warm nights favor the disease. Spores of the pathogen are dispersed readily by wind. Even though this disease flourishes with high humidity, wet leaves can actually inhibit germination of the fungal spores, thus preventing infection. Because of this, the disease should be minimized during periods of heavy rainfall. Also during this time, the numerous spores that are on the leaves will be washed away.

Control – Stopping powdery mildew in its earliest stages of development provides the best control. This can be done by sanitation: remove and destroy infected parts of the plant. Other measures of control are as follows: plant in the sunniest locations, provide good air circulation through pruning, and avoid excessive applications of fertilizer.

Fortunately there are a number of relatively safe and effective materials to use against this fungus - wettable sulfur, horticultural oils, including neem, and Kaligreen (potassium bicarbonate a relative of baking soda). Serenade is a biological fungicide, a bacterium, that helps prevent the powdery mildew from infecting the plant. Cucurbits (melon, squash, cucumbers) can be sensitive to sulfur.  Do not apply when the temperature is near or over 90 degrees and do not apply within 2 weeks of an oil spray. Use these materials in the earliest stages of disease development for best results.  Before spraying, it would help to remove the leaves that are heavily infested.  Do not dispose of them on the ground since they are loaded with fungal spores. Powdery mildews can attack healthy plants; but older plants, less vigorous plants and those that are stressed are more susceptible to infection.  

Most important, choose plant varieties (vegetables, fruit trees and ornamentals) which are tolerant or resistant to the powdery mildew fungus. For more information about seeds from locally developed vegetable varieties, contact the University of Hawai‘i at Manoa: http://www.ctahr.hawaii.edu/seed/seeds.asp. Or, contact your local university cooperative extension office. Note: Some of the resistant varieties will exhibit powdery mildew symptoms, but the disease is less severe. 

Photos: Cornell University, Colorado State University and USDA

Rats in Fruit Trees

Hollowed out oranges, papaya, bananas and other fruits, whether hanging on the tree or on the ground, are definitely signs of rat feeding.  In Hawaii, there are four rodents that cause economic hardship: the roof or black rat, the Norway or brown rat, the Polynesian or Hawaiian rat, and the house or field mouse. In addition, they are carriers of contagious diseases including plague, murine typhus, leptospirosis, and salmonellosis. Today, most notably, they are part of the disease cycle known as Rat-Lungworm. See health.hawaii.gov/docd/disease_listing/rat-lungworm-angiostrongyliasis/  for more information and prevention of this serious condition.

Rodent control is not easy due to their ability to adapt to changes and their capacity to reproduce.

Rats are nocturnal. They have excellent memories and very repetitious habits. They are easily frightened of new things placed in their environment. The roof rat is most pronounced in this tendency. Rats have a keen sense of smell and hearing, and only a fair sense of sight with the ability to see in the dark.

Besides the hollowed out fruits, other common signs of rats are droppings, rubmarks, gnawings, nests, and unpleasant odors.  

                                                         Rat Damage on Oranges - UC IPM

In controlling these pests around the home, the first step is to clean up the environment by removing accesses to food and shelter. Physical barriers such as screens may need to be installed. When only a few rats are involved, trapping can be successful, both live and snap traps. Devices that kill rats by electrocution (e.g., Rat Zapper or Victor Electronic traps) are expensive but effective.

Paraffin-type bait blocks containing anticoagulants are also effective in controlling rats. They appeal to the rats gnawing instinct, especially those blocks with numerous ridges. Baits should be replaced immediately as they are eaten, since a single feeding on the first generation anticoagulants such as diphacinone, will not control rats; multiple doses over several successive days is required. The newer “second-generation” compounds such as brodifacoum, bromadiolone, difenacoum, and difethialone, which can be fatal after a single feeding. Prepackaged, ready-to-use bait stations containing some of these toxicants can be purchased by homeowners. 
   

Some poisons have a secondary effect which will affect animals that consume dead or nearly dead rodents. Thus, it is imperative that strict safety precautions be used in the placement and disposal of poison baits for rodents.

For more information on rats see the UC IPM website, http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74106.html

Nematodes

 Nematodes are microscopic roundworms. There are both good and bad nematodes. Many nematodes live in our soils and are beneficial.  But there are a number of harmful species that attack plants. These nematodes insert their stylet mouth-part, much like a  hypodermic needle, into the  tissue and suck out the plant juices.  

