FRUIT IPM UPDATE #11

Transcription

1 FRUIT IPM UPDATE #11 August 29, 2017 What s New? When to Stop Spraying..1-3 Apple Pest Counts..6 Apple Scab Infections... 7 Final 2017 Newsletter Deciding When to Stop Spraying Thaddeus McCamant, Specialty Crops Management Specialist, Central Lakes College Many apple growers started selling early varieties like Paula Red and Duchess on August 20, and as Labor Day approaches, growers will start harvesting Zestar. As growers gear up for the main season, many are asking, "When should I stop spraying?" Pest pressure has been high this summer: apple scab, apple maggot, and codling moth have all been at high levels in some orchards, forcing some growers to spray every two weeks all summer, rather than at select times. Unfortunately, there are few guidelines on deciding when to spray the last insecticide. In Minnesota, each orchard has different pest pressure. In some orchards, codling moth is the worst pest in late summer, while in other orchards, apple maggot numbers are higher than codling moth. A few orchards have lesser apple worm instead of codling moth. Each grower must look at their own trap counts, and take into account varieties that are most threatened by the pests while following the correct preharvest intervals for different insecticides. Finally, each grower has a different perception of risk when it comes to insect pests. Always check the pre-harvest and reentry intervals before spraying any pesticide. At this time of year, growers are spending a great deal of time in their orchards and they may not be able to wait a week to harvest or wait several days before sending workers into the orchard. With temperatures in the mid 80's, it will take about 10 days for the eggs of codling moth adults that are flying now to become worms in the apples. Codling moths that hatch in the coming weeks could be alive when the apples are being bought and nobody wants to surprise their customers with a large, pink worm in the apple core. Consumers these days are more squeamish about wormy apples than ever. Most apple growers can readily spot an apple with frass in the calyx (Figure 1), but many customers don't realize that the frass means that the apple either has or has had a worm (Figure 2). Figure 1. Frass on the calyx of an apple, indicating codling moth. Apple Pest Update #11, August 29,

2 Figure 2. Codling moth damage. The larva had already departed. Figure 3. Frass at the calyx of an apple indicating lesser apple worm. On the other hand, most trap counts are fairly low. Many of the growers who participate in the weekly trap counts for apple pests for the MDA's IPM project are finding four to six moths per week. The official threshold for controlling codling moth is five moths in a pheromone trap over the course of one week. Some growers have avoided spraying at trap counts as high as ten adults with few bad consequences. In the August 22 IPM newsletter, lesser apple worm counts were quite high. Growers rarely worry about lesser apple worms, because the two pests are closely related, and lesser apple worms are usually controlled at the same time as codling moth. Like codling moth, lesser apple worms leave frass in the calyx (Figure 3), but the larva typically stay near the hollow area at the calyx and do not burrow into the core (Figure 4). Lesser apple worms tend to leave less frass than codling moths, but from my experience, it is impossible to distinguish the two without cutting the apple in half. Figure 4. Lesser apple worm in an apple. The official threshold for apple maggots is also five adults per trap if the lure is baited with apple essence, and lower for an unbaited trap. Again, most of the participants in the IPM project have numbers that are hovering around the threshold as August ends. We are at the end of the apple maggot season, and if maggots weren't controlled earlier, the apples will already show damage. With light damage, the apple will have a few sunken spots, with the small but distinctive oviposition scars (Figure 5). Apples with major damage are shrunken and knobby. Apples with a few maggots have white flesh with a few dark lines where the maggots have crawled through the flesh of the apple (Figure 6). If the apple has enough maggots to make the apple knobby, the flesh will be a solid brown and the apple will not be suitable for any use. Apple Pest Update #11, August 29,