Plants which are infested with nematodes may exhibit symptoms of stunting, yellowing and wilting. Further symptoms may be smaller and fewer leaves, and smaller fruit.  Also, due to this feeding, the root knot nematode, in particular, will cause the plant cells to rapidly enlarge, thus developing characteristic bumps or knots on the roots. 

What to do?  If root knot nematodes are in your garden, take it seriously. Eradicating the garden of nematode pests is highly unlikely, but they can be managed with good sanitary procedures. Here are some measures:

·      Apply organic matter (OM) to the soil.  OM will encourage the growth of beneficial organisms, fungi, bacteria and good nematodes. They, in turn, will ‘attack’ the plant parasitic nematodes and deplete their numbers. A note of interest, there is one species of fungus that forms loops with its fungal threads. Nematodes will become stuck in these loops, which will actually tighten, and eventually the nematode dies. 

·      Crop rotation  Do not replant the same crop or any susceptible crop into infested soil.  For instance, if you discover root knot nematodes on your cucumbers, plant a resistant crop, like garlic or corn, in place of the cucumbers. For best results, do not plant with a susceptible crop for 3 years.

·      Keep the soil fallow. A more drastic form of crop rotation is simply keeping the infested area fallow. This starves the nematodes. Do not allow any crop, even weeds to grow there. But do add organic matter. Again, 3 years is best.

·      Soil sterilization  This involves covering the infested area with a piece of clear plastic for at least one month and preferably two. This should be done during the hottest part of the year.  In hot climates with clear skies, enough heat can be produced underneath this tarp to kill most of the nematodes. 

·      Plan resistant varieties. The best control against nematodes is to use resistant varieties. You may notice on a tomato seed package the letters VFN.  This means that the variety is resistant to the fungus, Verticillium and Fusarium and to the root knot nematode.  Such varieties include Celebrity and Better Boy. Asparagus, corn, garlic, onion, strawberry, zinnia, salvia and marigolds are also resistant to root knot nematode.  

Most Important: DO NOT SPREAD CONTAMINATED SOIL

Perhaps the most important aspect of controlling nematodes is to stop them from spreading throughout the garden.  If your tomato patch is newly infested with nematodes, keep those nematodes there. Nematodes will slowly spread in the soil by themselves. Don’t help them by carrying soil from the infested area to other areas, or by carrying around contaminated dirt clinging to garden tools and shoes. Work in an infested area last. Always clean tools and shoes.  Be cautious when receiving potted plants from other gardens or even from a nursery that has non-sterilized soil.    

Marigolds have been reported for many years as a way of reducing the nemotode populations. The French marigolds are most effective; varieties include Nemagold, Petite Blanc, Queen Sophia, and Tangerine. Unfortunately, planting a few marigolds next to each susceptible plant will NOT eliminate the problem. To be effective, the entire infested area needs to be planted in marigolds, each plant about 7 inches apart. The flowers must be planted for a minimum of two months and then turned under. The area also needs to be kept free of weeds. Then, root knot sensitive plants can be planted back into that area the following planting season. Replanting marigolds every other year will almost certainly be necessary.   

Pesticides Leftover

Often times gardeners will  mix up an insecticide spray and store any leftovers for the next application.  But is it still good?  Maybe.  Mixed pesticides lose effectiveness depending on their type and the time stored. When powdered pesticides are kept dry and sealed, they can last for quite some time; the same is true for liquid formulations. But once they are mixed with water the solution may quickly lose it's potency.  

 It is the pH level of the water used in the spray mix that influences the chemical breakdown of many pesticides. The pH of a liquid is the measurement (ranging from 0 – 14) of its acidity or alkalinity. Seven is neutral, above 7 is alkaline and below 7 is acidic. Some insecticides, particularly the groups known as carbamates and organophosphates, undergo a chemical reaction in the presence of alkaline water. The reaction is known as alkaline hydrolysis, and reduces the effectiveness of the pesticide’s active ingredient. 

The chemical breakdown of a pesticide is commonly referred to in terms of its half-life. A half-life is the period of time it takes for one-half of the amount of pesticide in the water to degrade. A common insecticide, sevin, has a half life of 100-150 day in solutions whose pH is at 6. That number rapidly decreases to 24-30 days at a pH of 7. At a highly alkaline solution of 9, its half life is only 1-3 days.  Malathion’s half life goes from 8 days to 19 hours as the pH shifts from 6 to 8. One of our old fungicides, captan, last about 8 hours at neutral (7), and only 2 minutes when the spray solution is at a pH of 9.  When mixed pesticide solutions are stored for days, weeks or months, their effectiveness can greatly diminish.