3 Unlike codling moths, apple maggots have strong preferences for certain apple varieties. Their favorite Minnesota variety appears to be Fireside, but Honeycrisp, State Fair, and Chestnut are not far behind. In one study I did ten years ago, trap counts were higher in traps placed in Fireside trees than in trees with no apples. Apple maggots are small, blend in easily with the flesh of an apple and are nearly impossible to see in most apples. Still, there are some customers who would be horrified if they found out that there were small maggots in their apples. If apples are left at room temperature after picking long enough for the apples to become mushy, the maggots can be easy to see. One of the gray areas of pest management is deciding to spray when two distinctly different pests like apple maggot and codling moth are both slightly below threshold. This is where the grower's perception of risk plays a critical role. Some growers will do anything to prevent codling moths from entering the fruit during picking. Others will take a chance. There are also growers who will tolerate small numbers of apple maggots. All growers should consider spraying some varieties while leaving others. There is no reason to spray Zestar if they going to be harvested within a week. On the other hand, Honeycrisp won't be fully ripe for another three weeks while Fireside could be ripe in another month. If the weather warms in September, there will be time for both codling moths and apple maggots to hatch and burrow into your Honeycrisp. Figure 5. Oviposition scars on a Chestnut Crab apple with apple maggot. Figure 6. The inside of the apple from Figure 5. The maggots are still very young, and there are few brown lines where the maggots have fed. Apple Pest Update #11, August 29,

4 County Location 2017 Degree Days (Dates)* Apple Insect Pests: Weekly Trap Counts for 2017 Dates Traps were Out in Orchard CM DWB LAW OBLR RBLR STLM AM Chisago Shafer (05-01 to 08-28) to , 4, 5, , 0, 0 Cook Grand Marais CO (05-01 to 08-22) to Cook Grand Marais RB (05-01 to 08-22) to Fillmore Preston (04-01 to 08-29) to , 2, 6 Morrison Little Falls to , 2, 0 Rice Faribault (04-21 to 08-28) to ~800 2,1,1,0,1,0 Scott Veseli (04-01 to 08-29) to , 13, 21 St. Louis City of Duluth to St. Louis Duluth (04-01 to 08-30) to Wabasha Lake City - Site (04-01 to 08-29) to , 1, 6, 10, , 3, 2 74, 15, , 9, 1 Washington Hastings Site (04-01 to 08-28) to , 0, ~215 0,0,0,0,0,0,0,0,0 Washington Hastings Site (04-01 to 08-29) to , 9, ~160 ~170 4, 2, 1 KEY: AM = apple maggot, CM = codling moth, DWB = dogwood borer, LAW = lesser apple worm, OBLR = obliquebanded leafroller, RBLR = redbanded leafroller, STLM = spotted tentiform leafminer, AM = apple maggot --- = no data provided. * Degree days using Spec 7 Pro or Spec 8 Pro = cumulative or integral degree days (uses hourly temperatures), with calculation parameters of Base Temp = 50 o F and Upper Limit = 86 o F. Apple Pest Update #11, August 29,

5 2017 Apple Scab Infections (Cornell Model) Produced using Spec 7 Pro or Spec 8 Pro (Spectrum Technologies, Inc.) (Parameters: Base temp. = 33 degrees F, Upper limit temp. = 78 degrees F, Wetness threshold = 6) Orchard # of Infection Periods Infection Period Dates Overall Hours of Wetness 2017 Dates Duluth , 08-27, thru Hastings Site , 08-27, thru Hastings Site , 08-26, 08-27, 08-28, thru Lake City Site , thru Shafer , 08-25, 08-26, thru Veseli , thru DISCLAIMER References to products in this Newsletter are not intended to be an endorsement to the exclusion of others which may have similar uses. Any person using products listed in this publication assumes full responsibility for their use in accordance with current manufacturer directions. 625 Robert Street North St. Paul, MN In accordance with the Americans with Disabilities Act, this information is available in alternative forms of communication upon request by calling TTY users can call the Minnesota Relay Service at 711. The MDA is an equal opportunity employer and provider. Apple Pest Update #11, August 29,